There are few concepts that appear in the history of ideas that are common to many realms of thought in social and natural science and to philosophy in general. Evolution is such a concept, and its origin as a doctrine was deeply embedded in the social and economic conditions of the industrial West. Like all such world views, it embodies many principles, not all of which are admitted in its various uses, so that even those concerned with organic evolution are unable to agree on the essence of the idea of evolution.

Toward a definition

There is a hierarchy of principles in the evolutionary world view: change, order, direction, progress, and perfectibility. Evolutionary theories are distinguished by how many of these are successively included as essential. Some evolutionists include only change and order, others add direction, and some few, like Teilhard de Chardin, believe in perfectibility as well.

Change. The idea of evolution, in its simplest form, is that the current state of a system is the result of a more or less continual change from its original state. The qualification that the change be continual, or at least frequent or regular, is an essential one and distinguishes evolutionary from static world views. For example, many opponents of organic evolution in the nineteenth century accepted the authenticity of fossils but regarded them as antediluvian, as evidence of one or several floods that caused a total replacement of the world’s fauna. Diluvianism, like the closely related vulcanism, which postulated a series of inundations of the earth by lava, was a theory of catastrophic and irregular change. There is no fundamental difference between a theory of special creation that populates the world once with unchanging beings and one that populates it several times. They are both theories of special intervention of unique forces in an otherwise normally static system. The distinction between such a world view and an evolutionary one is important for our understanding of the social and economic origins of evolutionism.

Closely related to the idea that change is a characteristic of a system is the principle of uniformitarianism, the principle that the forces causing change are themselves unchangeable general laws that govern the system. Thus, geological evolution is seen as the result of processes of mountain-building, sedimentation, and erosion that have gone on throughout the history of the earth, at least since the time when liquid water was present in appreciable quantities. In like manner the processes of natural selection and mutation that can be seen occurring in the organic world today are assumed to have been the operative forces in all the past history of life. Moreover, since such forces are operating at present, it must be concluded that evolution is still going on. A commitment to an evolutionary viewpoint represents a commitment to the instability of the present order as well as the past. In its simplest and irreducible form, evolutionism is the doctrine that change of state is an unvarying characteristic of natural systems and human institutions and that such change follows immutable laws.

Order. While all evolutionary thought assumes continual change, there is some problem of distinguishing “real” change from a stasis that has only the appearance of change. Let us imagine a deck of cards being shuffled over and over again. In one sense it is obvious that the state of the deck is undergoing continual change, since the cards are being rearranged. But is the deck evolving? For Bergson and Whitehead it is not, because only alternate states of chaos are succeeding each other. Plus ça change, plus c’est la même chose. Out of this chaos some organization must appear to be true evolution, and the appearance of new organization, in the view of Whitehead and his followers, is the characteristic of an evolutionary process. In Science and the Modern World White-head says: “Evolution, on the materialistic theory, is reduced to the rôle of being another word for the description of changes of the external relations between portions of matter. There is nothing to evolve, because one set of external relations is as good as any other set of external relations. There can merely be change, purposeless and unprogressive”([1925] 1960, p. 157).

But how can we know order as against chaos, except that we have a preconception of order and purpose? To return to the analogy of the deck of cards, if after repeated shuffling, the deck turned out to be grouped by suits, we would certainly say that order had been created. An even higher degree of order would be ascribed to the deck if, in addition, the cards within the suits were arranged in ascending sequence. After all, in poker a royal flush wins and a mixed hand loses. Yet, on any objective criterion a royal flush has exactly the same probability as any given mixed hand, and a completely ordered deck is not less probable than any other given arrangement. The appearance of order is the correspondence between the arrangement of objects and a preconception.

The demand that an evolutionary process create order, or at least that there be a change from one order to a different order, shows clearly that evolution, in this sense, is neither a fact nor a theory, but a way of organizing knowledge.

In contrast to Whitehead, some modern evolutionists are willing to accept any rearrangement of the parts of a structure as evolution. Thus Dobzhansky (1937) defines organic evolution as “a change in the genetic composition of populations,” and he speaks for most students of organic evolution when he says that there are no differences between organisms that cannot be accounted for in this way. But a change in the genetic composition of populations is not different in essence from a reshuffling of cards in that it is for the most part only a change in the relative frequency of elements, all of which are already present. The question of order marks the separation between the completely positivistic evolutionism inherent in Dobzhansky’s definition and the creative evolutionism of Bergson and Teilhard. For, once it is proposed that order is the natural outcome of an evolutionary process, ideas of direction, progress, and perfectibility follow swiftly.

Direction. By direction in evolution we mean the concept that there is some natural linear order of states of the system and that an evolutionary process can be described as passing through successive states in this linear order. That is, evolution can be described by a line on a two-dimensional graph with time on one axis and some description of the system on the other. Moreover, this line is supposed to be always ascending or descending. But this description limits evolution to those attributes for which the human mind can make a sensible linear order. It must be possible to describe an evolutionary process as one in which something or other “tends to increase.” Thus, it is insufficient to describe the evolution of human culture in terms of a change from hunting and gathering to agriculture, from agriculture to industry. These modes of organization must somehow be placed on a graded scale as, for example, the degree of division of labor (Durkheim) or the degree of complexity (Spencer).

The attempt to find the proper scale on which such a directionality can be measured has been a preoccupation of nonpositivist evolutionists and is the chief point at issue among them. Complexity is the scale most appealed to in organic and social evolution. For organic evolution it is supposed that modern organisms have a more complex structure than primitive ones, just as mammals are thought to be more complex than bacteria. Coupled with this idea of increase in structural complexity is the theory that the information content of modern organisms is greater than for past forms. Evolution is, on this theory, a process of accumulation of information about the environment in the complex structure of organisms. Finally, the supposed accumulation of information is thought to be a reversal of the second law of thermodynamics, which prescribes an increase of entropy with time and thus an increase in the randomness of the universe. Evolutionists sometimes talk of the accumulation of “negentropy” in organic evolution, marking off life from the inorganic cosmos.

This view of organic evolution, which is supposed to apply not only to the structure of organisms but to the interrelations between organisms in the total biosphere, suffers from a number of serious difficulties. First, it would be difficult to show exactly in what sense mammals are more complex than bacteria. There is no doubt that there are many more kinds of tissues in mammals, but bacteria are capable of carrying out many synthetic reactions not possible for mammals. At the level of cell physiology and metabolism, bacteria– bringing a greater synthetic repertory–must be regarded as more complex. Moreover, even if we assume that modern organisms are more complex structurally than those of the Cambrian, no criterion of complexity can distinguish between mammals and bony fish, although there are 270 million years between the first appearance of each. Second, the relation between structural complexity and information about the environment is not perfectly clear. No one knows exactly how to measure the information contained in any organism. It might be done, as in the Shannon-Wiener solution, by regarding the genes as a code made up of three-letter words with a four-letter alphabet, corresponding to what is known about the molecular basis of heredity. If this is done, however, many invertebrates turn out to have more information than many vertebrates, and among bacteria there is a very great variation. The real difficulty is that the equation between complexity and information has been chiefly a metaphorical rather than an exact one.

Finally, the equation of information and complexity with a thermodynamic measure of entropy is based on a misunderstanding of the kinetic theory of gases. The second law of thermodynamics, in the early nineteenth century, represented the beginning of modern evolutionary cosmology. The term “entropy,” used for a property of the universe that always increases, has the same meaning, etymologically, as “evolution.” Originally, the increase in entropy only signified that different parts of a physical system became more and more alike in their energy content, so that less and less useful work could be obtained from an interaction between them. The kinetic theory of gases provided a picture of molecules moving and colliding, and thus explained the gross observation of heat and work. This, in turn, led to a new interpretation of the second law as guaranteeing that a collection of molecules in any region of space would eventually have the same distribution of kinetic energies as in any other region. Two confusions have arisen about the kinetic theory of gases that have had an important effect on evolutionary thinking. First, it is supposed that individual molecules will all have the same kinetic energy rather than that assemblages of molecules will have the same statistical distribution of energies. Second, there is a confusion of kinetic energy of molecules with general kinetic and potential energies, especially gravitational and electromagnetic potential. These two confusions give rise to an erroneously derived generalized second law stating that all the molecules in the universe will eventually be equally spaced out from each other. Given such a formless and orderless end, the evolution of life does indeed seem to go in the opposite direction.

The tendency to turn the second law of thermodynamics into a generalized evolutionary world view (with life as an exceptional countercurrent) has been further encouraged by confusion of thermodynamics with yet another evolutionary cosmology, the “expanding universe.” According to this cosmogony, the material universe came into being on the order of 10 thousand million years ago in a small region of space, the matter exploded outward, and the material cosmos will continue to expand forever from this original point in space. A consequence of a fixed amount of matter occupying a larger and larger volume is that matter is becoming more thinly spread globally but not necessarily locally. The theory of the expanding universe does not demand, for example, that the earth break up and its pieces spread apart.

In its own way the theory of the expanding universe is another example of the search for directionality in evolutionary systems. More recently, Bondi, Gold, Hoyle, and others have given currency to nondirectional theories of the cosmos. One is a steady-state theory that allows for expansion but holds density everywhere in dynamic equilibrium by continual creation of new matter. The other is an oscillation theory, which postulates a cyclic expansion and contraction of the material cosmos.

Homeostasis, introduced by Cannon as a principle of physiology and evolution, is related to complexity. Homeostasis is the property of a system to hold constant certain of its elements despite external disturbing forces. What is held constant are those qualities of the system that are necessary for its maintenance, such as body temperature in a mammal or ionic strength of the blood. This property of homeostasis is then extended by evolutionists to include communities of organisms occupying different but coordinated positions in the natural economy. Thus, the relation between numbers and efficiency of carnivores that prey on herbivores and herbivores that crop the grass is thought to be stabilized by the process of evolution, so that fluctuation in the abundance of any of these organisms is compensated by changes in the others. The result is a stable community structure. The notion of stability is appealing to modern evolutionists, who see evolution as self-fulfilling, as a stabilization of life in a capricious universe. For the nineteenth century it was quite another matter. Is Nature

“So careful of the type?” But no.
From scarped cliff and quarried stone
She cries, “A thousand types are gone:
I care for nothing, all shall go.”
Tennyson, In Memoriam, Part 56, Stanza 1

Perhaps the only evolutionist doctrine that contains no important element of direction is evolutionary geology, which is entirely a cyclic theory. Mountain-building revolutions are followed by erosion, the formation of featureless peneplains, the deposition of sediments in the seas, followed by new uplift and new mountain-building. Of course, it is supposed that the final cooling of the earth’s core will at last put an end to the cycle, but this cooling is the only vestige of directionality that is apparent. Geology remains, among the historical sciences, obdurately materialistic and positivist.

Progress . It is not always easy to differentiate evolutionist doctrines of simple direction from those with an element of progress. I distinguish them by the moral or, better, moralistic tone of progressivism, but moralism is sometimes well hidden. For example, the doctrine that homeostasis gives direction to evolution is sometimes arrived at because man is assumed a priori to be the measure of evolution, and it is fairly easy to make the case that man, the rational mammal, is most homeostatic. “L’Homme, seul parametre absolu de l’Évolution” is even more anthropocentric than it seems at first sight, for Teilhard de Chardin (1956) is referring not simply to the history of life but cosmic evolution! But this is simply Whitehead brought up to date, Whitehead who, in Modes of Thought, divides occurrences in nature into six types: “The first type is human existence, body and mind. The second type includes all sorts of animal life, insects, the vertebrates, and other genera… . The third type includes all vegetable life… . The sixth type is composed of the happenings on an infinitesimal scale, disclosed by the minute analysis of modern physics” (1938, p. 214). Man leads all the rest. The shibboleths of progressivism are the superiority of man in the cosmos, industrial man in the world economy, and liberal democratic man in world society.

Spencer makes extensive use of the term “progress,” but in a way that seems not to have a normative or moralistic overtone: “From the earliest traceable cosmical changes down to the latest results of civilization, we shall find that the transformation of the homogeneous into the heterogeneous is that in which progress essentially consists” ([1857] 1915, p. 10). In “Progress: Its Law and Cause,” Spencer shows that this transformation has occurred in music, poetry, society, government, manufacturing, commerce, language, and so on. He cautions against normative definition of progress. “Leaving out of sight concomitants and beneficial consequences, let us ask what progress is in itself” (ibid., p. 9). But this is a very curious question to ask, what progress is in itself, for does not progress, as opposed to simple change, imply a moral direction? What Spencer has done is to equate progress with change, to say that change, whatever its direction may turn out to be, is progressive by its very nature. We come again to that nineteenth-century belief that change is good, in Spencer’s words, “a beneficient necessity.”

Most modern students of natural evolution, both organic and social, have taken a step toward materialism in omitting the idea of progress from their systems. An exception is B. Rensch (1947), who distinguishes higher and lower forms of life and devotes a special category of evolutionary change, anagenesis, to evolution of higher from lower. While he includes under this rubric increases in stability, homeostasis, and complexity that are discussed here simply as directional rather than progressive, Rensch clearly regards man and especially human freedom as the highest and best product of evolution.

Perfectibility. With the exception of the philosopher Teilhard, modern evolutionism does not contain a utopian element. On the contrary, evolution is generally envisaged as an endless process with no particular perfect end or goal. There is some logical difficulty, however, in maintaining that evolution leads to greater homeostasis, greater cerebralization, greater adaptation, while ignoring the possibility of perfect homeostasis, complete cerebralization, or absolute adaptation. It is not at all obvious how homeostasis of individuals or communities can continue to increase forever. Nevertheless, this issue is generally ignored or, as in the case of Spencer, deliberately set aside. The single important exception is in evolutionary economics, especially various utopian socialisms. Marxism, especially as interpreted by Lenin and Trotsky, is a straightforward progressivist, perfectionist evolutionary theory. A stage of primitive capitalist accumulation through exploitation of the workers and colonies enriches the society. This is accompanied by the bourgeois revolution that leads to liberal bourgeois democracy. In turn comes the proletarian revolution, proletarian democracy, a breakdown of national interests in the face of class interests, and a final total leveling of the class structure. In this utopian scheme there will still be division of labor, but the “entropy” of the social order will be at a maximum. The parallel is with thermodynamic evolution, which is also a leveling theory and is in contrast with those views of organic evolution that depend upon an increase in differentiation, complexity, storage of information, and a decrease in entropy.

Evolution and history

There are close parallels between the methods and statements of evolutionist doctrines in the natural sciences and the methods and statements of historiography. Geology, cosmology, and organic evolution are historical sciences in that they are descriptions of, and attempts to explain, past events in the light of present occurrences. The problems of making laws or lawlike statements about the past and prediction of the future are the same whether the focus of interest is human history or the history of all organic life. Karl Popper, in The Poverty of Historicism (1957), asks the question, “Can there be a law of evolution?” and answers, “‘No,’ the search for the law of the ‘unvarying order’ in evolution cannot possibly fall within the scope of scientific method, whether in biology or in sociology… . The evolution of life on earth, or of human society, is a unique historical process” (pp. 107, 108).

The chief difficulty of the historical sciences is that they fail to meet what is widely accepted as the norm for a science, Popper’s criterion of falsifiability. For Popper, scientific laws are universal statements (“All swans are black, all planets move in ellipses,” etc.) and therefore are really prohibitions (“A white swan cannot be found, no planet moves in a circle,” etc.). Such prohibitions provide a program for testability, for if one wishes to test the universal law about swans, he does not look for black ones but white ones. If a white swan is found, the law is disproved. A white swan is a potential falsifier of the law, and any statement that has no potential falsifier (any existentially quantified statement falls in this category) is metaphysical and ought to be excluded from a science.

The trouble with historical sciences like organic evolution is that they are almost entirely made up of existential rather than universal statements. We may take modern Darwinism as an example. It asserts that the organisms now living have evolved from ancestral organisms of a different nature and offers the fossil record as direct evidence. Moreover, it asserts that the mechanism of this change is embodied in three principles: (1) different individuals in a species have different morphologies, physiologies, behaviors, that is, there is variation; (2) there is a correlation between the form of the parents and the offspring, that is, the variation is heritable; and (3) different variants have different rates of survival and reproduction in different environments.

Let us now examine these assertions in the light of Popper’s criterion. The evidence that evolution has in fact occurred is contained in the succession of fossils found in different geological strata. From the fossil record we can state with confidence that there are many kinds of animals and plants that, having once existed, no longer exist. But that statement of itself, far from being a universal statement, is an existential one; in fact it is a historical statement, exactly corresponding to the assertion that Napoleon once lived or that Martin Luther died on February 18, 1546, at Eisleben.

Can we push this observation further and say that all animals and plants in the fossil record are of a kind no longer represented? No, because that does not happen to be true. But can we at least make the much more interesting and important hypothesis that all kinds of animals and plants eventually will be supplanted by other forms? This also is not falsifiable and is really identical to the assertion that every man has his price. It says, in fact, that every species is mortal, that there exists a time in the future at which any given species will no longer exist. If statements about the universality of evolution are historicist rather than scientific, it still might be that the principles underlying the mechanism of evolution are falsifiable in explaining any particular case of evolution. But that is clearly not the case. The statement of natural selection is that there exists an environment–a combination of temperature, humidity, food, soil, competition of other forms–in which different variants will have different relative reproductive rates. But applied to the past or the future, such a statement has such vast explanatory and predictive power that it is empirically empty. To say that the dinosaurs became extinct because some change in environment caused their rate of reproduction to be lowered below the replacement point, or to say that certain amphibia gave rise to reptiles because some environment existed which favored heritable variation in that direction, is, by Popper’s criterion, to say nothing.

An example of this difficulty is the argument of the selectionist in explaining the observed differences among populations of present-day organisms. Why are the frequencies of blood types different in different human races? The selectionist says these have arisen by natural selection, and even if no differential survival can presently be discovered (as none can), things used to be different and, at one time, the different races lived in such different environments that they were differentially selected. Such an argument bears more than a little similarity to the claim of Hegel that one cannot act on principles deduced from history, because “each period is involved in such peculiar circumstances, exhibits a condition of things so strictly idiosyncratic, that its conduct must be regulated by considerations connected with itself, and itself alone.”

Both historical explanation and evolutionary sciences can be concerned only with offering sufficient explanations for past events and with prescribing possible future events on the basis of the observation of present processes.

Evolutionism–social and economic matrix

While there is no doubt that the publication of Darwin’s On the Origin of Species (1859) led to an almost immediate explosion of evolutionary thought, it was only the percussion cap for a charge already set. Because a theory of organic evolution touched upon man’s origin and presented a materialistic challenge to his preordained primacy in the universe, it was bound to excite great interest. Nevertheless, the Origin of Species appeared in the middle of a period of rampant evolutionism and radical political and social change. It served as the issue over which the battle between stasis and change could be fought, a battle for the final supremacy of a world view that had been making steady gains since the beginning of the eighteenth century.

Darwin was the inheritor, not the creator, of the general preoccupation with evolutionism. This is made clear by Spencer in his Principles of Biology (1864–1867), when he argues that one of the chief evidences for organic evolution is that, after all, everything else evolves. It is now universally admitted by philologists, that languages, instead of being artificially or supernaturally formed, have been developed. And the histories of religion, of philosophy, of science, of the fine arts, of the industrial arts show that these have passed through stages. … If, then, the recognition of evolution as the law of many diverse orders of phenomena, has been spreading, may we not say that there thence arises the probability that evolution will be recognized as the law of the phenomena we are considering? ([1857] 1915, pp. 432–433)

The theory of organic evolution will be in its proper historical perspective if it is remembered that evolutionary cosmology had been founded in Kant’s Metaphysical Foundations of Natural Science of 1786 and in Laplace’s nebular hypothesis of 1796. At about the same time, Hutton was forming modern geology by his rejection of the catastrophic theories of the origin of geological formations and his introduction of the principle of uniformitarianism. Although the term “entropy” was not introduced until 1865 by Clausius, the second law of thermodynamics was formulated by Sadi Carnot thirty years earlier. By the time of the publication of the Origin of Species, the physical sciences were already thoroughly evolutionist in outlook. Moreover, the fact of the evolution of living forms, although not a mechanism for that evolution, was accepted widely in scientific and literary circles. Darwin’s grandfather, Erasmus Darwin, in the Temple of Nature (1803, p. 3) invokes the Muse to say “How rose from elemental strife, organic forms, and kindled into life.” And his Muse reports that even “imperious man, who rules the bestial crowd, … arose from rudiments of form and sense” (p. 28). Less romantic students of natural history like Buffon and especially Lamarck, had, by the beginning of the nineteenth century, fully developed theories of the transformation of species. Even Diderot in 1769 in Le rêve de d’Alembert asks: “Qui scait les races d’animaux qui nous ont precedes? Qui scait les races d’animaux qui succederont aux notres? Tout change, tout passe, il n’y a que le tout qui reste” ([1830] 1951, p. 56, my italics). Seventy years later we hear the echo in Tennyson: “The old order changeth, yielding place to new” (“Morte d’Arthur,” I. 408).

It is often thought obvious that scientific discovery influences the direction of social and economic change, or at least its rate. But what must be even more true is that social and economic world views must permeate science. No appeal to a Zeitgeist is implied by such a relationship, for the meaning of Zeitgeist is that science and other social activities respond equally to some spirit of the age whose source and power are unknown. To appeal to Zeitgeist is to reject any legitimate theory of historical causation. On the other hand, there is nothing mystical about the way in which notions of cause and effect, choice and chance, determinacy and freedom, spread from one science to another. Equally, it is entirely within the normal picture of historical causation that general social attitudes and economic relationships between social classes should have a profound effect upon the acceptability and apparent reasonableness of scientific hypotheses. Science is, after all, a social activity.

Prior to the eighteenth century, European social systems were characterized by a determinist world view. A man was born to his estate and occupied it by divine providence. Fixity and static stability were the mark of society, and radical changes in position could occur only as exceptional withdrawals or extensions of divine grace. Although Charles I was king of England Dei gratia, he could be deposed because, as Cromwell said, divine grace had been removed from him. The fact of his severed head was sufficient proof of that. Occasionally a man might rise from low estate to be the counselor of kings, but again only an extraordinary grace made this possible. Species were fixed as was the position of the earth in the universe. Galileo’s heresy was not that the earth was not at the center of the cosmos but that it moved. Men reason by analogy from the condition of their lives to the condition of the universe, and a static society could hardly believe in a dynamic cosmos.

In the eighteenth century a change became felt in the condition of society as the influence of the industrial revolution spread. Social mobility became more common, and classes of parvenus acquired political and social power.

“And it is a remarkable example of the confusion into which the present age has fallen” … says Sir Leicester, … “that Mrs. Rouncewell’s son has been invited to go into Parliament.”
Miss Volumnia utters a little sharp scream… . “Good gracious, what is the man?”
“He is called, I believe–an–Ironmaster.”
Charles Dickens, Bleak House, 1853

The phase of bourgeois revolution had begun and from it developed bourgeois revolutionary science. As change became the rule and characteristic of society, catastrophism lost ground in natural science, and a uniformitarian principle of change took its place. It is surely no coincidence that Josiah Wedgwood, who began as a potter’s apprentice and ended as one of the great eighteenth-century magnates, was Charles Darwin’s maternal grandfather. Darwin’s paternal grandfather, Erasmus, belonged to the circle of new Midland industralists: James Watt, James Keir, Matthew Boulton, and, of course, Wedgwood. Although from the middle class, Erasmus was a self-made man and his son, Robert (Charles’s father), emulated him by accepting 40 pounds of his father’s money and building it into a respectable fortune with no further aid.

The bourgeois revolution not only established change as the characteristic element of the cosmos but added direction and progress as well. A world in which a man could rise from humble origins must have seemed, to him at least, a good world. Change per se was a moral quality. In this light, Spencer’s assertion that change is progress is not surprising. Moreover, for those still rising or hopeful of improvement, there is a vested interest in the perpetuity of change, in a uniformitarian principle of replacement of the old by the new.

The bourgeois revolution reached its peak in England in the Reform Bill of 1832 based on Bentham’s principle of the “greatest happiness for the greatest number,” while on the Continent it took the more violent form of the revolutions of 1848. By the time Darwin published the Origin of Species, the ascent of the middle classes was complete and the supremacy of change and progress established in all the natural sciences except biology. The furor against Darwinism was only the last hopeless struggle of an already fatally wounded adversary. It is, of course, true that the principle of natural selection, converted by Spencer to the “survival of the fittest,” was used in the last half of the nineteenth century as a justification for laissez-faire practices. But it was only the borrowing of a metaphor to further justify a system already in full operation. It would be quite wrong to propose that Darwinism was an effective agent promoting unlimited economic competition.

Like all revolutions the bourgeois revolution gave way slowly to a period of consolidation, a period in which we still find ourselves. Once the new classes had gained power, it was clearly to their advantage to prevent the revolution from going further. The static hereditary society could hardly be reconstituted, but in its place a system of dynamic stability was erected. Change and social mobility are still accepted as characteristic of society, but it is a running-in-place rather than an overturn of the existing order. Liberal democracy of the twentieth century has a vested interest in maintaining the world social order but allowing individuals, on the basis of relative competitive ability, to find their own place in the social structure.

It is not remarkable, then, that evolutionary theories of the twentieth century are marked by a concern for equilibrium condition and dynamic stability, a playing down of progressivist and perfectionist elements, and a general reliance on the principle that plus ça change, plus c’est la même chose. In cosmogony there has been the rise of the steady-state theory of perpetual creation and also of the cyclic expansion-contraction model. Both are characterized by constant movement and change, but neither allows that the universe is going anywhere in particular. In thermodynamics and statistical mechanics there has been emphasis on the local rather than global nature of the law of increase of entropy. It is now admitted that entropy may decrease in other parts of the universe or at other times and that a global statement of the second law of thermodynamics may be too strong. In the realm of organic evolution, progressivism has been entirely abandoned except for a few metaphysical writers. The direction in which evolution is supposed to lead, when a direction is admitted, is that of greater complexity and greater integration leading to greater stability. Modern students of evolution are preoccupied with dynamic stability and equilibrium in a global sense. The technical literature of evolutionary genetics is filled with reference to and studies of stable equilibria. This preoccupation would have seemed strange to the evolutionists of the nineteenth century, who saw, reflected in the process of organic evolution, the tendency toward a better world. The evolutionist of the twentieth century presumably sees, in his view of evolution, “the best of all possible worlds.”

R. C. LEWONTIN

BIBLIOGRAPHY

BERGSON, HENRI (1907) 1944 Creative Evolution. New York: Modern Library. → First published in French.

DARWIN, CHARLES (1859) 1964 On the Origin of Species. Cambridge, Mass.: Harvard Univ. Press.

DARWIN, ERASMUS 1803 Temple of Nature: Or, the Origin of Society; a Poem With Philosophical Notes. London: Johnson.

DIDEROT, DENIS (1830) 1951 Le rêve de d’Alembert, Entretien entre d’Alembert et Diderot, et suite de I’entretien. Edited by Paul Vernlere. Paris: Didier. → Written in 1769, but first published in 1830.

DOBZHANSKY, THEODOSIUS G. (1937) 1951 Genetics and the Origin of Species. 3d ed., rev. New York: Columbia Univ. Press.

MAYR, ERNST 1963 Animal Species and Evolution. Cambridge, Mass.: Belknap Press.

POPPER, KARL R. 1957 The Poverty of Historicism. Boston: Beacon. → A paperback edition was published in 1964 by Harper.

RENSCH, BERNHARD (1947) 1960 Evolution Above the Species Level. New York: Columbia Univ. Press. → First published as Neuere Probleme der Abstammungs-lehre: Die transspezifische Evolution.

SPENCER, HERBERT (1857) 1915 Progress: Its Law and Cause. Volume 1, pages 8–62 in Herbert Spencer, Essays: Scientific, Political, and Speculative. New York: Appleton. → First published in the Westminster Review.

SPENCER, HERBERT (1864–1867)1914 The Principles of Biology. 2 vols. New York: Appleton.

TEILHARD DE CHARDIN, PIERRE (1956) 1963 La place de I’homme dans la nature: Le qroupe zoologique humain. Paris: Editions du Seuil. → First published as Le groupe zoologique humain: Structure et directions evolutives.

WHITEHEAD, ALFRED NORTH (1925) 1960 Science and the Modern World. New York: Macmillan.

WHITEHEAD, ALFRED NORTH 1934 Nature and Life. Univ. of Chicago Press.

WHITEHEAD, ALFRED NORTH 1938 Modes of Thought. New York: Macmillan. → Lectures delivered between 1934 and 1938. A paperback edition was published in 1958 by Capricorn.

II PRIMATE EVOLUTION

When Darwin published the Origin of Species in 1859, only a handful of fossil primates had been found and recognized as such. But during the last three decades of the nineteenth century a considerable series of fossil primates of Tertiary age were recovered in Europe and the Americas. The earliest scholarly attempts to analyze the course of primate evolution from the evidence of these fossils were made by Schlosser in Munich (1887), by Osborn in New York (1902), and by Wortman at Yale (1903–1904 ). Among the earliest monographic studies of early Tertiary primates were those of William K. Gregory of the American Museum of Natural History, who best summarized his views on primate evolution in a detailed review of the North American lemurlike primate Notharctus (1920). Only a little earlier Stehlin (1912–1916) at Basel had published a complementary review of European Eocene prosimians.

Inadequacies of material and methods. There have been several factors holding back full analysis of the course of primate evolution. A major problem results from their presumed early emergence in the tropical forests of the equatorial zones. Because Tertiary vertebrate fossils from these regions are poorly known, the early history of primates remains relatively obscure. Primates of the past, like those of today, were apparently restricted to relatively warm climates, often to tropical forests, and prosimian remains dating after warm, early Tertiary times are not generally found in the northern continents. Exploration for sites in the tropics where Tertiary land vertebrate fossils do occur is being actively carried out today, and it is from these paleontologically little-known regions that most important future additions to knowledge of primate evolution may be expected.

Most of the primate fossils from the early Tertiary (Paleocene and Eocene epochs) consequently are not found in the equatorial regions, where presumably the mainstream of primate evolution has always been located. Therefore, reconstruction of primate history must, at present, be extrapolated in part from fossil evidence from marginal areas. This gap in knowledge of ancient tropical faunas is somewhat lessened because we know that many extinct primates were wide-ranging species and that the higher Old World primates, particularly apes and pre-men, never diversified into a host of separate lineages, as did some groups of mammals, such as bats and rodents.

Thus, differentiation of the main groups and their interrelationships can be reasonably well understood, even from the occasional sampling typical of our imperfect paleontological finds. This is particularly true for the late Tertiary relatives and ancestors of man. However, two theories, originating outside the realm of paleontological evidence, appear to have been responsible for creating a contrary impression. Although the outlook is seldom specifically articulated, some students seem to believe that in order to produce the brain capacities of modern man there must have been many competing early hominid species. With this goes the assumption that the mental capacities of apes and those presumed for pre-men would have made them near masters of their environment and, therefore, so successful that many species would have arisen. These views, together with an overeagerness to create new species and genera, led some early anthropologists and a few modern students to the false conclusion that a great diversity of fossil apes and prehuman species once existed. A confusing welter of ill-founded names exists in the literature on such extinct primates. Those applied to Tertiary apes and hominids have recently been revised by Simons and Pilbeam (1965). Although over a thousand individual specimens of extinct great apes (pongids) and early hominids are now known from European, Asian, and African Miocene-Pleistocene deposits, only a few genera, Pliopithecus, Oreopithecus, Dryopithecus, Ramapithecus, and Giganto-pithecus, can be shown to be distinct and valid. Thus, the main types of close fossil relatives and forerunners of man (Hominoidea) that can convincingly be demonstrated to have existed in the past are about the same in number as the genera of this group that exist today–Hylobates (gibbon), Symphalangus (siamang), Pan (chimpanzee), Pongo (orangutan), Gorilla (gorilla), Homo (man).

As a consequence of the foregoing problems of approach, the study of primate evolution has been hampered. First, the early history of primates is obscured by inadequate knowledge of equatorial vertebrate faunas; and second, understanding of the later history of the particular group that included man’s ancestors has been confounded by the tendency to proliferate invalid genera and species.

Origin of primates

Undoubted primates first appear in the fossil record in North American deposits of early Paleocene age, which are more than sixty million years old. By middle Paleocene, the order had already diversified into three or four different main groups, or families. This would suggest that the initial separation of the order from primitive, insectivore-like stock was considerably earlier, perhaps in the late Cretaceous. Indeed, a species possibly primate, but based only on one tooth, has been reported from late Cretaceous beds in Montana.

The best-known primates of Paleocene times appear to represent specialized side branches, which did not long survive the beginning of the Eocene epoch. Although not directly ancestral, these early species do indicate the starting point of the basic primate arboreal adaptation and give evidence of the gradual transition from nonprimate to primate. It may be that some of the less completely known American Paleocene primates, such as Palaechthon and Plesiolestes, will eventually prove to be near the ancestry of one or more of the primate families of initial early Eocene appearance.

Origin of major surviving groups

Lemurs. At the beginning of the Eocene, pro-simians of lemurlike and tarsierlike aspect appear, nearly simultaneously, in the vertebrate assemblages of Europe and North America but are as yet unknown elsewhere. Three major sorts of primates can be characterized.

The most generalized of these early Eocene pro-simians are the loosely defined Adapidae, including such well-known forms as Pelycodus, and later Notharctus and Smilodectes in North America, and Adapis in Europe. Species of these genera, together with allied forms, apparently did not advance during the Eocene beyond the “lemuroid” condition represented today by the Malagasy lemurs, whom they resemble both in limb-bone structure and in the structure of the ear region of the skull. However, dental patterns in notharctines and adapines do not show trends evolving toward those of modern lemurs; this appears to indicate that known Eocene species of Adapidae were not directly ancestral to the living varieties of lemurs. On the other hand, the family Adapidae–now rather broadly defined–could well represent the family from which modern lemurs and possibly the lorises differentiated (Simons 1962a).

How and when the true lemurs reached their present limited range of distribution in the island of Madagascar is a most intriguing and nearly insoluble problem on the basis of present evidence. If their introduction to this island was by way of the African continent, it might be expected that some evidence of true lemurs would by now have been recovered from the Egyptian Oligocene or east African Miocene deposits, both of which contain warm-climate forest faunas. Although small, evidently arboreal primates have been found in some abundance in these deposits in Egypt and east Africa, none show any significant similarities to the Malagasy lemurs. On the other hand, similarities between lorises and two European late-Eocene primates, Pronycticebus and Anchomomys, at present classified as Adapinae, suggest that the introduction of this group into Africa could have been by way of Europe.

Tarsiers. Enough is now known of the craniology and dentition of the so-called “tarsioid” primates to demonstrate the falsehood of the extravagant opinion of Wood-Jones that Hominidae are to be derived directly from Tarsius-like forms through an ancestral line not shared by the Old World apes and monkeys. What is perhaps more significant is that the Eocene tarsioids so far discovered could not have given rise to the common stem of monkeys, apes, or men either, for they possess non-catarrhine specializations, and all had apparently lost one pair of lower incisors. All surviving members of Anthropoidea have retained these teeth. Nevertheless, these early tarsioids are of great interest because they presumably represent the general level of organization the unknown forerunners of the living higher primates must have reached as far back as the beginning of the Eocene epoch.

The first Anthropoidea. Another major group of primates appeared in the early Eocene, in Europe, Asia, and North America, and is, with one or two exceptions, restricted in temporal range to this epoch. (Gazin 1958 has reviewed the distribution of middle-Eocene members of this family, together with other contemporary North American primates.) Collectively, members of Omomyidae show greater resemblances to Anthropoidea than do other early prosimians. They could well represent the taxon from which all true Anthropoidea differentiated, a probability that is also sound zoogeographically, in view of their Holarctic distribution. Certainty on this point must wait, however, until better knowledge of cranial and postcranial anatomy is available for members of this family. Partial evidence, derived mainly from remains of a species of the North American middle-Eocene omomyid genus Hemiacodon gracilis, shows that this species cannot be regarded as “tarsioid” postcranially (Simpson 1940). This observation, in turn, implies that Anthropoidea in its earliest differentiation may not have passed through a definably tarsioid grade of organization. Rather, the anatomy of both Eocene Necrolemur and Holocene Tarsius clearly indicates that so-called “tarsioids” and the Anthropoidea probably originated in the same segment of early Prosimii. Although serious gaps in the geologic record of primates prevent any very definite attachment of early Tertiary species successions to those of the later Cenozoic, it is likely that some known Paleocene and Eocene types, at least at the generic level, do pertain to the ancestry of living forms. Because climates were much more equable throughout the early Tertiary, with regions as far north as Montana and England supporting a subtropical flora, we can suppose that many of these Paleocene-Eocene genera contained species having a broad north-south range of distribution. Some of the species we now know from northern areas must have had very close allies in the unknown southern faunas. An example of this sort of distribution is provided by one of the large Eocene herbivores. Skulls and jaws of species of the Wasatch panto-dont genus Coryphodon found in the Big Bend region of Texas, near the Mexican border, cannot be distinguished from remains of this animal found in northern Wyoming. This represents a north-south separation of about one thousand miles; if paleontological information on Wasatch faunas were available from Canada to Guatemala, this distribution could probably be extended even farther. Nevertheless, as with most attempts to trace phyletic lineages in given groups of fossils, the need to discover a great many more connecting links remains, and the search in the more nearly equatorial regions for early Cenozoic faunas containing primates is one of our major objectives.

For the Oligocene two sets of primate data from equatorial regions have become available through recent research projects.

The first of these projects has provided new evidence on the question of differentiation of cerco-pithecoid monkeys and hominoids. Discoveries made in Oligocene deposits in Egypt between 1961 and 1966 by Yale expeditions under the writer’s direction have added new data on the nature of the initial appearance of these two superfamilies. A jaw fragment of a new small primate, Oligopithecus savagei (Simons 1962b), shows the typical lower dental formula of Miocene-Holocene Old World Anthropoidea, in combination with what are the most primitively constituted lower molars known in this suborder. Premolar heteromorphy and slightly bilophodont molars, plus certain other characters, suggest relationships, on the one hand, to Eocene Omomyidae, and on the other, to Miocene-Holocene cercopithecoids. Further finds of this small mammal could strengthen the possibility, which now rests almost entirely on the evidence of this single fragmentary jaw, that cercopithecoids arose from Old World omomyids. Miocene monkeys are known from deposits in Egypt and east Africa. The early history of Old World monkeys remains very poorly understood, and much of the known earliest material has not been described.

The emergence of Hominoidea

To date no significant information whatever on upper tooth structure of earliest (Eocene-Oligocene) African or Eurasian Anthropoidea has been published. This has been a serious gap in our knowledge because upper premolars and molars, being somewhat more complex than lower-cheek teeth, allow for more accurate appraisal of taxonomic affinity. Restudy of the previously poorly known upper molars of Pondaungia cotteri from the late Eocene of Burma, together with discovery by the recent Yale expeditions of several partial upper dentitions of Apidium and of isolated upper teeth of Aegyptopithecus and Parapithecus, has greatly added to the data available for consideration of the prosimian sources of the higher primates of the Eastern Hemisphere. Briefly, the dental evidence of these three early species bears on the crucial point of emergence of Hominoidea from Prosimii: the upper molars of Apidium and Pondaungia show the three primary cusps of the trigon unconnected by ridges. The hypocone is large, and a pitted lingual cingulum is usually present. In both these genera several small accessory cusps occur in the trigon between the three main cusps. In their over-all upper molar morphology, species of these two genera equally resemble prosimians and higher primates. Cheek teeth of Apidium, particularly, also show similarities to those of Oreo-pithecus. This may be due to retention of a comparatively primitive molar cusp pattern in the otherwise advanced latter hominoid. Upper molars of Propliopithecus, on the other hand, show the typical pattern characteristic of much later apes, near men, and men. However, the cingulum on the inner side of the upper molars is large, which strengthens the view that this is a primitive character of ape dentitions.

Pondaungia cotteri was described by Pilgrim (1927). This specimen was recovered from the Pondaung sandstone of Burma together with a small mammal fauna, which indicates a late Eocene age for this as well as for another primate, Amphipithecus. Comparisons based primarily on the anthracotheres of the Pondaung sandstone and those of the Fayum early Oligocene of Egypt suggest that the Burmese fossils may be earlier, but the question is by no means definitely settled. At present it seems best to regard Amphipithecus and Pondaungia as of late Eocene age, and since both appear to be hominoids, this indicates the upper limit of differentiation for this major group of primates. As Colbert (1937) has already pointed out for Amphipithecus, both appear to show close ties with Pongidae. Tentative assignment of Amphipithecus and Pondaungia to this family does not seem questionable on grounds of their antiquity, inasmuch as a considerable number of presently existing mammalian families have now been traced back to the late Eocene.

Pilgrim’s illustrations and comments on Pondaungia left much to be desired, and in fact, on the basis of the information he provided, it was hardly possible to accept the species as belonging to the primates. As he mentioned, part of the source of his weak case lay in the fact that a web-like erosion of the enamel of the lower teeth in the type of Pondaungia had obscured their crown patterns. He supposed this to be also the case with the upper molars. Microscopic examination of this material indicates that, while the lower molars do appear to have suffered some erosive damage, the crenulations on the upper teeth represent the natural surface except in one or two broken areas. This sort of crenulation is not unusual among primates, being of frequent occurrence in Eocene Omomyidae and in Miocene–Holocene apes. In conclusion, these features of the upper molars, together with some distinguishable details of the lower teeth and mandibles, suggest an assignment to the Pongidae. Indeed Pondaungia may not be far from the direct ancestry of such forms as Propliopithecus and its less well-known allies from the Oligocene of Egypt. A less likely possibility is that Pondaungia is an advanced omomyid primate with teeth paralleling those of early pongids.

All finds of primates from the Old World Oligocene are distributed throughout some six or seven hundred feet of mainly continental sediments of the Qatrani formation, Fayum province, Egypt–generally regarded as being of early Oligocene age. The classic finds of Parapithecus, Propliopithecus, and Moeripithecus are from the lower part of this series, in the “Fossil Wood Zone,” while the type of Apidium came from about five hundred feet higher. The new species of Apidium and one of Propliopithecus have been found stratigraphically between the two earlier known levels. There are then three primate-yielding levels, of unknown age separation. Enough time had elapsed between their successive depositions, however, to bring about evolutionary changes among the respective primate lineages represented. In December 1963 the Yale expedition recovered from the upper Fayum deposits two new genera and species of primates. One of these, Aegyptopithecus, appears to be a good candidate for the ancestry of Dryopithecus, of Miocene–Pliocene age, and was possibly a forerunner of all subsequent apes. The second new find, Aeolopithecus, resembles gibbons, living and fossil.

Turning to a consideration of primates of the Miocene epoch, which began perhaps about 25 million years ago, there is more abundant information. From South American deposits, located mainly in Argentina and Colombia, a series of monkey species is known. These have most recently been discussed by Stirton (1951). At least partial skulls and jaws are known for species of three genera, Homun-culus, Dolicocebus, and Cebupithecia. These materials show that by Miocene times South American monkeys were structured much as they are today, and therefore they do not provide much evidence as to the origins of this group. Old World monkeys of Miocene age are known only from sites in east Africa and Egypt. The east African Miocene monkey has not yet been named, but that from Egypt was given the rather unfortunate generic name of Prohylobates by Fourtau (Egypt 1918).

Broadly contemporary with these monkeys are species of the Dryopithecus group of apes (including subgenera Sivapithecus and Proconsul), which are apparently close to the ancestry of the living African apes. Also represented in Miocene faunas of Europe and Africa are species of Pliopithecus, an ancient relative of the present-day gibbons and siamangs of southeast Asia.

Toward the end of the Miocene epoch in Eurasia and Africa, perhaps about fourteen million years ago, species of Dryopithecus are contemporary with the oldest undoubtedly manlike primate, Ramapithecus punjabicus. Originally discovered in the Siwalik Hills of north India in beds of Miocene-Pliocene age, this species was named by Pilgrim in 1910, but he did not recognize that it belonged to a major new variety of primate and so considered it a species of Dryopithecus. Lewis (1934) defined the genus Ramapithecus, to which Pilgrim’s species has subsequently proved to belong. Lewis initially pointed out that Ramapithecus has many manlike features in the upper tooth series. Thus, it can reasonably be placed in Hominidae, the taxonomic family of man (Simons 1963; 1964). To date, however, only parts of upper and lower jaws of this animal have been found. Apparently Ramapithecus was a successful and wide-ranging primate. Outside north India, teeth and jaws just like those of the Ramapithecus punjabicus have been found at Fort Ternan in Kenya, east Africa; in Yunnan, China; and just possibly in Europe.

Apart from the fact that facial displays and diet in Ramapithecus must have been more similar to that of true man than to that of the apes (in view of incisor, canine, and premolar reduction–relative to cheek teeth) little can be inferred about this earliest hominid. It cannot be called man or human, because there is no evidence that R. punjabicus manufactured tools. Moreover, nothing is known of its brain size and limb or body skeleton. After this we know little of fossil primates until the earlier Pleistocene.

The Pleistocene and hominid evolution

The Pleistocene was a time of rapid hominid evolution, during which toolmaking first appeared among the ancestors of modern man. In 1925 Dart described an infant skull that he named Australopithecus africanus. In spite of the name, which means “African southern ape,” Dart recognized Australopithecus to be a hominid belonging to the same taxonomic family as modern man, Homo sapiens. Nevertheless, his views were not generally accepted until a large number of fossil men or near men of similar age and structural type were recovered by Dart, Broom, and Robinson in south Africa.

More recently, Australopithecus has apparently been identified in Java and in north and east African deposits. The proposed name Homo habilis, recently coined by Leakey and his associates (1964) on the basis of specimens from Olduvai Gorge in Tanganyika, covers mixed materials, some assignable to the prior south African species Australopithecus africanus and some to Telanthropus capensis and to Homo erectus. In the view of many students (Campbell 1964a), even the first two species are not distinguishable, both belonging to the same small and gracile variety of early man. In addition to the species A. africanus, there is a large and more specialized early hominid form in east and south Africa, which should most correctly be called Australopithecus robustus. Both of these species appear to have been habitual bipeds; the postcranial skeletons are similar to that of Homo sapiens and little like those of apes. Their jaws and teeth, although relatively large, are strongly reminiscent of teeth of later and better-known fossil men. However, in both species, known brain size was apparently little more than a third that of the average present-day Homo sapiens. The small brain size initially caused many students to place Australopithecus close to or with the apes. This early and erroneous placement of Australopithecus has continued to affect balanced understanding of their relationship to living man. Several poorly known or juvenile Australopithecus specimens have been made the “types” of new taxa, said to be more advanced and more like Homo sapiens. Both Telanthropus capensis and Homo habilis fall in this category. As Campbell (1964b) demonstrates, they have not been shown to be different enough from each other or from members of Australopithecus africanus to warrant species identification.

Clarification of these taxonomic problems is crucial to discussion of the course of the mainstream of the early evolution of man and is one of the prime areas for future advance in this science. Definition and delineation of species populations is requisite to understanding of the ancestral lineage of modern man. Unfortunately, most anthropologists and anatomists who have been the namers of fossil men and near men were not adequately trained to define taxa in harmony with the concepts of modern systematics and population genetics. That these inadequacies are still leading to confusion in understanding of human evolution was confirmed in the technically incorrect diagnosis of Homo habilis. Such neglect of the new taxonomy and systematics has seriously affected understanding of primate and human history. In all discussion of earliest men and pre-men, mammalian taxonomy is an area that needs much more sober scientific attention than it has been given so far.

ELWYN L. SIMONS

BIBLIOGRAPHY

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CAMPBELL, BERNARD G. 1964b Science and Human Evolution. Nature 203:448–451.

CLARK, WILFRID E. LEGROS (1955) 1964 The Fossil Evidence for Human Evolution. Rev. ed. Univ. of Chicago Press.

CLARK, WILFRID E. LEGROS 1960 The Antecedents of Man. Chicago: Quadrangle Books.

COLBERT, EDWIN H. 1937 A New Primate From the Upper Eocene Pondaung Formation of Burma. American Museum of Natural History Novitates 951:1–18.

EGYPT, SURVEY DEPARTMENT (1918) 1920 Contribution à l’étude des vertébrés miocÉnes de l’Égypte, by René Fourtau. Cairo: Government Press.

GAZIN, C. LEWIS 1958 A Review of the Middle and Upper Eocene Primates of North America. Smithsonian Miscellaneous Collections, Vol. 136. Washington: The Institution.

GREGORY, WILLIAM K. 1920 On the Structure and Relations of Notharctus, an American Eocene Primate. American Museum of Natural History, Memoirs New Series 3:49–243.

GREGORY, WILLIAM K. 1922 The Origin and Evolution of the Human Dentition. Baltimore: Williams & Wil-kins.

LEAKEY, L. S. B.; TOBIAS, P. V.; and NAPIER, J. R. 1964 A New Species of the Genus Homo From Olduvai Gorge. Nature 202:7–9.

LEWIS, G. EDWARD 1934 Preliminary Notice of New Man-like Apes From India. American Journal of Science Fifth Series 227:161–179.

OSBORN, HENRY F. 1902 American Eocene Primates and the Supposed Rodent Family Mixodectidae. American Museum of Natural History, Bulletin 16:169–214.

PILGRIM, GUY E. 1910 Notices of New Mammalian Genera and Species From the Tertiaries of India. India, Geological Survey, Records of the Geological Survey of India 40, no. 1:63–71.

PILGRIM, GUY E. 1927 A Sivapithecus Palate and Other Primate Fossils From India. India, Geological Survey, Palaeontologia indica New Series 14:1–24.

SCHLOSSER, MAX 1887 Die Affen, Lemuren, Chiropteren usw. des europäischen Tertiärs. Beiträge zur Paläontologie und Geologie Österreich-Ungarns 6:1–162.

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III HUMAN EVOLUTION

The modern or synthetic theory of evolution considers evolution to be the result of changes in the gene frequencies in populations. Gene frequencies are altered by means of mutation, selection, migration, and certain chance factors such as drift. When mutations are considered relative to their usefulness to the organism and to the population, mutations occur at random. They are not at random relative to the chemical structure of the organism, and mutations are commoner at some locations on the chromosomes than at others. Mutations are mostly disadvantageous, and their frequency is increased by radiation and by certain chemicals. Selection is the primary agent for bringing order to this process. Over the course of time, those genetic combinations that favor reproductive success become more common; and, conversely, those combinations that are less viable decline. It should be stressed that selection is for reproductively successful populations, and, from the viewpoint of evolution or of the species, the individual is only important insofar as he helps or hinders the success of the population. Variability itself is very important, as it permits the species to adapt to changing conditions; and selection is often for variability rather than for homogeneity which demands a single, limited environment. Migration from one population to another may introduce new genes or change gene frequencies, and this means that genes that are favored by selection may spread throughout the species. Partial reproductive isolation permits adaptation to local conditions and the formation of races, but in the long run it is the species that is the evolutionary unit. Especially when the number of individuals in the breeding population is small–as, in fact, was the case throughout most of human history–chance may cause the gene frequencies of one generation to differ from those of the preceding one. This has been called drift, and it can be a source of variability between small populations of a single species. If populations are founded by very few individuals, on the basis of statistical probabilities their gene frequencies are unlikely to be the same as those of the parent population. In summary, evolution is the result of changing gene frequencies. These changes originate by chance and are ordered by selection. They are modified by migration and certain chance factors. The process of natural selection has both directly and indirectly favored highly variable populations, and there has been no tendency for evolution to produce genetically uniform types.

Biological history

The synthetic theory of evolution has many important implications for social science, but it must be remembered that the concepts most frequently borrowed by the social scientists long antedate modern evolutionary theory. The greatest misunderstanding comes from the notion that evolution is directed toward a particular goal, frequently man himself, and that its course is determined by trends (orthogenesis). The reproductive success of populations is determined by the conditions of the moment, not by ultimate desirability or by the remote future. Reptiles succeeded the amphibians because of structural, physiological, and behavioral advantages that led to numerical and adaptive success, not because reptiles were to give rise to mammals, one of which was to become man. The trends seen in evolution are descriptions of what has actually taken place, and if a trend continues it is because selection in each generation has favored a particular course of events. When there are only a few fossils, the course of evolution may appear simple; but when the record is rich, biological history appears as an incredibly complex web of adapting organisms.

These points are made particularly incisively by Simpson (1964). As he emphasizes, evolution is biological history. The mechanism is revealed in the geneticist’s laboratory, and the record is discovered in the rocks. But there are no laws in the usual scientific sense determining the course of evolution. In biological history there are no ultimate goals, inevitable trends, or vitalistic explanations.

The contrast between thinking about evolution in terms of inevitable trends and as the result of selection may be illustrated by consideration of the length of the toes of man and his ancestors. Since human toes have become much shorter compared to those of any possible ancestral primate, it is often asked whether this trend will continue; and men of the future are pictured as having still smaller toes. But the length of human toes has not been determined by a trend which is continuing into the future, regardless of the circumstances. When our ancestors became terrestrial animals to a greater extent, selection favored more efficient bipedal locomotion. Selection for shorter toes was a part of this process, but the fact that selection was for different proportions in the foot does not mean that selection continues to be for even shorter toes. Probably an equilibrium was reached more than a million years ago, and selection for shorter toes has not continued. There is no trend determining the foot proportions of the future; the notion of an inevitable man of the future with a huge brain, tiny face, small limbs, and so forth finds no support in modern evolutionary theory. Future gene frequencies will be determined by selection (i.e., by reproductive success), and man will become more intelligent only if those individuals with the combinations of genes favoring the development of high intelligence leave more offspring. Evolutionary trends are statements of what has actually taken place in the history of forms of life, but there is no biological momentum that carries past trends into the future.

The study of human evolution is particularly beset with the notions of goal and trend and of evolution as a magic process. It might clarify thinking on the subject to omit the word “evolution” for the moment and to consider the history of the forms of life as revealed by the fossil record, the mechanism of change as shown in the laboratory, and the interpretations of these two kinds of data. The study of man’s biological history means precisely the same thing as the study of human evolution, and it does not carry the suggestion of inevitable trend or progress toward some desired goal. The statement that we may study human biological history, human cultural history, and the interrelation of the two should mean precisely the same thing as that we may study human biological and cultural evolution. Understanding comes from the study of the data, and no information is added by using the word “evolution” in preference to the word “history.”

The importance of using modern concepts is well illustrated by the change in the meaning of the word “origin.” In human evolution “origin” has usually meant a relatively restricted time and place in which a new type arose. Thus one theory held that Neanderthal man arose in Europe, another that he evolved in central Asia. But if the populations of the genus Homo were not reproductively isolated during the latter half of the Pleistocene period, mutations and gene combinations favored by selection might have spread throughout the whole species. The origin of the Neanderthal populations that inhabited Europe during the last interglacial period was not limited to Europe but depended on the extent to which European populations were in contact with populations in other parts of the Old World. In the traditional typological, local sense there really is no “origin” of Neanderthal but many origins at different times and places which became incorporated in the European gene pool some 50 to 150 thousand years ago. The origins of Neanderthal were going on steadily over large areas for long periods of time, and the populations named Neanderthal were in part the result of events (mutations, drift, selection, migration) taking place in other races of the species.

Stages in human evolution

The characteristics of the main stages of human evolution are based on the fossils and on the structure and behavior of the primates that exist today. Both kinds of evidence are necessary to reconstruct the course of human evolution, and it is important to keep in mind the nature of the understanding that each kind of evidence can give. The fossils are the only direct clue to what the ancestors were like, the only evidence on many forms that have no close living relatives, and the only evidence as to the actual time of appearance of the various groups of primates. But the fossils are limited to those hard parts that do not easily deteriorate, largely jaws and teeth. Study of the contemporary primate forms supplements this record in two ways. The changes that are seen in the fossils may be interpreted more meaningfully if knowledge of the living animals is used. For example, changes in the form of the ethmoid bone may be directly related to changes in the sense of smell; or a particular form of limb bone may be interpreted as showing a special loco-motor pattern. In a quite different way the contemporary forms may indirectly suggest characteristics that probably existed in extinct forms. For example, primitive mammals and contemporary prosimians have tactile hairs, scent glands, and a well-developed sense of smell. It is highly probable that these conditions were general for all the primates of the first half of the Age of the Mammals, but there is direct evidence only on the sense of smell. It is probable that this whole complex was reduced at the end of the Eocene or the beginning of the Oligocene, but this may have happened much earlier or later, and different parts of the complex may have changed at different times. If one is interested in general statements about what happened, the evidence is very good. But the more detailed questions one asks, the more the answers are limited to those that may be derived from teeth and bones.

For the first half of the Age of the Mammals the ancestors of man were small, long-snouted pro-simians, not distinguished from the other primates of their time in any remarkable way. The prosimians of the Eocene were successful and highly diversified, and the group underwent a major adaptive radiation resulting in the formation of many distinct families. Many of the most successful groups evolved elongated incisor teeth; and if one examined only the Eocene primates, it might be concluded that these were to be the ultimately successful forms. But the majority of these Eocene primates became extinct after the true rodents appeared, and it may be that competition with the rodents was a major factor in their extinction. Unfortunately, the fossil record is particularly scant at the end of the Eocene and at the beginning of the Oligocene, so it is impossible to determine just which of the families of the early primates are ancestral to the later ones. But the main changes, as outlined below, would be similar regardless of just which lineage is eventually proved to be the correct one. While maintaining hands and feet adapted for climbing by grasping and a primitive quadrupedal posture, some primates in both the New World and the Old World independently evolved a new organization of the special senses and the brain. The primitive sense of smell was reduced, and binocular, stereoscopic color vision evolved. The reduction of the sense of smell is directly reflected in the fossil bone. The changes in vision are inferred from the conditions found in contemporary primates and are mirrored, at least to some extent, in the structure of the bony orbit. The brain increased in size at least four or five times and changed in organization from a primitive dependence on the sense of smell to an organization based on vision as the dominant sense. The change from prosimian to monkey is primarily in the brain and special senses, but in addition the face became shorter and deeper. The special senses and the brain determine what aspects of the external world can be perceived and appreciated and how sensations are organized. In a very real sense the world that we think of as normal (a stereoscopic world of color, in which activity is diurnal) began with the monkeys of the end of the Eocene or the beginning of the Oligocene. Judging from contemporary prosimians, the increase in the size of the brain made a great difference in intelligence. But it must be remembered that this reorganization of the structural basis of experience was an adaptation to a particular kind of arboreal life which took place independently in both the New World and the Old World. Although this organization forms the basis for human experience, it initially evolved because it was useful to monkeys, not to prepare the way for man.

Monkeys (Cercopithecidae) have remained quadrupedal, but the apes (Pongidae) evolved a different locomotor pattern. Apes climb by reaching far up above the head and may hang by one arm, swing below a branch, or reach to the side. This manner of locomotion in the trees probably evolved first as a way of feeding out near the ends of small branches. The structures that make this mode of living possible are complex and include a shallow, wide chest, a long clavicle, a special shoulder joint that involves modification of all bones near the joint, and changes in the elbow, wrist joint, and hand. The fossil record is exceedingly scanty, but this whole complex probably evolved during the Miocene, long after the separation between monkeys and apes. At least many monkeylike features persist in the arm bones of such forms of early Miocene ape as Proconsul and Pliopithecus. Man is similar to the apes in this structure of the trunk and arms (including the form and numbers of vertebrae, the disposition of the viscera, the form of joints and muscles, and the proportions of the trunk and limbs). Again it must be remembered that this complex evolved for a special life in the trees, and it is shared by all the contemporary apes. Man has these features because his ancestors were arboreally adapted apes, not because there was a trend toward man.

By the beginning of the Pleistocene, possibly two million years ago, the family of which man is the sole living representative (Hominidae) was represented by a genus of bipedal, small-brained creatures (Australopithecus). The direct fossil evidence shows that the Hominidae must be at least two million years old; but Australopithecus already had small canine teeth of human form and a pelvis closely approximating that seen in man (Homo), and the foot differed from that of Homo only in details. Prior to the Pleistocene, the Hominidae must have been separated from the Pongidae for some substantial period of time during which these characters evolved. Fragments of jaws dating from the end of the Miocene in India (Ramapithecus) and Africa (Kenyapithecus) suggest that the line leading to the Hominidae may have been distinct at that time; but since the rate of evolution may be very different for different functional complexes, there is no way to tell if these forms were beginning to be bipedal. It has been repeatedly shown that conclusions drawn from such fragmentary remains are likely to be wrong. [See EVOLUTION, article on PRIMATE EVOLUTION.]

Stone tools and animal bones have been found with the remains of Australopithecus; and it is probable that these creatures made the tools and hunted small animals, just as from their anatomy it appears that the behavior of Australopithecus was far more human than apelike. There were at least two species, A. africanus and A. robustus, a small one and a large one. Judging from the evidence of the teeth and from the associated archeological remains, both probably made tools. The small species may have been directly ancestral to the genus Homo of the Middle Pleistocene. The large form lived at the same time as the small one and continued on to be a contemporary of Homo before becoming extinct. As man is approached, there is a tendency to label each specimen as distinct and to emphasize differences instead of similarities. The result is a multiplication of species and genera the more the forms resemble man. Actually, the reverse should be the case. The more a primate is bipedal, tool-using, and hunting, the less likely the form is to speciate and the more likely it is to occupy wide areas with only racial differences.

By the Middle Pleistocene men of the genus Homo were fully bipedal, and there is every indication that their locomotor system had evolved to virtually its present form. Brains were approximately twice the size of those of Australopithecus. Tools were made according to complex traditions of manufacture, which were widely distributed geographically. Large animals were hunted, and fire was used. This fully human way of life appears to have been established by half a million years ago, but it changed very slowly. Approximately fifty thousand years ago men of modern form appeared. Undoubtedly evolution continued, and the populations adapted to local conditions by biological as well as cultural means. But the main events of human evolution had taken place before that time, and culture became increasingly more important in human adaptation. With agriculture and particularly with modern science the whole pattern shifted, numbers increased from a few millions to billions, and recently it is man who has altered the world. Mutation, selection, and migration are changed by the human way of life and at least in part may be brought under human control.

Since the fossil record is so fragmentary, it is important to note that the latest biochemical and cytological evidence supports the classification of the primates and the general stages of human evolution noted above. Immunochemical studies (especially Williams 1964 and Goodman 1963) show that man’s closest living relatives are the great apes, especially the chimpanzee and the gorilla. The small apes, the gibbons, are much less similar both in their immune reactions and in chromosome number and types. The Old World monkeys are still further removed, and the prosimians are both very different and highly diversified. For the first half of the Age of the Mammals man’s ancestors were prosimians whose primary adaptation was climbing by grasping. At the end of the Eocene a monkey-ape group evolved, characterized by changes in the brain and special senses. This group diversified in the Oligocene, and the special ape locomotor and feeding patterns evolved in the Miocene. A human bipedal group separated later and was fully evolved in locomotor and dental characters by the beginning of the Pleistocene, some two million years ago. It should be stressed that the Eocene prosimians evolved into many different forms, most of which became extinct. Subsequently, monkeys and apes evolved into dozens of different genera. No general trend dominates the evolution of the primates, and most of primate evolution has no relation to that of man.

Evolution of behavior

The direct evidence for the course of human evolution outlined above is fragmentary and limited primarily to teeth and jaws. Changes in locomotor patterns are directly reflected in the bones, and in a general way the stages outlined above are probably correct; but so few limb bones are preserved that a very wide variety of interpretations is possible. Turning to the evolution of other behaviors, the evidence is even more indirect and is based primarily on the contemporary forms. The general logic in the use of information from the contemporary primates is that if a structure or behavior is common in a group of living primates it was probably present in closely allied fossil forms. Both parallel evolution and convergence may render such conclusions invalid; and since it is highly adaptable, behavior is particularly liable to these sources of error. An example may make the situation clearer. Primitive mammals probably had a litter of several young. In the primates the number of young is usually reduced to one at a time, and that infant is carried by the mother. The general kind of change is clear, but the details cannot be determined from the record. Marmosets do have twins and galagos more than one young. It is probable that the reduction took place several times, and it is possible that the ancestral apes had more than one young. But the most probable interpretation is that in arboreal primates a single infant carried by the mother was more likely to survive, and that this behavioral adaptation had evolved in the prosimians prior to the evolution of the later primate forms.

The human infant is remarkable in being unable to hold onto its mother, and this not only alters the pattern of human mother-child relations but introduces a division of labor and many social problems that are unique to man. George B. Schaller (1963) observed that the female gorilla also must help her infant for the first six weeks. Thus man is a little less different than had been thought, and perhaps the particular group of apes from which we are descended were more like gorillas in this regard than like all the other apes and monkeys. The matter cannot be proved one way or the other, but it is most likely that the main difference between man and the other primates came with the evolution of large brains in the Pleistocene, long after the human lineage had separated from the other apes.

A problem exists in the examples that follow which is similar to that encountered in our consideration of the number of young in a litter and the degree of dependency of the newborn: that is, comparison of the living forms highlights a situation of importance in the behavior of man, but the time of origin of the behavior can be suggested in only the most general way. However, many important aspects of human evolution can be appreciated only in this way. For example, length of life has increased greatly in man. In small primitive mammals maturity is a matter of months and old age of two or three years. Comparable figures for monkeys are maturity in three or four years and a life span of well over twenty years. In chimpanzees full maturity is in approximately eight to ten years, and the life span is more than forty years. Comparable figures for man are nearly twice as long. Although there is much variation, the time during which the human young enjoy a protected and privileged position has increased, and a human of eight or ten is still learning and playing at an age when most primates would be fully adult. Clearly, selection has favored the long period of development and learning, even at a great biological cost. It might appear that a process based on reproductive success would favor several young and rapid growth, but in the human way of life selection has been for a single infant, growing slowly. Prolonged dependency and the presence of experienced adults have been major factors in the evolution of human society.

In monkeys and apes there is a menstrual cycle of approximately a month in duration. Sexual activity is concentrated in a period of estrus close to the time of ovulation. Females do not come into estrus for a period of some months during the later part of pregnancy and the first part of lactation. These physiological facts have the effect of concentrating sexual activity when conception is most probable and of spacing the infants. The spacing may be further reinforced by a breeding season, or at least a much greater frequency of births in one part of the year. In the human female the basic cycle continues, but estrus behavior has been lost.

Human females experience a period of lowered fertility during early lactation which is probably comparable to that in many monkeys. However, this period is not long enough for the requirements of human infant spacing; the human infant cannot feed itself when it is six months or even a year old. In man the spacing of infants must be extended by taboos and customs which supplement the physiological mechanisms of birth spacing in the non-human primate. The loss of the estrus cycle in the human female may be related to the economic division of labor and the interdependency of the sexes, which is unique to man. This loss may be due to the need of the human female to keep the interest of the food-sharing male; but there may well have also been selective pressure against estrus behavior, which would have been disruptive to the stable, interdependent relationships of a family unit.

The monkeys and apes are almost entirely vegetarian, although most will eat birds’ eggs and insects. Hunting of small mammals has been observed rarely in chimpanzees and baboons. The range in which monkeys and apes forage is small, varying from much less than a square mile in gibbons to some fifteen square miles in baboons and gorillas. From a human point of view it is remarkable that animals with keen eyesight, who are capable of climbing into trees and surveying the scene and who are well adapted for locomotion on the ground, so restrict their normal activities. It is probable that hunting is the behavioral adaptation that caused the change in man’s relation to his physical environment. Intensive hunting would drive game from a small range, and the location of game and the pursuit of wounded animals would lead to the establishment of large territories. In most non-human primates ranges are not defended, and the areas occupied by groups of monkeys or apes usually overlap. Human defense of territory may also be the result of hunting.

Tool use is one of man’s most distinctive attributes, and the skillful use of objects is unique to man. Manipulative skills depend on the brain as well as on the hand, and the large area in the human brain devoted to the hand is probably the result of the new selection pressures that came into being with the beginnings of tool using. Both the evidence of the teeth and of the associated tools and animal bones suggest that Australopithecus was a tool user and had been so for a long time. If this interpretation is correct, then the increase in size of the human brain came long after the use of tools and probably in response to the new ways of life that tools made possible.

Intelligence and learning are not general but are related to specific abilities. For example, human children play with objects as well as with other children. They enjoy practicing using and throwing, and many games are built around objects. In marked contrast, monkeys and apes are tool-dumb, so to speak. Tasks that the human child easily masters are very difficult or impossible for non-human primates. Only the chimpanzee uses some minor tools (Goodall 1964). The same principle can be seen in language. In spite of major efforts, it has proved impossible to teach monkeys or apes to speak. This is because the nonhuman primates lack the neural mechanisms necessary for speech. The sounds of the nonhuman primates convey emotion (such as fear) and the location of the calling animal, and they are important especially when combined with gestures in social interaction. The sound systems of primates are not more complex than those of many other mammals; and, like other mammals, primates can be trained to respond to human sounds. The distinctive character of human speech is the naming of objects, and this requires the linking of visual and auditory parts of the cortex of the brain (Geschwind 1964). The necessary connections are not present in the brains of the nonhuman primates. Once naming of objects had commenced in even the most minor way, the success of this revolution changed selection pressures so that the course of evolution of the brain changed, and structures evolved that ultimately made possible language as we now know it. It is likely that the situation which led to object naming was tool use, and it is the uniqueness of this combination of tools and language to man that accounts for why the other primates did not develop even the simplest languages. If language began at the time the brain was doubling in size between Australopithecus and Homo, this would give a minimum of two million years of stone toolmaking by small-brained bipeds as a time in which the first naming of objects might have occurred. It is tempting to attribute this great increase in brain size to language and all the ways of life that language made possible. But it must be remembered that it is not just increase in size that separates the brain of modern man from that of the contemporary apes. There have been changes of organization in the brain too, and without these changes skills in tool use, language, and social planning would be impossible. Human intelligence is built on specific abilities which are the products of the evolutionary process.

But the world in which man evolved was a very different one from that in which we are living today. Our bodies had evolved to practically their present form some fifty thousand years ago, and since then human adaptations to the environment have been increasingly by technology and custom. This does not mean that evolution has stopped, but it does mean that the direct interrelation that selection had forged between man and his ways of life is no longer functional. As stone tools improved over vast intervals of time, the biology of the users had time to evolve along with their way of life. In the human head we see the product of the interrelations of biology and a succession of ways of life in which selection was for smaller faces and bigger brains. But since the agricultural and scientific revolutions customs have changed so rapidly that there has been no time for corresponding biological evolution to fit the human actors for the modern world. Human biology evolved to be adaptive in a world of small society, great hazards, and personal skill. The human actor in modern society is too aggressive, too dominance seeking, too acquisitive. The kind of planning necessary in the modern world is difficult for an organism built along the lines of Homo sapiens. Many acts which now would be judged undesirable (acts of selfishness, cruelty, and war) are easily learned because they are in accord with basic human biology (Hamburg 1963).

Natural selection can bring about adaptation between biology and a way of life only in very long periods of time, and there is no orthogenesis carrying trends of the past into the future. The fit between organism and society must now be determined by science, and for the first time in all of biological evolution both biology and social life can be planned. But planning can be more efficient if planners remember that the actors in modern technical society are products of the past, of times and ways of life long gone.

S. L. WASHBURN AND JANE B. LANCASTER

BIBLIOGRAPHY

BUETTNER-JANUSCH, JOHN (editor) 1963–1964 Evolutionary and Genetic Biology of Primates. 2 vols. New York: Academic Press.

DEVORE, IRVEN (editor) 1965 Primate Behavior: Field Studies of Monkeys and Apes. New York: Holt.

DOBZHANSKY, THEODOSIUS 1962 Mankind Evolving: The Evolution of the Human Species. New Haven: Yale Univ. Press.

GESCHWIND, NORMAN 1964 The Development of the Brain and the Evolution of Language. Georgetown University, Washington, D.C., Institute of Languages and Linguistics, Monograph Series on Languages and Linguistics 17:155–169.

GOODALL, JANE 1964 Tool-using and Aimed Throwing in a Community of Free-living Chimpanzees. Nature 201: 1264–1266.

GOODMAN, MORRIS 1963 Man’s Place in the Phylogeny of the Primates as Reflected in Serum Proteins. Pages 204–234 in Sherwood L. Washburn (editor), Classification and Human Evolution. Chicago: Aldine.

HAMBURG, D. A. 1963 Emotions in the Perspective of Human Evolution. Pages 300–317 in Symposium on Expression of the Emotions in Man, New York, 1960, Expression of the Emotions in Man. Edited by Peter H. Knapp. New York: International Universities Press.

Index medicus. → Published since 1960. Articles are listed by subject matter and author.

MAYR, ERNST 1963 Animal Species and Evolution. Cambridge, Mass.: Belknap Press.

NAPIER, J. R. 1964 The Evolution of Bipedal Walking in the Hominids. Archives de biologic (Liege) 75 (Supplement): 673–708.

OAKLEY, KENNETH P. 1964 Frameworks for Dating Fossil Man. Chicago: Aldine. → A comprehensive discussion of methods of dating and the dates of fossil man.

PIVETEAU, JEAN (editor) 1957 Traite de paleontologie. Volume 7: Vers la forme humaine… . Paris: Masson. → The best general source on fossil primates, including man.

SCHALLER, GEORGE B. 1963 The Mountain Gorilla: Ecology and Behavior. Univ. of Chicago Press.

SIMONS, ELWYN L. 1963 Some Fallacies in the Study of Hominid Phylogeny. Science 141:879–889.

SIMONS, ELWYN L.; and PILBEAM, D. R. 1965 Preliminary Revision of the Dryopithecinae (Pongidae, Anthropoidea). Folia primatologica 3:81–152. → Classification of apes and the origin of man.

SIMPSON, GEORGE G. 1964 This View of Life: The World of an Evolutionist. New York: Harcourt.

TOBIAS, PHILIP V. 1965 Early Man in East Africa. Science 149:22–33. → Most recent review of Australopithecus and the problems of the origin of man.

WASHBURN, SHERWOOD L. (editor) 1963 Classification and Human Evolution. Chicago: Aldine.

WILLIAMS, C. A. JR. 1964 Immunochemical Analysis of Serum Proteins of the Primates: A Study in Molecular Evolution. Volume 2, pages 25–74 in John Buettner-Janusch (editor), Evolutionary and Genetic Biology of Primates. New York: Academic Press.

Zoological Record. → Published since 1865. Covers classification and fossils, topics not in the Index medicus.

ZOOLOGICAL SOCIETY OF LONDON 1963 The Primates. Proceedings of the Symposium held on April 12–14, 1962, Symposia 10. London: The Society.

IV CULTURAL EVOLUTION

The grand movement of origin, transformation, and differentiation of our universe, our earth, and life itself is called evolution. Within this total process we are concerned with the transformations that occurred when the biological, or organic, phase arose out of the inorganic and when the later, cultural phase arose from the organic. Despite their interconnections, however, each of these stages has its own characteristic mode and tempo of evolution.

The cultural phase transcended the organic and inorganic when populations of men created new ways of adapting to each other and to the environment. These adaptations occurred after certain gradual changes in the size and complexity of the hominid forebrain made symbolic thought and communication possible. The capacity for, and use of, symbolic manipulation brought forth unprecedented kinds of social behavior. These new ways of behaving were suprabiological in the sense that such natural primate characteristics as jealousy, fear, sex and food appetites, and so on, were so often channeled, sublimated, or otherwise altered by means of social rules. In a few striking respects, in fact, the new modes of social behavior were contrabiological inasmuch as they actually repressed such powerful urges as the sexual, for example, and required sharing rather than competing for scarce food.

The sum total of the social and political rules, technological inventions and economic institutions, the arts, shared beliefs and practices–that is, the culture–tends to persist through time because any particular society maintains these parts integrated with each other and with its environment. Evolutionary changes, therefore, do not correspond to a single world-wide pattern, and each society maintains a certain distinctiveness in the course of change. Thus the culture of mankind generally is an evolutionary stage in the universal process, while particular societies differentiate into cultural genera and species, creating heterogeneity. Sometimes this adaptive process brings forth striking advances that permit greater dominance, all-round adaptability, and growth.

All anthropologists would agree that the earliest human societies must have been small and simple in social organization, poor in technological equipment, without formal legal or governmental institutions, and with an ideology based more on the supernatural than on science. Since these characteristics contrast greatly with modern industrial states, we think of evolution as directional: generally from small to large societies, from simple to complex organizations, from informal to formal political institutions, and so on. The idea of directionality is important because it provides the criteria for classifying separate societies into general stages of the evolution of culture as a totality.

A second characteristic of evolution is the relatedness of the sequence of forms. An important aspect of the interpretation of any particular unit is the investigation of the ancestral forms from which it “unfolded”-its phylogeny.

Not many social scientists are evolutionists, and even the anthropologists who agree that there has been general evolutionary growth disagree about whether there has been enough orderliness in the process for the theory to be useful in classifying cultures or in interpreting a purely historical succession of discrete events. But then, of course, even if the orderliness is agreed upon, it is natural to want to know what causes it, and here further disagreement arises. Most of the different conceptions of the nature and causes of cultural evolution arose in the eighteenth and nineteenth centuries, and a brief historical sketch is useful in describing them.

History of the concept

As primitive societies in various parts of the world became known to Europeans during the age of discovery, two different explanations of their primitiveness were offered. The most widespread belief was theological: they had “degenerated” further from an original state of grace than had civilized peoples. The other, the rationalist explanation that became usual among intellectuals in the seventeenth and eighteenth centuries, was that civilization had evolved from earlier primitive types that must have been similar to the culture of contemporary savages and barbarians. There had been, to be sure, evolutionary notions held by philosophers of the classical traditions–by the Greek Epicurus and the Roman Lucretius, for example– but modern ideas about cultural evolution were propounded most influentially by Turgot, Montesquieu, Rousseau, Condorcet, Helvetius, Diderot, and others in France; Kant and Herder in Germany; Vico in Italy; and Hume, Hobbes, and Ferguson in Britain.

As opposed to degenerationism, all of these writers held to a theory of progress. Although modern anthropologists have commonly thought that this theory was merely a happy giddiness induced by the great economic and political advances of the period, most European scholars were actually pessimistic about progress in the future. While all agreed that progress had taken place in the past, and that discernible, orderly stages of its evolution could be demarcated, only Kant, Turgot, and Condorcet thought that progress was inevitable. There were variations in the names and numbers of stages that were proposed, but most accepted either Turgot’s stages of hunting, pastoralism, and farming, or Montesquieu’s similar typology of savagery, barbarism, and civilization.

Although the modern holistic concept of culture was lacking, the elements that were seen to be evolving were the human institutions of which culture is composed. There was one emphasis in the evolutionary theory that was peculiar to the times –rationalism. Human institutions were viewed as products of the human mind (as indeed in some sense they must be), a mind that was mistaken or irrational in the past, but increasingly less superstitious and more reasonable as time went on. Since civilized man had literally thought himself out of a “state of nature,” the degree of orderliness that lay in human history was due to the progressive improvement of mentality.

In the nineteenth century, evolutionary thought became less philosophical and more influenced by empirical aspirations. Tylor in England and Morgan in the United States were prominent in the creation of ethnological evolutionism; Spencer in England, Saint-Simon, Comte, and Durkheim in France were pioneers in sociology; the reworking of Hegel by Marx and Engels was a creative contribution to political-revolutionary theory. They differed from their predecessors and from one another mainly with respect to the following problems: (1) What is it that evolves–culture in general or only the institutions of specific societies? (2) How does it evolve–by orthogenetic, inevitable progress, by rational thought and intention, survival of the fittest, or an unconscious dialectical struggle? (3) Where is the locus of the evolutionary impulse– in the improvements of technology and the material aspect alone, in the division of labor, the political or ideological aspect, or as a force in the cosmos?

The nature of culture. The ethnologists of the late 1800s, convinced of orderliness in evolution, directed themselves to the delineation of stages of culture, which E. B. Tylor defined in the anthropological sense in 1871. L. H. Morgan (1877) did not use that word, but in his usage, “society” and “ethnical periods” (for stages) referred essentially to the subject matter of Tylor’s concept. Tylor and Morgan, and most ethnologists of their time as well, were concerned with the world-wide manifestation of cultural stages, not with the culture of a particular society. A specific primitive tribe was of interest only as an illustration of aspects of the culture of an entire stage and of a large geographical area.

This concern with culture in its most general sense rather than with particular societies caused a misunderstanding of the ethnological evolutionists by later commentators. Modern anthropologists frequently criticize the nineteenth-century evolutionists as “unilinear,” meaning that the latter believed that all societies inevitably progress through the same stages. This would be a powerful blow at nineteenth-century evolutionary theory, if it were true, but it seems doubtful that any of the evolutionists believed such manifest nonsense. In the statement that seems best to serve modern critics, Morgan said:

Since mankind were one in origin, their career has been essentially one, running in different but uniform channels upon all continents, and very similarly in all the tribes and nations of mankind down to the same status of advancement. It follows that the history and experience of the American Indian tribes represent, more or less nearly, the history and experience of our own remote ancestors when in corresponding conditions. ([1877] 1964, pp. 6–7)

This sounds “unilinear” to a modern ethnologist, whose concern has been restricted to the structure and functioning of unit systems, but inasmuch as ethnologists were not making such studies in the nineteenth century, it seems apparent that Morgan must have meant nothing more than that wherever barbarism (defined by the traits of horticulture or pastoralism) was found, a general stage of hunting-gathering society (savagery) had preceded it and that stages of both had preceded civilization on continents that had achieved civilization. Such a judgment is attested by archeology now as well as by common sense and should evoke no comment, but in Morgan’s day it was worth stating because theories of degeneration and catastrophe were still commonly opposed to evolutionism.

The sociologists tended toward the organismic model for society. A society was thought of as a contained unit made up of interdependent parts, each subserving the others. Evolution was seen as the development of more parts and greater differentiation of them. The “parts” are individuals, groups, and specialized persons and groups. Religions, morals, and political, social, and economic institutions function largely to bolster, integrate, and smooth the relations between the social parts. This early model, refined as nonevolutionary structural functionalism, became characteristic of modern American sociology and British social anthropology but had its roots in the eighteenth-century concern with progress.

The third group, the Marxists, was closer to ethnology than to sociology, at least in its beginning phases under Marx and Engels. In fact, Engels’ Origin of the Family, Private Property and the State (1884) was inspired by Morgan’s Ancient Society (1877) and borrowed heavily from it. The theory of general developmental stages was the same, and the conception of the evolving unit did not have the organismic particularism of the sociologists. As in Morgan’s case, the concept of culture was absent but would have been appropriate, for institutions (especially technological and economic) were not merely subserviently integrative in function but had more of an initiating “prime mover” status than the sociologists believed.

How does culture evolve? The eighteenth-century evolutionists thought of the progressive improvement of the human condition as a mentalistic evolution and thus took an idealist view of the evolutionary process. Some of the language of nineteenth-century ethnological evolutionism reflects this, so that we find such expressions as Morgan’s “growth of the idea of government” and Tylor’s frequent use of “mind” and “mental life” as near synonyms of many aspects of culture. But a very important change occurred in the nineteenth-century view of evolution. As it became more scientifically oriented it posited causal and functional connections between different aspects of culture. Tylor, in his greatest book, Primitive Culture (1871), devoted much of the introductory chapter (“The Science of Culture”) to describing not only cause-effect relations in culture but also the determining of these cultural relations and the thought and will of the individual. The ethnological school of evolution thus made significant moves toward determinism and against assumptions of free will in human affairs.

The “how,” the mechanics of the evolutionary process, was not explicitly described, beyond the suggestion that it was unconscious. We are told that new elements tend to replace older ones if they are better, sometimes, but beyond that one has the impression that evolution was taken as a “given,” that orthogenetic forces had moved mankind ever upward, however fitfully.

There is a recurrent note reminiscent of the eighteenth-century rationalist ancestry: “Now that we understand evolution we can more consciously control it.” Tylor called anthropology a “reformer’s science,” and Morgan said, “The time will come … when human intelligence will rise to the mastery over property… .” This was the deterministic paradox: We can scientifically analyze the evolution of culture because it is orderly (because it is determined); knowing this we can somehow influence the future as we pass, as Tylor put it, “from the age of unconscious to that of conscious progress.”

The sociological wing of nineteenth-century evolutionism pursued the implications of the biological analogy. The rationalist optimism of Condorcet was outdated by the obvious attendant evils of industrialization, especially in England, as illustrated by Malthus’ pessimistic Essay on the Principle of Population. After the great intellectual success of Darwin’s theory of selection by survival (itself suggested by Malthus’ essay), a theory of “social Darwinism” arose, which, whatever its demerits, at least provided a “how” for the evolutionary process: as a result of conflict between societies superior ones replace the inferior. Some added an “internal conflict” aspect: out of the struggle between classes, groups, and even individuals within the society comes the improvement of the society. Walter Bagehot, Auguste Comte, Herbert Spencer, and Ludwig Gumplowicz were the leaders in this mode of thought.

Marx and Engels were even more consistently deterministic and materialistic than the ethnologists and had a much more definite theory of the mechanics of evolution. This theory was orthogenetic in that the impetus for change came from within the society, from the “dialectic” of the class struggle, the resolution of contradictions in terms of either failure or a higher unity. It should be noted that this internal-conflict theory is not like social Darwinism: the ruling or propertied class is not superior. Marx and Engels, like the ethnologists, were insistent on the lawful, determined nature of evolution, and they also believed that evolution could be oriented by the conscious action of man once he understood its processes. Then, by abolishing the capitalist form of production, “Man, at last the master of his own form of social organization, becomes at the same time the lord over nature, his own master–free” (Engels [1882] 1935, p. 75).

Most of the sociologists became less interested in evolution itself than in the more immediate problems of the organismic nature of a society, particularly that of integration. What holds a society together? Following the social contract theories of the eighteenth century, there were psychological theories, mental interaction theories, and imitation theories, all of which took society to be an organismic entity somehow mentally constituted. This was the time when sociology became nonevolutionary, as it mostly remains to this day.

The locus of the evolutionary impulse. Morgan thought that cultural evolution consisted of two distinct aspects-“inventions and discoveries” (the technical order), which evolve in connected, progressive, cumulative relations to one another, and “institutions” (the forms of the family, of government, religion, architecture, property), which stand in “unfolding relations.” By this expression he meant that social institutions originate in a few “primary germs of thought” and thereafter independently change form as well as replace previous forms. Tylor also, although not so explicitly, thought that technology, science, and other aspects of material culture undergo evolution rather independent of religion and “intellectual and moral” progress. Nowhere is it demonstrated, however, that one of these aspects is the “prime mover” and the other a dependent variable or superstructure. But again it should be remembered that Morgan and Tylor were not talking about the process of systemic change in any particular society, hence the matter of functional priority of one part over another simply did not concern them.

The sociologists, preoccupied with social integration, psychology, and mentalism, did not see the initiating locus of evolutionary change in any aspect of culture at all. Spencer and his followers saw evolution as a grand cosmic force that generated complexity out of simplicity and heterogeneity out of homogeneity, aided somewhat by Darwinian “conflict and survival.” A few Frenchmen, most notably Emile Durkheim (1893), posited that the division of labor in society, like the functional specialization of organs in biological entities (again, the organismic model), is related to population increases, greater social density, and larger, stronger societies. But it is not clear what the causes of these developments are, and Durkheim explicitly denied that the division of labor is increased for utilitarian reasons like “the greatest happiness for the greatest number” or by any other kind of intention or plan.

The Marxians, on the other hand, were firm in the conviction that the locus of evolutionary change lies in the technoeconomic (or material) sector, which then affects the nature of the social classes and their interrelations, and that ideology is mere superstructure. As such, it is the last part of culture to change. This form of the old materialist versus idealist philosophical argument persists strongly to this day.

Twentieth-century evolutionism

Most American ethnologists in the first half of the twentieth century repudiated an evolutionism that they misunderstood in favor of a raw ethnographic, “natural history” approach to the study of primitive culture. In Britain and France, and in sociology nearly everywhere, evolutionism succumbed intellectually to a structural functionalism that had greater utility for the practical solution of social problems through political administration in the colonies and at home.

A. G. Keller, an American sociologist, Leslie A. White and Julian H. Steward, American ethnologists, and V. Gordon Childe, a British archeologist, were virtually alone in opposing the antievolutionary temper of the times. It was not until after mid-century that there was any larger shift of opinion toward an evolutionary outlook again, but this took place only in America, only in anthropology, and there only in part.

Twentieth-century evolutionism differs from previous theories in two major respects. The first concerns the cultural adaptations through which evolutionary changes are believed to occur. The concept of cultural adaptation has supplanted the orthogenesis of earlier evolutionists, who had found the only generative impulse in the internal class-struggle dialectic proposed by Marx and Engels and in the social Darwinism of some sociologists. Second, a new theoretical synthesis has been made by the reworking and integration of some of the earlier perspectives that had been thought to be contradictory.

The significance of the adaptation of culture to the natural environment as an important aspect of the evolutionary process was presented as “cultural ecology” by Julian H. Steward. Others have proposed further that in the process of adaptation, the environment includes not only the natural environment but adjustments to other social systems as well. According to this view, inventions and discoveries, borrowings, unconscious historical “accidents,” changes from whatever source, are the raw materials for evolutionary change in culture. Some of these “fit” as improvements in the internal functional arrangements, while others solve external environmental problems with respect to nature or competition: thus they are selected simply because they are superior instruments. The advance of this perspective over eighteenth-century and nineteenth-century ideas of the “inevitability of progress” and orthogenesis is manifest: the evolutionary perspective is not mystical and can accommodate and make more intelligible the variety of historical data we now possess.

One of the historical facts of life that has plagued all orthogenetic schemes is that different societies manifest great variation in rates of evolution, from drastic revolution to the other extreme of non-evolution–i.e., stabilization. And in so many instances a society makes a very rapid rise only to reach a long-term plateau. The theory that evolution proceeds by adaptation, however, allows for all of these eventualities, taking stabilization as much for granted as progress. Stabilization, after all, merely bespeaks the success of the adaptive process: when the culture is successfully adapted, it tends to reject subsequent possible changes. This can render explicable what might seem paradoxical: that a culture “high” in one stage might fail to advance to further heights in the next simply because of its earlier success. And, of course, the more specialized its form of adaptation, the more deeply entrenched and committed to its present environment it becomes.

The perspective of cultural adaptation and selection is particularly useful in reconciling opposing viewpoints derived from the nineteenth century.

What is it that evolves? Is it culture in general, through grand stages, or only particular social systems? The reconciliation of these two views is easy: both are correct. The evolution of the totality is the product of the evolution of particular societies. To be sure, there is but a single evolutionary process, the selection of traits and their functional adjustment via adaptation in particular systems. This is the way societies become differentiated one from the other, but it is also the way some become superior to others in measurable ways. Thus, two different theoretical perspectives are possible with respect to the same data. These are what Sahlins (Sahlins & Service 1960) calls the specific evolutionary perspective as compared with the general evolutionary perspective. The former refers merely to the creation of diversity by adaptive modification of related particular societies. The latter is the measurement of progress; some specific evolutionary changes are significant breakthroughs that can be measured by such objective directional criteria of progress as greater all-round adaptability, greater dominance, or greater complexity of organization. In short, specific evolution refers to our concern with descent-with-modification or adaptive variation; general evolution refers to the progressive emergence of superior forms, stage by stage, which can be related to the directional evolution of the total culture of the human species.

How does culture evolve? Is it in some measure intentionally planned, or is it an unconscious and nonrational process, determined by events outside human awareness? Surely an improvement in ideas has something to do with it, and sometimes ideas must be conscious and rational; this is most obvious in science and engineering but also holds true in the institutional realm. Many political institutions, for example, result from attempts to solve social or economic problems purposefully. But, of course, there are often latent and unintended consequences of even the most manifest political expediency.

Again, it would seem that a reconciliation of the opposed views can be made by means of the adaptation-selection perspective. New culture traits or modifications can have any number of sources: inventions, purposeful borrowings, accidents, unconscious functional shifts, and so on. The selection or rejection of any of these could also involve conscious intentionality, even if but rarely. The selective process in cultural evolution is only roughly analogous to natural selection in biology; certainly the capacity of a person to analyze his own behavior, predict future events, and rearrange his affairs on that basis is a distinctively human trait. It is more difficult to plan and arrange things on a social or political basis, and the greater the demographic scope the more difficult it is; but it does happen. The adaptation-selection perspective has the great virtue of not prescribing either conscious intention or unconsciousness; it can accommodate either and still lead to greater comprehension of cultural change. And further, determinism in human affairs is not equated with unawareness, indeterminism with awareness. Determinism is a perspective that the analyst takes, not a property of the subject matter under investigation.

Where is the locus of the evolutionary impulse? Does it lie in the mode of production, in technology, in the relations of production, in the class struggle, in the division of labor, in man’s view of destiny; or is it a mystical force in the cosmos? It would seem that those who posited mode of production, class struggle, technology, or division of labor were much influenced by the industrial revolution, which has been, of course, a most striking evolutionary prime mover for the past century and a half and promises even more wondrous cultural transformations almost immediately. But has the material, technoeconomic aspect always been the prime mover? The change from primitive chiefdoms to early states and then to empires in Mesoamerica, Peru, and probably elsewhere seems to have been first in the political sector; even the important inventions of writing and mathematics could have originated in the occult mumbo jumbo of priests. The modern evolutionist accordingly wants to know more about particular instances of change and finds no need to insist that the initial loci must be always in the same sector of culture.

Evolutionists of the nineteenth century were more empirical than their predecessors, but their use of ethnographic data was often mere illustration rather than proof of hypotheses. The “comparative method” of some, such as Sir James Frazer, was simply an uncritical, but energetic, “clip and paste.” E. B. Tylor (1888) was a notable exception: he originated a method of statistical correlations in his comparative study of marriage and descent rules. Hobhouse, Wheeler, and Ginsberg’s work (1915) was another important application of statistics to problems of cultural evolution. Otherwise, both evolutionary theory and the comparative method nearly perished from inattention until mid-twentieth century.

The use of the comparative method began to revive in America during the 1950s, particularly stimulated by George P. Murdock’s creation of the Human Relations Area Files and later the “Ethnographic Atlas.” Formal graphical means of showing correlations and sequences of culture traits in the course of evolutionary changes have attracted attention recently. (See Naroll 1956; Freeman & Winch 1957; Gouldner & Peterson 1962; and Carneiro & Tobias 1963.) The results of these efforts are meager so far, and several unsolved difficulties attend them. The problems of how to define the significant cultural units to be counted and how to select a random sample of them in the absence of an adequate number of ethnographies of unacculturated societies are serious.

The gravest difficulty of all is caused by the tendency of a culture to become specialized as it adapts to its environment, for to the extent that it is special it is incommensurable. Walter Goldschmidt has aptly called this “the Malinowskian dilemma”: Malinowski argued (and successfully demonstrated) that every cultural institution must be understood as a unique product of the cultural whole within which it developed. It would seem to follow, therefore, that the comparative method is wrong, comparing incomparables. Yet, paradoxically, Malinowski often extrapolated from his insights into Trobriand culture to the primitive world in general. Goldschmidt argues that the solution is to compare functions, not institutions: “What is consistent from culture to culture is not the institution; what is consistent are the social problems. What is recurrent from society to society is solutions to these problems” (1966, p. 31).

Julian H. Steward’s studies (1955) of “multilinear evolution” show an awareness of these difficulties. He recommends comparative studies of specific holocultures in evolution, rather than comparisons of isolated traits, in order to find the “regularities” of evolution.

There is a different test of theory that is bound to be used more frequently–the test of fruitfulness. Since one of the main purposes of evolutionary theory is to provide intelligibility to historical data, then the better it fulfills this function, the greater must be its empirical as well as logico-didactic virtues. Guy Swanson’s study of religion (1960) is a good example: light is cast on the development of religion, and at the same time evolutionary theory proves to be useful.

Such empirical applications of evolutionary theory can result in its refinement only to the extent that evolutionists maintain an empirical orientation, willing to change the theory in the service of its intellectual functions. Some of the older evolutionary philosophies, particularly those of Marx and Spencer, were too grand in scope and too schematic to be useful. They also became stultified dogmas as they were used by political parties and academic “schools of thought.” A better fate may be expected of recent evolutionism, judging from the evidence of new empirical attitudes, particularly if its proponents remain guarded against unnecessary and untested preconceptions that can so easily impede a true evolutionary science of culture.

ELMAN R. SERVICE

[See alsoANTHROPOLOGY, articles on THE FIELD and THE COMPARATIVE METHOD IN ANTHROPOLOGY; ARCHEOLOGY, article onTHE FIELD; CULTURE; ECOLOGY. Also related are the entriesINTEGRATION; SOCIAL DARWINISM; SOCIOLOGY, article onTHE DEVELOPMENT OF SOCIOLOGICAL THOUGHT; and the biographies ofCHILDE; MORGAN, LEWIS HENRY; SPENCER; TYLOR.]

BIBLIOGRAPHY

CARNEIRO, ROBERT L.; and TOBIAS, STEPHEN F. 1963 The Application of Scale Analysis to the Study of Cultural Evolution. New York Academy of Sciences, Transactions Second Series 26, no. 2:196–207.

CHILDE, V. GORDON (1936) 1965 Man Makes Himself. 4th ed. London: Watts.

CHILDE, V. GORDON 1951 Social Evolution. New York: Schumann.

COTTRELL, WILLIAM F. 1955 Energy and Society: The Relation Between Energy, Social Change, and Economic Development. New York: McGraw-Hill.

DOBZHANSKY, THEODOSIUS 1962 Mankind Evolving: The Evolution of the Human Species. New Haven: Yale Univ. Press.

DURKHEIM, ÉMILE (1893) 1960 The Division of Labor in Society. 2d ed. Glencoe, Ill.: Free Press. → First published in French.

ENGELS, FRIEDRICH (1882) 1935 Socialism: Utopian and Scientific. New York: International Publishers. → First published as Die Entwicklung des Sozialismus von der Utopie zur Wissenschaft.

ENGELS, FRIEDRICH (1884) 1942 The Origin of the Family, Private Property and the State. New York: International Publishers. → First published in German.

FERGUSON, ADAM (1767) 1819 An Essay on the History of Civil Society. 8th ed. Philadelphia: Finley.

FREEMAN, LINTON C.; and WINCH, R. F. 1957 Societal Complexity: An Empirical Test of a Typology of Societies. American Journal of Sociology 62:461–466; 63:78–79.

GOLDSCHMIDT, WALTER R. 1959 Man’s Way: A Preface to the Understanding of Human Society. Cleveland: World.

GOLDSCHMIDT, WALTER R. 1966 Comparative Function-alism: An Essay in Anthropological Theory. Berkeley and Los Angeles: Univ. of California Press.

GOULDNER, ALVIN W. and PETERSON, R. A. 1962 Notes on Technology and the Moral Order. Indianapolis, Ind.: Bobbs-MerriU.

HOBHOUSE, LEONARD T.; WHEELER, GERALD C.; and GINSBERG, MORRIS (1915) 1965 The Material Culture and Social Institutions of the Simpler Peoples: An Essay in Correlation. London School of Economics and Political Science Monographs on Sociology, No. 3. London: Routledge.

HUXLEY, JULIAN S. 1942 Evolution: The Modern Synthesis. London and New York: Harper.

HUXLEY, JULIAN S. 1955 Evolution, Cultural and Biological. Pages 3–25 in Yearbook of Anthropology. New York: Wenner-Gren Foundation.

KELLER, ALBERT G. (1915) 1931 Societal Evolution. Rev. ed. New York: Macmillan.

MAINE, HENRY J. S. (1861) 1960 Ancient Law: Its Connection With the Early History of Society, and Its Relations to Modern Ideas. Rev. ed. New York: Dutton; London and Toronto: Dent.

MONTAGU, ASHLEY 1962 Culture and the Evolution of Man. New York: Oxford Univ. Press.

MORGAN, LEWIS H. (1877) 1964 Ancient Society. Cambridge, Mass.: Harvard Univ. Press.

MUNRO, THOMAS 1963 Evolution in the Arts and Other Theories of Culture History. Cleveland (Ohio) Museum of Art.

NAROLL, RAOUL S. 1956 A Preliminary Index of Social Development. American Anthropologist New Series 58:687–715.

SAHLINS, MARSHALL D.; and SERVICE, ELMAN R. (editors) 1960 Evolution and Culture. Ann Arbor: Univ. of Michigan Press.

SERVICE, ELMAN R. 1962 Primitive Social Organization: An Evolutionary Perspective. New York: Random House.

SPENCER, HERBERT 1915 Works. 18 vols. New York and London: Appleton.

STEWARD, JULIAN H. 1955 Theory of Culture Change: The Methodology of Multilinear Evolution. Urbana: Univ. of Illinois Press.

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SYMPOSIUM ON THE EVOLUTION OF MAN’S CAPACITY FOR CULTURE, CHICAGO, 1957 1959 The Evolution of Man’s Capacity for Culture. Detroit: Wayne State Univ. Press.

TYLOR, EDWARD B. (1871) 1958 Primitive Culture: Researches Into the Development of Mythology, Philosophy, Religion, Art and Custom. 2 vols. Gloucester, Mass.: Smith. → Volume 1: Origins of Culture. Volume 2: Religion in Primitive Culture.

TYLOR, EDWARD B. (1888) 1961 On a Method of Investigating the Development of Institutions: Applied to Laws of Marriage and Descent. Pages 1–28 in Frank W. Moore (editor), Readings in Cross-cultural Methodology. New Haven: Human Relations Area Files Press.

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WHITE, LESLIE A. 1949 The Science of Culture: A Study of Man and Civilization. New York: Farrar, Straus. → A paperback edition was published in 1958 by Grove.

WHITE, LESLIE A. 1959 The Evolution of Culture: The Development of Civilization to the Fall of Rome. New York: McGraw-Hill.

V SOCIAL EVOLUTION

Evolutionary theory dominated sociological thought in the nineteenth and early twentieth centuries, but since about 1920 interest in it has, on the whole, given way to preoccupation with systematic analysis of social systems, analysis of broad social and demographic trends, and investigation of the social determinants of behavior (Ginsberg 1932). The recent tentative revival of interest in an evolutionary perspective is closely related to growing interest in historical and comparative studies. It does not, however, denote a mere return to the assumptions of the classical evolutionists, but rather it implies revision and reappraisal of evolutionary theory in the light of recent advances in sociological theory and research.

From the point of view of sociological analysis, the older evolutionary models broke down mainly over two stumbling blocks. The first was the assumption that the development of human societies is relatively cumulative and unilinear and that the major “stages” of development are universal–even if there are many differences in detail and even if not all societies reach every stage of evolution. The second stumbling block was the failure to specify fully the systemic characteristics of evolving societies or institutions or the mechanisms and processes of change through which the transitions from one “stage” to another were effected. Most of the classical evolutionary schools tended, rather, to point out general causes of change (economic, technological, spiritual) or some general trends (for example, the trend to complexity) inherent in the development of societies. Very often they confused such general tendencies with the causes of change or assumed that these general tendencies explained concrete instances of change (Bock 1963).

Attempts to reappraise evolutionary perspectives, therefore, must address themselves to several basic problems inherent in the new analytical developments in sociological theory, on the one hand, and in the general setting of the evolutionary problem, on the other (Wolf 1964; Parsons 1964; Eisenstadt 1963a). The first crucial problem concerns the extent to which change from one type of society to another is not accidental or random but, rather, evinces over-all evolutionary or developmental trends. Second is the question of the extent to which such changes are cumulative within any given society and within any given institutional sphere in different societies (Wolf 1964). Third is the problem of the extent to which such changes do indeed enhance the adaptive potential of a society in relation to its cultural and natural environment–however such adaptation and environment are defined (White 1959; Sahlins & Service 1960).

Furthermore, even if some such common characteristics or trends can be found within different and disparate societies, the question remains as to the validity of talking about the evolution of human society or culture as a whole. Here three sub-problems exist: the first is the extent to which other societies constitute the “environment” of any society–that is, the environment to which any given single society has to adapt and which can enhance the general reservoir of its techniques of adaptation. The second is a question of the extent to which institutions and forms of organization that have adaptive value can be borrowed and transplanted from one society to another, thus enhancing their adaptive potential. And, finally, we must ask to what extent human society is a “system of points” with some common adaptive and integrative mechanisms. As distinct from the general theory of cultural evolution, which in a way assumes the unity of mankind and hence also the internal transferability of institutions or techniques, the focus of sociological analysis is on the relations between the systemic characteristics of societies in interaction with their natural, social, and cultural environments, on the one hand, and some possible broader trends of changes and transformations in their “transbiological” or superorganic abilities and traditions, on the other (Mead 1964).

The starting point of all these discussions, especially from the point of view of the relation between the transformative capacities of any single society and any possible general trends in the development of human societies and human society in general, is the problem of the extent to which such changes may be envisaged as crystallizing into developmental “stages”-a key concept in classical evolutionary thought (Ginsberg 1932). In the older evolutionary school such stages were construed mostly in terms of “specialization” and “complexity,” whereas in recent works these concepts have been to a large extent replaced by that of “differentiation.”

Differentiation and institutional growth

Differentiation, like complexity or specialization, is first of all a classificatory concept. It describes the ways through which the main social functions or the major institutional spheres of society become dissociated from one another, attached to specialized collectivities and roles, and organized into relatively specific and autonomous symbolic and organizational frameworks within the confines of the same institutional system. In broad evolutionary terms, such continuous differentiation has usually been conceived as a continuous development starting from the “ideal” type of the primitive society or band, in which all the major roles are allocated on an ascriptive basis and the division of labor is based primarily on family and kinship units. Development then proceeds through various stages of specialization and differentiation.

Specialization is first manifest when each of the major institutional spheres, through the activities of people placed in strategic roles within it, develops its own organizational units and complexes and its specific criteria of action. The latter tend to be congruent with the basic orientations of a given sphere, facilitating the development of its potentialities: technological innovation, cultural and religious creativity, expansion of political power or participation, or development of complex personality structure.

Second, different levels or stages of differentiation denote the degree to which major social and cultural activities as well as certain basic resources –such as manpower and economic resources– have been disembedded or freed from kinship, territorial, and other ascriptive units (Parsons 1964; Bellah 1964; Eisenstadt 1963a). Although these “free-floating” resources pose new problems of integration, they may also become the basis for a more differentiated social order that is, at least potentially, better adapted to deal with a more variegated environment. Thus, a new set of problems–those of integration–emerges as the very crux of the way in which such resources can be utilized for the crystallization of some general transformative potentials within a society.

Problems of integration. As the more differentiated and specialized institutional spheres become more interdependent and potentially complementary in their functioning within the same over-all institutionalized system, this very complementarity creates more difficult and complex problems of integration. The growing autonomy of each sphere of social activity, and the concomitant growth of interdependence and mutual interpenetration among them, pose for each sphere ever more difficult problems in crystallizing its own tendencies and potentialities and in regulating its normative and organizational relations with other spheres. And at each more “advanced” level or stage of differentiation, the increased autonomy of each sphere creates increasingly more complex problems of integrating these specialized activities into one systemic framework.

The growing autonomy of the different institutional spheres and the extension of their organizational scope not only increase the range and depth of social and human problems but also open up new possibilities for technological development, expansion of political power or rights, and cultural creativity. Growing differentiation also enhances systemic sensitivity to a much wider physical-technical environment and to more comprehensive intersocietal relations. But the growth of systemic sensitivity to new problems and exigencies does not necessarily imply a concomitant development of the ability to deal with these problems, nor does it indicate the ways in which these problems may be solved. At any given level of differentiation an institutional sphere may or may not achieve an adequate degree of integration, and the potentialities unfolded through the process of differentiation may be “wasted”-that is, they may fail to become crystallized into an institutional structure.

Recognition of the integrative problems that are attendant on new levels of differentiation constitutes the main theoretical implication of the concept of differentiation, and it is in the light of the analytical problems raised by this implication that the various questions pertinent to a reappraisal of the evolutionary perspective in social science have to be examined. We are as yet far from any definitive answers to these questions, but at least we can point out some of the most important problems of research in this direction.

Responses to differentiation

The passage of a given society from one stage of differentiation to another is contingent on the development within it of certain processes of change which create a degree of differentiation that cannot be contained within the pre-existing system. Growing differentiation and the consequent structural breakthroughs may take place through a secular trend of differentiation, or through the impact of one or a series of abrupt changes, or both. These tendencies may be activated by the occupants of strategic roles within the major institutional spheres as they attempt to broaden the scope and develop the potentialities of their spheres. The extent to which these changes are institutionalized and the concrete form they take in any given society necessarily depend on the basic institutional contours and premises of the pre-existing system, on its initial level of differentiation, and on the major conflicts and propensities for change within it (Eisenstadt 1964b).

We need not assume that all changes in all societies necessarily increase differentiation. On the contrary, the available evidence shows that many social changes do not give rise to over-all changes in the scope of differentiation but result, instead, mainly in changes in the relative strength and composition of different collectivities or in the integrative criteria of a particular institutional sphere. Largely because the problem has not yet been fully studied, we do not know exactly what conditions facilitate or precipitate these different types of change in different societies and what makes for variations in innovative or transformative capacities among different societies (Eggan 1963; Sahlins 1964).

Even when social change increases differentiation, the successful, orderly institutionalization of a new, more differentiated social system is not always a necessary outcome. Moreover, at any level of development, response to the problems created by the process of differentiation may take one of several different forms (Weber 1922a; Eisenstadt 1963a). The most extreme outcome is failure to develop any adequate institutional solution to the new problems arising from growing differentiation. Aside from biological extinction, the consequences may be total or partial disintegration of the system, a semiparasitic existence at the margin of another society, or total submersion within another society.

A less extreme type of response tends to lead to “regression,” that is, to the institutionalization of less differentiated systems within the more differentiated system that has broken down. Examples include the establishment of small patrimonial or semifeudal chiefdoms on the ruins of the Achaemenid Empire, the development of dispersed tribal-feudal systems at the downfall of the Roman Empire, and similar developments on the ruins of Greek city-states. Many such regressive developments are only partial, in the sense that within some parts of the new institutional structure some nuclei of more differentiated and creative orientations may survive or even develop. Sometimes, but certainly not always, these nuclei “store” entrepreneurial ability for possible–but not inevitable–future developments.

Another possibility, which perhaps overlaps with the last one but is not always identical with it, is the development of a social system in which the processes of differentiation and change go on relatively continuously in one part or sphere of a society without becoming fully integrated into a stable, wider framework. In such situations a continuous process of unbalanced change may develop, resulting either in a breakdown of the existing institutional framework or in stabilization at a relatively low level of integration. Perhaps the best examples of such developments can be found in various “dual conquest” societies (for example, conquest of the sedentary population by nomads in the Mongol Empire) and especially in the pre-independence stages of modern colonial societies.

A fourth, and perhaps the most variegated, type of response to growing differentiation consists of some structural solution that is on the whole congruent with the relevant problems. Within this broad type a wide variety of concrete institutional arrangements is possible. Such different solutions usually have different structural results and repercussions. Each denotes a different structure crystallized according to different integrative criteria and is interpenetrated in a different way by the other major social spheres.

Thus, drawing on examples from the great centralized empires of history, we see that although the initial stages of socioeconomic differentiation were relatively similar in Byzantium, in the later (Abbasside) caliphate, and in post-Han China, each of these societies developed different over-all institutional structures (Eisenstadt 1963a). The Byzantine Empire became a highly militarized and politically oriented system, whereas the caliphate developed a theocratic structure, which was based on continuous attempts to institutionalize a new type of universalistic politicoreligious community. China developed a centralized system based on the power of the emperor and the bureaucracy and, at the local level, on the relative predominance of the gentry; the selective channels of the examination system and the elite formed by the literati were the major mechanisms for integrating the local and central levels.

One very interesting structural solution is the development of a relatively stable system in which the major institutional spheres vary in degree of differentiation. One of the most important examples of such variation occurs in feudal systems, which are characterized by a relatively high degree of differentiation in some of the central cultural roles as against a much smaller degree of differentiation in the economic and political roles.

In cases of such uneven differentiation the more differentiated units of such related societies (for example, the church in feudal or patrimonial systems) often tend to develop a sort of international system of their own, apart from that of their “parent” societies.

The variety of integrative criteria and institutional contours at any level of differentiation is, of course, not limitless. The very notion of interdependence among major institutional spheres negates the assumption that any number of levels of differentiation in different institutional spheres can coalesce into a relatively stable institutional system. The level of differentiation in any one sphere necessarily constitutes, within broad limits, a precondition for the effective institutionalization of certain levels of differentiation in other social spheres. But within these broad limits of mutual preconditioning a great deal of structural variety is possible.

The intersocietal environment. The processes of change and of differentiation, on the one hand, and the development of different integrative responses to them, on the other, do not take place within single, closed societies. They are closely related to the international system that constitutes the broader environment of any society. Each society is related to many others geopolitically, ecologically, and socioculturally. These relations constitute the environment to which each society has to adapt and which may also influence its ability to evolve institutional responses to the processes of change.

Such international geopolitical factors, in the broadest sense, not only provide the general setting for any given society but also give rise to many of the concrete pressures upon it, such as external pressures of population, problems of military security, or adjustment to international trade. Such an intersocietal environment need not always consist of societies of the same type or level of differentiation; it may, indeed, contain many different types of societies. In general, it can be assumed that the more differentiated a society is, the greater is its systemic sensitivity–although not necessarily its ability to cope with these problems–to a wider and more variegated international setting.

Thus, at any given level of differentiation, the crystallization of different institutional orders is shaped by the interaction between the broader structural features of the major institutional spheres, on the one hand, and, on the other, the development of elites or entrepreneurs in some of the institutional spheres of that society, in some of its enclaves, or even in other societies with which it is in some way connected.

The variability in the concrete components of such interaction helps to explain the great (but not limitless) variety of structural and integrative forms that may be institutionalized at any given level of differentiation. Although different societies may arrive at broadly similar stages of evolution in terms of the differentiation of their major institutional and symbolic spheres, yet the concrete institutional contours developed at each such step, as well as the possible outcomes of such institutionalization in terms of further development, breakdown, regression, or stagnation, may differ greatly among them.

Reappraising evolutionary theory

The preceding analysis of processes of change and differentiation and of concomitant institutionalization of new structures indicates some of the problems that are posed by any attempt to reappraise the evolutionary perspective in sociological theory.

First is the exploration of the different mechanisms of social change and the distinction between those conditions and processes of change that create potentialities for transformation and those that do not. It is obvious, as indicated above, that not all processes of social change necessarily give rise to changes in over-all institutional systems. Although the potentialities for such systemic changes (as distinct from changes in patterns of behavior, in the composition of subgroups, or in the contents of the major integrative criteria of different spheres) exist in all societies, the very actualization of these potentialities, as well as the tempo and direction of such changes, varies greatly among different societies (Eggan 1963; Eisenstadt 1963a).

Second, and closely connected with the first, is the problem of how cumulative the development is of different types of institutional organization. Here it seems that in some institutions there may indeed be a “scale” or “semiscale” order of development, although the application of scale analysis to this type of phenomenon has so far failed to detect any perfect regularities (Carneiro 1962; Good-enough 1963). Such scale order is probably to be found least in the sphere of kinship or family institutions, whereas it is more pronounced in those institutional spheres, such as economics, politics, and law, that are most closely connected with technology or with organizational problems (Wolf 1964; Schwartz & Miller 1964).

However, even the existence of such scale order does not necessarily imply that developments in any institutional sphere are necessarily cumulative in the sense that they can be transferred easily from one society to another at a similar general level of differentiation; neither does it imply that their development within any single society or their transfer from one to another must necessarily proceed in a certain order or that “jumps” are not possible. Studies of the modernization of traditional societies are especially relevant from the point of view of the possibilities of such jumps, although similar cases can probably be found in other types of societies also (Eisenstadt 1963b). At most, studies of such scale order indicate that a certain trait or organizational type may be a necessary, but certainly not a sufficient, condition for the emergence of another; even here, the findings of “quasi scales” indicate the possibility of many functional equivalents of any such trait in a sequential series–especially in cases of rapid social change (Schwartz & Miller 1964).

Third is the question of the extent to which the problems arising from growing differentiation and the institutional solutions to these problems are indeed the same in different societies, thus creating some common trend of development. This problem is very close to that of the relation, to use Sahlins’ and Service’s nomenclature (1960), between “specific” and “general” evolution or that of the feasibility of the assumption, as put by Eggan (1963, p. 355), “that these particular developments necessarily add up to ‘the succession of culture through stages of overall progress,’ which is general evolution.” But there appears to be no reason why all societies should reach certain stages of differentiation or why they should necessarily develop the same types of institutional contours once they attain such stages. The most that can be claimed at present is that the processes of differentiation in different societies exhibit similar formal and structural characteristics and that these create somewhat similar integrative problems.

It is in these common characteristics and problems that the fact of the common humanity of all human societies, as well as the possibilities of some common understanding and of intersocietal borrowing and transfer of institutions, is rooted; moreover, these characteristics indicate the existence of some “evolutionary universals” in the development of different human societies (Parsons 1964). However, the variety of possible “functional equivalents” of institutionalized solutions to such problems, as well as the possibilities of “regression,” stress the fact that the paths of development of different societies are neither necessarily common nor given. In other words, there is no reason to assume that there is a necessary relation and congruity between the mechanisms of genetic (here cultural or social) “transmission and change and the route of development of this or that organism or species” (Gellner 1965, p. 17).

This discussion is closely related to the fourth, very crucial problem involved in the reappraisal of evolutionary perspectives: the explanation of the variability of institutionalized solutions to the problems arising from the development of a given level of structural differentiation. Here it should be recognized that the conditions giving rise to structural differentiation and to “structural sensitivity” to a greater range of problems do not necessarily create the capacity to solve these problems.

Creative entrepreneurial elites. The crucial factor is the presence or absence, in one or several institutional spheres, of an active group of special “entrepreneurs”-that is, an elite that is able to offer solutions to the new range of problems. Among modern sociologists Weber came closest to recognizing this factor when he stressed that the creation of new institutional structures depends heavily on the “push” given by various “charismatic” groups or personalities and that the routinization of charisma is critical for the crystallization and continuation of new institutional structures (Weber [1922b] 1963, chapters 4, 10, and 11). The development of such “charismatic” personalities or groups constitutes perhaps the closest social analogy to genetic mutation. It is the possibility of such mutation that explains why, at any level of differentiation, a given social sphere contains not one but several, often competing, possible orientations and potentialities for development.

As yet, we know little about the specific conditions (as distinct from the more general trend toward structural differentiation) that facilitate the rise of new elites–that is, the conditions which influence the nature of their basic orientations as well as their relations with broader groups, strata, and trends of development, and their ability to forge out and maintain a viable institutional order. There are indications, however, that factors beyond the general trend toward differentiation are important. For example, various special enclaves, such as sects, monasteries, and sectarian intellectual groups or scientific communities, play an important role in the formation of such elites. Furthermore, a number of recent studies (see, for instance, McClelland 1961; Hagen 1962) have indicated the importance of certain familial, ideological, and educational orientations and institutions.

Within this context, it is necessary to re-examine the whole problem of the extent to which institutional patterns are crystallized through diffusion from other societies rather than through independent invention within a society. Cases of diffusion might be partially due to successful importation, by entrepreneurial groups on the margins of a given society, of acceptable solutions to latent problems or needs within that society.

Intersocietal borrowing. The problems of the interaction between processes of change and “imitative” elites are closely related to a set of problems bearing on the intersocietal nature of evolution. We have seen that the international setting not only constitutes the environment to which any single society has to adapt itself but also provides a reservoir of responses that may be available to it; for instance, the setting may provide enclaves from which new elites or adaptive techniques and organizations can be borrowed.

The existence of such interrelationships–in terms of both common problems and the possibility of “borrowing” solutions–necessarily underlies the basic mutual resemblance of human societies, and it is in turn closely related to the problem of the extent to which it is possible to talk about general “social” evolution or the evolution of human society as a total entity. However, the existence of such mutual resemblance certainly does not ensure that the development of “human society” as a unified system with common adaptive mechanisms necessarily increases its ability to deal with over-all problems of adaptation. Paradoxically, the very interrelatedness of societies may create problems with which they may not be able to deal. What may seem to be a positive accumulation of available mechanisms and a repertoire of adaptations from the point of view of human society as a whole may yet, because of the lack of intersocietal inte-grative and adaptive mechanisms, constitute a very grave problem.

Limitations of evolutionary theory

The considerations presented above constitute the background for a reappraisal of the evolutionary perspective within the framework of recent sociological theory. An evolutionary perspective, from the point of view of human societies, makes sense only so far as at least some of the processes of change that are inherent in any social system create the potentialities for the institutionalization of more differentiated social and symbolic systems. From the point of view of human society or culture as a whole, such a perspective makes sense only insofar as there exist some mechanisms for the transmission of various institutional and adaptive techniques and for creating some common, intersocietal, adaptive and integrative capabilities and frameworks.

With regard to all these areas, several problems for which there exist as yet no adequate solutions have been pointed out above. They have, in a way, all focused on the interaction between processes of social differentiation, on the one hand, and the formation and activities of different elites, on the other. It is this interaction that makes possible the institutionalization of different integrative principles and concrete structures at a given level of societal differentiation. Any search for solutions to these problems must concentrate on the various aspects of these processes. In this endeavor broad evolutionary considerations indicate ranges of possibilities and types of potential breakthroughs but do not in themselves provide answers.

SHMUEL N. EISENSTADT

[Directly related are the entries on EMPIRES; FEUDALISM; SOCIAL INSTITUTIONS. Other relevant material may be found in ANTHROPOLOGY; DIFFUSION.]

BIBLIOGRAPHY

BELLAH, ROBERT N. 1964 Religious Evolution. American Sociological Review 29:358–374.

BOCK, KENNETH E. 1963 Evolution, Function and Change. American Sociological Review 28:229–237.

CARNEIRO, ROBERT L. 1962 Scale Analysis as an Instrument for the Study of Cultural Evolution. Southwestern Journal of Anthropology 18, no. 2:149–169.

EGGAN, FRED 1963 Cultural Drift and Social Change. Current Anthropology 4:347–355.

EISENSTADT, SHMUEL N. 1963a The Political Systems of Empires. New York: Free Press. → Contains an extended bibliography.

EISENSTADT, SHMUEL N. 1963b Modernization: Growth and Diversity. Bloomington: Indiana Univ., Department of Government.

EISENSTADT, SHMUEL N. 1964a Social Change, Differentiation and Evolution. American Sociological Review 29:375–386.

EISENSTADT, SHMUEL N. 1964b Institutionalization and Change. American Sociological Review 29:235–247.

GELLNER, ERNEST 1965 Thought and Change. London: Weidenfeld & Nicolson.

GINSBERG, MORRIS (1932) 1961 The Concept of Evolution in Sociology. Volume 1, pages 180–199 in Morris Ginsberg, Essays in Sociology and Social Philosophy. London: Heinemann.

GOODENOUGH, WARD H. 1963 Some Applications of Guttman Scale Analysis to Ethnography and Culture Theory. Southwestern Journal of Anthropology 19: 235–250.

HAGEN, EVERETT E. 1962 On the Theory of Social Change. Homewood, Ill.: Dorsey.

MCCLELLAND, DAVID C. 1961 The Achieving Society. Princeton, N.J.: Van Nostrand.

MEAD, MARGARET 1964 Continuities in Cultural Evolution. New Haven: Yale Univ. Press.

PARSONS, TALCOTT 1964 Evolutionary Universals in Society. American Sociological Review 29:339–357.

SAHLINS, MARSHALL D. 1964 Culture and Environment: The Study of Cultural Ecology. Pages 132–147 in Sol Tax (editor), Horizons of Anthropology. Chicago: Aldine.

SAHLINS, MARSHALL D.; and SERVICE, ELMAN R. (editors) 1960 Evolution and Culture. Ann Arbor: Univ. of Michigan Press.

SCHWARTZ, RICHARD D.; and MILLER, JAMES C. 1964 Legal Evolution and Social Complexity. American Journal of Sociology 70:159–169.

WEBER, MAX (1922a) 1957 The Theory of Social and Economic Organization. Edited by Talcott Parsons. Glencoe, Ill.: Free Press. → First published as Part 1 of Wirtschaft und Gesellschaft.

>WEBER, MAX (1922b) 1963 The Sociology of Religion. Boston: Beacon. → First published in German.

WHITE, LESLIE A. 1959 The Evolution of Culture: The Development of Civilization to the Fall of Rome. New York: McGraw-Hill.

WOLF, ERIC R. 1964 The Study of Evolution. Pages 108–119 in Sol Tax (editor), Horizons of Anthropology. Chicago: Aldine.

VI EVOLUTION AND BEHAVIOR

The fundamental postulate of the modern biological theory of evolution is that the guiding agency of evolutionary changes is adaptation to the environments that a species inhabits. This is equally true of changes in structural features, physiology, and behavior of organisms. Evolutionary developments maintain the adaptedness of the species when the environments change, or they improve the adaptedness if the environments are more or less stationary. The environment does not, however, impose changes on the organism, as was believed by some early evolutionists, particularly by the adherents of the now almost completely abandoned Lamarckian hypothesis. It is more accurate to say that the environment presents challenges, to which a living species may respond by adaptive modification of its genetic endowment (genotype). If a response is elicited, the adaptedness is preserved or improved; if the species fails to respond, its fitness declines and it may become extinct. The genetic raw materials from which evolutionary changes may be constructed are mutations, that is, alterations in the gene or chromosome structures. The effects that mutations produce vary in magnitude all the way from alterations so drastic that the mutant is inviable (lethal, as a fatal hereditary disease causing death before sexual maturity) to changes so slight that the change in fitness, if any, that they produce can be detected only by means of refined statistical study. Mutant genes may be favorable only in heterozygous carriers (hybrid vigor, or heterosis) but unfavorable in double dose (in homozygous condition). Or a genetic change may be favorable in some environments and unfavorable in others. Or, finally, a mutant gene may be favorable in combinations with some genes but unfavorable in combination with others.

The great evolutionary importance of sexual reproduction lies in that it constantly combines and recombines the various genes present in the species population, enabling the favorable gene combinations to arise and to be tested by natural selection. In sexually reproducing and outbreeding species, such as man, no two individuals (except identical twins) have the same genotype. We inherit our genes from our parents and pass them to our children, but the gene constellation, the genotype, of every individual is unique. By and large, the greater the change a mutation produces, the greater the chance it will be harmful to the organism.

The adaptive evolutionary changes are compounded almost exclusively of the slight mutational changes (sometimes termed polygenic changes). Changes of greater magnitudes are important rather as the source of the genetic pathology, incapacitation, or weakness. Such diseases as phenyl-ketonuria, with its associated mental defects, and schizophrenia, to name only two, are examples of deleterious mutants affecting behavior. Apart from their negative importance for public health, mutations that produce strikingly visible alterations are also important as materials for genetic studies. Most of the pioneering work in genetics was done with genetic variants that must be classed as more or less pathological deviants and that play only negative roles in evolution, contributing to the “genetic load” the population carries.

Natural selection. Mutational changes, those affecting behavior as well as those responsible for structural and physiological alterations, are mostly harmful to the organism. This is the reason why any increase of the frequency of mutations in human populations (for example, through exposure to X rays, other mutagenic radiations, or chemical mutagens) can result only in a reduction of the average fitness of the populations concerned. How, then, can mutations serve as building blocks for adaptive evolutionary changes? The answer is that mutational changes and their combinations are sorted out by natural selection. The majority of mutations that decrease the fitness of their carriers in all environments and in all combinations are cast out of the populations by natural selection; those that are useful in at least some environments and in some combinations are preserved and multiplied.

The action of natural selection is sometimes compared to that of a sieve, separating the useful genetic variants from the harmful ones. This analogy is misleading if it is taken too literally. Human populations, and those of most sexually reproducing and outbreeding species, always carry great stores of genetic variants which arose by mutation in the immediate or remote past. The genetic reassortments, combined with natural selection in changing environments, become a cybernetic process in which the genetic developments that occur at a given time depend upon the changes that have taken place earlier, and, in turn, they condition the developments that may take place in the future.

Mutation, the process that supplies the raw materials from which evolutionary changes can be constructed, is repeatable and reversible; it is a physiological and, in the last analysis, a mechanical process. But the evolution controlled by natural selection becomes a creative process that is unlikely to be reversed or to be repeated. Evolutionary transformations, such as the transformations that have led to the emergence of the human species, are chains of unique, nonrecurrent events.

Importance of environment. Another consideration, particularly relevant in relation to the evolution of behavior, is that heredity determines not fixed “characters” or “traits” but reactions of the developing organism to the environment. The trait “behavior” obviously cannot be transmitted in inheritance, because it is not present in the sex cells, which are the only material bridge connecting the parents with their progeny. But neither is the skin color inherited in this sense, because no skin pigment is present in the sex cells. What the sex cells do carry are genes, and the genes determine the pattern, or path, that the development of an individual will follow in a given sequence of environments. Another individual with similar genes, an identical twin, may develop differently if his environments are different. A carrier of a different set of genes might also develop differently in similar environments.

We observe that human beings vary with respect to a great many traits–skin color, height, weight, head shape, intelligence, temperament, and special abilities, among countless others. As a broad generalization, it is fair to say that whenever the variation in any trait has been adequately studied genetically, it has been found to be influenced by both genetic and environmental factors. In the past, investigators have often tried to determine which traits are genetic or hereditary and which are environmental; such a dichotomy is now recognized as naive and misleading. All traits, or at any rate a great majority of them, are both genetic and environmental. If everybody had the same genes, as identical twins do, people would look and behave more nearly alike than they actually do. Likewise, if the environments in which people grow and develop were made uniform, this would also result in a reduction of the observed structural and behavioral diversity.

Fixity and conditioned plasticity. No living species inhabits an absolutely uniform and constant environment. Organisms have to face many environments, variable both in space and in time. For example, the inhabitants of the temperate and cold climates must survive in both summer and winter environments. Man achieves more and more effective control over his physical environments, but he must face a great and growing diversity of sociocultural environments. There are two ways to become adapted to a diversity of environments, and both have actually been used in the evolutionary process, including human evolution. One is genetic fixity and genetic specialization; the other is genetically conditioned developmental plasticity. In general, genetic fixity is characteristic of traits whose presence and precise form are indispensable for survival and reproduction. The developmental processes giving rise to such traits are said to be homeostatically buffered, so that they can occur in all environments that the species normally encounters in its habitats. Thus, with very few exceptions, all infants are born with two eyes, a four-chambered heart, physiological systems which digest food and maintain a constant body temperature, ability to learn a symbolic human language, etc. Genetic specialization makes the species polymorphic (consisting of two or more genetically distinct forms living and interbreeding in the same territory) or polytypic (consisting of races that inhabit different territories and are genetically adapted to the environments of their respective territories). For example, the darker and the lighter skin pigmentation of some human races is plausibly supposed to fit them to the climatic conditions of the lands in which they originally lived.

Genetically conditioned developmental plasticity is advantageous when the organism profits by having some traits shaped differently in the different environments that it encounters. The tanning of human skin on exposure to sunlight is an example of such a plasticity. It is important to realize that both fixity and plasticity are genetically determined. Natural selection favored the spread and establishment of mutant genes which in some populations make the skin permanently darkly pigmented and in other races make the pigmentation contingent on sun exposure.

Culture. The most significant product, and the paramount determining factor, of human evolution is culture. The relationships between the biological evolution and culture are frequently misunderstood, and it is important to make them clear. Culture is not transmitted biologically through some special genes; it is acquired anew in every generation by learning and instruction, in large part through the medium of the symbolic language. However, the capacity to learn and to instruct and, most essential of all, the capacity to use the symbolic language, is biologically and genetically vouchsafed to every nonpathological human being. An individual whose genes deprive him of these capacities is an obvious misfit, and his genes are likely to be eliminated by natural selection. Conversely, it is safe to assume that the genetic equipment that made the human species capable of developing and maintaining culture has been compounded by natural selection in the course of the prehuman, subhuman, and human evolution.

Of the many existing forms of human culture, the particular one an individual acquires is determined by the society in which this individual is brought up, rather than by his genes. And yet, not only the ability to acquire any culture at all but also the capacity and inclination to choose this or that occupation, role, or trade within a culture may well be genetically conditioned, facilitated, or hindered. The ability to speak is genetically determined, but what a person will actually say is largely independent of genetics.

Human acquired, extrabiological culture is man’s most potent adaptive instrument; it is chiefly by brain, not by brawn, that man controls his environments. Since, however, human environments are preponderantly created by culture, the possession of a genetic endowment that enables members of human populations to adapt themselves to these cultural environments becomes overwhelmingly important. The evolution of the biological basis of human behavior has been controlled by this fact. In all cultures, primitive as well as advanced, the vital ability is, and always was, for every individual to be able to learn whatever is necessary to become a competent member of the culture of which the individual is a part. For this reason, natural selection has favored in human evolution a remarkable plasticity of the behavioral development; man’s cardinal adaptive trait is his educability, that is, his capacity to adjust his behavior to circumstances in the light of experience.

Most individuals can be trained, with a greater or lesser facility, for many or most of the occupations and roles that a given culture requires to be filled. Almost everybody could become, if properly brought up, a fairly competent farmer, craftsman, soldier, sailor, teacher, or priest. This is the valid premise of the tabula rasa theory, from which this theory draws an erroneous conclusion. First clearly stated by John Locke in 1690, this theory is still popular in many circles. It asserts that a human being at birth is a clean slate on which the environment will inscribe a collection of attributes and qualities. The genetically secured developmental plasticity of human behavior, however, is not at all incompatible with genetic diversity. It is eminently probable that an infant at birth is not a clean slate and that some individuals are, because of their genetic endowments, more easily trainable for some occupations than for others. This is certain for some specialized professions; by no means does everybody have the genetic wherewithal to become a fine singer or a first-class composer or performer of music, or to achieve peak performance in sports or in art.

Biologically, this makes sense; the development of cultures and civilizations has not caused the diversity of vocations to become smaller; on the contrary, this diversity increases by leaps and bounds. The biologically adaptive response to this situation is obviously a combination of an educability or trainability, with an underlying genetic diversity to facilitate the division of labor.

The fallacy of racism. While the variants of the tabula rasa theory would make us believe that all the observed differences, especially all the differences in behavior, between people are the products of upbringing and education, the even more pernicious fallacy of racism would claim that these differences are genetically fixed and largely independent of the environment. One superficially plausible argument often given in favor of racist views is worth discussing here, because it will enable us to bring out clearly an important feature of the evolution of human behavior. It is claimed that the variation in psychic or behavioral traits among human individuals and races must be genetically fixed to about the same extent as it is among breeds of domestic animals. Different breeds of dogs, horses, or cattle are indeed clearly different in behavior, temperament, disposition, intelligence, trainability, etc. These differences are very largely genetically fixed, although by careful training and discipline one can modify them to some extent. Why then, it is argued, should the differences between humans be supposed to be anything but genetic?

This argument overlooks a profound dissimilarity between the evolutionary histories of the human species and of the animals that man has domesticated. The behavior of a breed of a domestic animal is an essential part of the complex of characteristics that are selected to fit this breed for its intended use. A work horse should not behave like a race horse, because this would make it dangerous and inefficient, and a race horse should not behave like a work horse if it is to win any races. The laboratory mouse and the laboratory rat are sluggish, unaggressive, and apparently dimwitted compared to the wild mouse and the wild rat. They would hardly survive under the conditions in which their wild ancestors thrive; it has been claimed that these species have degenerated when man has furnished their food and shelter. It is, however, obvious that wild mice and wild rats are inconvenient as laboratory animals, and that is why they are seldom used in laboratories.

What would be rated as degenerate in the wild state is a desirable trait in an animal living in a laboratory cage. Man has seen to it that the genes for fixing and stabilizing desirable behavior are established and that the genes for undesirable behavior are bred out of the animals he has domesticated. Although some writers have seen fit to call man a “self-domesticated” animal (a designation accepted by few biologists), his evolutionary pattern is in many ways just the reverse. As previously stated, natural selection may favor mutant genes that confer a developmental fixity on some traits and developmental plasticity on others; the latter is the case with human behavior. A person who is able to learn whatever modes of behavior fit various professions and vocations available in a human society, and to adjust himself to the ways of life that go with these professions and vocations, is likely to have both a social and a biological advantage. An individual set in his ways, always aggressive or always yielding, unable and unwilling to learn or to be trained, is likely to be discriminated against by natural selection. The great developmental plasticity of psychic traits in man is, thus, no biological accident but, on the contrary, a fundamental evolutionary adaptation that distinguishes man from nonhuman animals.

Ethics. A considerable amount of speculation has been devoted to the problem of whether human ethics could have arisen through the action of natural selection in the evolutionary process. Natural selection is obviously not a benevolent spirit guiding the evolution but a blind and opportunistic process. It is opportunistic in the sense that it promotes the establishment of genes which confer an advantage for survival or reproduction when and where the selection acts, regardless of whether these same genes may be disadvantageous later on. The extinction of countless species of organisms has been due to such a narrow, overspecialized adaptation to the environments that did not endure. Now, it is conceivable that natural selection might encourage genes for altruistic behavior in a species broken up into numerous small colonies or sub-populations. “Altruistic” is in this case to be defined as a behavior benefiting the group (family, clan, tribe) to which the individual so behaving belongs and detrimental to that individual himself. A small population in which genes for such behavior occur may prosper and multiply, despite some of the carriers of these genes sacrificing themselves for the sake of their fellows and thus not transmitting their genes to their own progeny. Conversely, in a large, undivided population, genes for “egotistic” or “criminal” behavior may secure an advantage, if their carriers survive and leave progeny at the expense of other members of the population.

Another possibility is that altruism and egotism are not products of some kind of special genes but, rather, products of cultural developments transmitted not by genes but by learning. C. H. Waddington (1960) has argued that natural selection acts to make man an “ethicizing being” and an “authority acceptor.” Particularly in childhood but also during his entire life, a person is able and even eager to acquire, from his parents or from other persons, ideas about what is good and what is evil and to accept instruction or counsel concerning the desirable ways of living in a society with other human beings. According to this view, man is not born virtuous or vicious but with a capacity for both virtue and vice. Biological evolution does not make man ethically better or worse, but it does promote intellectual alacrity and perhaps a sensitivity to ethical issues.

Future developments. Even more speculative and uncertain are the attempts to prognosticate the future evolutionary developments of human behavioral traits and capacities. This is evidently a part of a more general problem of the evolutionary perspectives of the human species. An opinion often expressed is that the biological evolution of man has virtually completed its course, and from now on any further development will be in the cultural realm. This is true to the extent that cultural changes are more rapid than the genetic ones, and this is, in fact, the reason why the development of the capacity for culture has conferred upon the human species an unprecedentedly high biological fitness. It should, however, be kept in mind that the maintenance, not to speak of further expansion, of cultural capacities is possible only on the basis of sound human genetic endowments. Improvement, maintenance, and prevention of deterioration of these genetic endowments is the task of the applied science of eugenics. Many eugenists have been extremely pessimistic about the genetic future of mankind, believing that the genetic processes which go on in human populations trend inexorably toward biological disaster. Others have urged various remedial schemes, such as sterilization of the unfit, or the artificial insemination of women by semen collected from biologically superior donors and preserved in frozen condition for extensive use over the years. During the first third of the twentieth century, eugenics was often used as a support of ultraconservative social philosophies and racist doctrines. All this has made many social scientists, and the public at large, properly skeptical and suspicious of eugenic schemes. Yet eugenics undoubtedly has a sound core; sooner or later man will be forced to take the management of his evolution in his own hands.

THEODOSIUS DOBZHANSKY

[Other relevant material may be found in EUGENICS; GENETICS; PSYCHOLOGY, article on COMPARATIVE PSYCHOLOGY; and in the biography of DARWIN.]

BIBLIOGRAPHY

DOBZHANSKY, THEODOSIUS 1962 Mankind Evolving: The Evolution of the Human Species. New Haven: Yale Univ. Press.

DOBZHANSKY, THEODOSIUS 1964 Heredity and the Nature of Man. New York: Harcourt.

FULLER, JOHN L.; and THOMPSON, W. ROBERT 1960 Behavior Genetics. New York: Wiley.

HALLER, MARK H. 1963 Eugenics: Hereditarian Attitudes in American Thought. New Brunswick, N.J.: Rutgers Univ. Press.

HIRSCH, JERRY 1962 Individual Differences in Behavior and Their Genetic Basis. Pages 3–23 in Eugene L. Bliss (editor), Roots of Behavior: Genetics, Instinct, and Socialization in Animal Behavior. New York: Harper.

ROE, ANNE; and SIMPSON, GEORGE G. (editors) 1958 Behavior and Evolution. New Haven: Yale Univ. Press.

WADDINGTON, CONRAD H. 1960 The Ethical Animal. London: Allen & Unwin.

International Encyclopedia of the Social Sciences

Evolution

views updated May 11 2018

EVOLUTION.

Although it can encompass cosmic and cultural change, evolution is a term usually associated with the modern scientific theory of species change and is most closely associated with the work of Charles Darwin (1809–1882) and, to a lesser extent, Alfred Russel Wallace (1823–1913). Darwin himself did not refer to his specific theory as "evolution" but instead used the phrase "descent with modification." Only the very last word of his famous work laying out the argument for his theory, On the Origin of Species, which appeared in 1859, was "evolved." The term gained widespread currency especially in the English language and came to be virtually synonymous with Darwin and Darwinism because of its use by contemporaries like the social theorist Herbert Spencer (1820–1903) and by the numerous commentators, advocates, and translators, such as Thomas Henry Huxley (1825–1895) and Ernst Haeckel (1834–1919), who were carrying meanings into the theory of species change from areas of biology concerned with developmental processes like embryology.

Before then, the term evolution had been used in a number of contexts. Stemming from the Latin verb evolvere, the term generally refers to an unrolling or unfolding. The substantive form evolutio refers to the unrolling of a scroll. Implied in these meanings is the fact that something is there to unfold, develop, or unroll. Its scientific use was first noted in mathematics in the sixteenth century, but it was soon applied to the development or unfolding of ideas or principles. In the seventeenth century the term began to take on a biological cast when it was first used by an anonymous English reviewer to characterize the embryological theories set forth by Jan Swammerdam (1637–1680). A preformationist, Swammerdam postulated a theory of insect development that relied on preexisting or preformed parts that expanded and grew in the embryonic larva. The semen of the male was required in this process, but only as a stimulus to realize the development of the adult form already encased in the semen of the female. The term evolution was thus coined initially to describe a developmental or embryological process of unfolding resulting in the reproduction of an adult form. Its application to the process of species change took place gradually in fits and starts over the next 150 years or so by a broad range of thinkers who increasingly carried over meanings of developmental or embryological unfolding in reproduction to theories of species change within historical, temporal, or geographical frame of references.

The extent to which the German Naturphilosophen ("nature philosophers") or German Romantic thinkers like Friedrich Wilhelm Joseph von Schelling (1775–1854) may have held evolutionary theories and the extent to which such views of dynamische Evolution ("dynamic evolution") shaped or even resembled subsequent evolutionary theories leading eventually to Darwinian evolution is the subject of lively discussion and controversy among historians of evolution of the eighteenth and early nineteenth centuries. The received view of the history of evolutionary thought does not usually locate the origins of Darwinian evolution in the German philosophical context, or in movements like Romanticism, but has instead located its intellectual origins in the context of Enlightenment views that included belief in progress, in theological movements like natural theology, and in the shifting views and practices of traditional natural history that led to reforms in taxonomic practices and in emerging related "sciences" like geology. Its social origins are generally linked to late-eighteenth-century economic theories that articulated laissez-faire policies, to the rise of industrial capitalist societies and states, and to the increasing linkage between natural history (and indeed science as whole) to colonialism and to imperialist ambitions, especially in Great Britain. In the received view, the history of evolution long predates Darwinian developments, though Darwin and his theory are given exceptional emphasis.

Evolution as a Theory of Species Change

The belief in a changing or dynamic universe can be first seen in ancient Greek philosophy. Heracleitis (c. 540–c. 480 b.c.e.), also known as the "flux philosopher," believed that change was a fundamental property of the universe. His successor, Empedocles (c. 490–430 b.c.e.), first articulated a crude but dynamic theory that postulated that the origin of life had taken place in a manner that suggested evolution. With the philosophical worldview established by Aristotle (384–322 b.c.e.), the belief in a changing universe fell into disfavor. Aristotle and his numerous medieval and Renaissance translators, commentators, and supporters instead believed in a static universe which held that living organisms were created initially by a designer (the Demiurge of Plato's Timaeus or the biblical Creator) and then remained essentially unchanged. These ideal types or species were arranged hierarchically in what came to be known as the scala naturae, or the ladder of creation. Like the rungs of a ladder, each species was arranged hierarchically, with lower forms of life on the bottom and higher forms of life on the top. During the Renaissance, the ladder of creation gave way to the popular metaphor of the "great chain of being," which referred to a progression of living forms linked in an orderly chainlike arrangement. Extinction, the sudden disappearance of a species, in such a scheme was unthinkable since it meant that the chain would lose a vital link. Belief in the fixity of species and in the species characterized by an ideal type therefore dominated thinking about living entities and was most clearly demonstrated in the modern classification scheme that originated with Carolus Linnaeus (1707–1778).

Belief in species change, or more precisely what was termed "transmutationism," slowly began to emerge during the Enlightenment. One reason for this was the recognition that the earth was of greater antiquity than previously thought and that fossils, long held to be curiosities of nature that adorned the shelves of Renaissance collectors were in fact the remnants of once living organisms. The organic origin of fossils had been suggested earlier by Nicholas Steno (1638–1686) and others who were concerned with them.

Another reason was that the Enlightenment also saw the emergence of the belief in a progressive world, both scientific and social, at the same time that it was slowly realized that the earth itself had a long and tumultuous history of its own. The closing of the eighteenth century saw the beginnings of attempts to understand the history of the earth in terms of natural causes and processes. These geological theories suggested that fossils were of organic origin and that uniform or constant processes rather than catastrophic or one-time events had shaped the Earth's history. In the eighteenth century two rival schools of thought existed: the first, known as the catastrophists, upheld the belief in the uniqueness of geological events, while the second upheld the belief that geological processes were not unique or catastrophic but instead were part of a uniform and largely gradual process of natural change. The latter school was associated with a "uniformitarian" theory of geological change and its advocates known as uniformitarians.

The French naturalist Georges-Louis Leclerc, comte de Buffon (1707–1788), was one of the first to embrace a uniformitarian philosophy, to question the fixity of species, and to suggest a transmutationist theory for species change. Although he was a respected naturalist, writing a forty-four-volume treatise on the natural history of the world known as L'historie naturelle (Natural history), his theoretical explanations for the origin of life and of species change were not accepted during his time; he provided no cogent mechanism for such changes. Buffon's transmutationist ideas were also not accepted because they were undermined by the philosophical teachings of his successor, Georges Cuvier (1769–1832), an anti-uniformitarian who thought successive "revolutions" or catastrophes had shaped the pattern of diversity of life on earth. Cuvier was a pioneer of comparative anatomy and is generally regarded as the father of modern vertebrate paleontology. He upheld the fixity of species despite fossil evidence of species change. Ironically, although he opposed transmutationism strongly, Cuvier was the first to recognize the phenomenon of extinction, or the view that species had disappeared from the biological record. His system of classification placed living organisms into four distinct groupings or what he termed embranchements: the Vertebrata, Articulata (arthropods and segmented worms), Mollusca, and Radiata (echinoderms and cnidarians). The four "branches" were distinct from one another and could not share any evolutionary transformation. If any similarities existed, this was due to shared functional circumstances and not to any common ancestry. Cuvier's influence in zoology in particular and in French science generally was enormous and played a role in discrediting efforts to formulate transmutationist theories.

The first to suggest a viable theory of transmutation was the Frenchman Jean-Baptiste de Lamarck (1744–1829), a contemporary of Cuvier's who faced notable opposition from him. First an expert on botany, Lamarck was given the lowly task of organizing the invertebrate collections at the Musée National d'Histoire Naturelle (National Museum of Natural History). In the process of working with the little-known group (Lamarck coined the term invertebrate ), he began to note progressive trends in the group. He became particularly interested in adaptation, or the manner and process by which organisms are able to adapt physiologically and morphologically to their environment, and he was especially interested in how well-adapted organs originated. His most celebrated example was the modification of the neck of the giraffe, which became elongated in response to stretching during feeding on the leaves of trees on the African plains. This and other examples were explored in works such as Philosophie zoologique (Zoological philosophy), published in 1809. According to Lamarck, the use, or in many cases the disuse, of such vital organs could lead to the origin of novel but well-adapted traits; the cumulative effect of these adaptations could eventually lead to new species. Lamarck never provided a cogent mechanism by which this physical transformation took place, however, though he did draw on contemporary theories from animal physiology to suggest that the body heat generated by physical exercise could lead to such structural transformation. Sometimes called "the inheritance of acquired characters," Lamarckian transmutationism, also later called Lamarckian evolution or "Lamarckism," was subsequently shown to be erroneous because changes acquired as a result of use and disuse were shown to be not heritable. The German experimental biologist Auguste Weismann (1834–1914) is generally credited with disproving Lamarckian inheritance through a number of experiments that included cutting off the tails of hundreds of mice, and through his famous theory that first made the distinction between germplasm (cells that passed on hereditary information) and somatic or bodily cells. The "Weismann Barrier," which eventually became one of the central dogmas of modern biology, postulated that hereditary information moves only from the genes to the somatic cells and not vice versa.

Lamarck's ideas were, however, very popular throughout much of the nineteenth century, especially among naturalists interested in adaptation, and continued to gain support in some communities well into the twentieth century, sometimes being associated with "neo-Lamarckian" theories of species change. Darwin himself relied heavily on the inheritance of acquired characters to explain many adaptations that he later outlined in laying out his own transmutationist theory as it finally appeared in 1859.

Transmutationism itself became increasingly acceptable in the early decades of the nineteenth century. It captured the interest of Darwin's own grandfather, Erasmus Darwin (1731–1802), who suggested that life had originated from "one living filament" in his two-volume work Zoonomia (1794–1796). Other transmutationists included the French anatomist Étienne Geoffroy Sainte Hilaire (1805–1861), who studied teratology, or the science of birth defects. He suggested that through such "monstrous births" new species might arise in a sudden or rapid process, a theory later challenged by modern genetics.

In the nineteenth century a series of scholars began to uphold not just transmutationist doctrines but theories suggestive of what eventually would become known as Darwinian natural selection. In 1813 William Wells delivered a paper to the Royal Society with the title "An Account of a Female in the White Race of Mankind." Wells suggested that new human races originated when groups moved into new territories where they encountered new conditions of life. In the process of adaptation to these new conditions, newer improved races of humans would emerge. In 1831 Patrick Matthew came even closer to formulating a view of natural selection in the appendix to an obscure treatise, On Naval Timber and Arboriculture. In this account Matthew invoked the extinction of species by catastrophic events, after which the survivors would diversify into new, better-adapted species that would remain stable for long periods of time. In 1835 yet another scholar, Edward Blyth (1810–1873), in a paper titled "An Attempt to Classify the Varieties of Animals," suggested a competitive process echoing natural selection whereby the elimination of the unfit groups would take place.

In 1844 the work of one transmutationist in particular drew the attention of wide Victorian audiences. Writing anonymously at first, Robert Chambers (1802–1871) outlined a transmutationist theory under the title Vestiges of the NaturalHistory of Creation. The book became an instant sensation for its many readers, who were greatly entertained by the provocative—and indeed some thought scandalous—account of the origins of the solar system and of the origins of humanity, which postulated evolution from the apes. Though it was widely read and discussed, it received devastating criticism from scientists; this was so much the case that Charles Darwin, witnessing the controversy precipitated by Vestiges, is thought to have been dissuaded from publishing his own transmutationist views for nearly fifteen years.

Charles Darwin and Descent with Modification
by Means of Natural Selection

Charles Darwin was the leading transmutationist of the nineteenth century. The grandson of Erasmus Darwin, and the son of a fairly successful country physician, Robert Waring Darwin, Charles was born into an educated and affluent English family that fostered his interest in natural history. His mother, Susanna Wedgwood, was the daughter of the famed industrialist and potter Josiah Wedgwood (1730–1795) and heir to a family fortune. Charles was the youngest of two sons and had three sisters, who doted on him after the premature death of his mother when Charles was only eight. His scholastic achievements were less than stellar, though he early on developed a passion for natural history. He first made an attempt to study medicine at Edinburgh University but gave up after witnessing an operation on a young child without the aid of anesthesia. Although his formal medical studies disappointed him, Darwin did enjoy interactions with local experts in natural history. In particular he fell under the influence of an ardent transmutationist, Robert E. Grant (1793–1874), a keen student of marine invertebrates who encouraged Darwin's interest in natural history and also encouraged him to consider the leading transmutationist theories of the day. Under Grant's tutelage at Edinburgh, Darwin was exposed seriously to the scientific theories of Lamarck and to the insights of his own grandfather Erasmus, which he entertained but did not enthusiastically accept, at least at that time. He also made some of his first scientific observations using a microscope on the mode of fertilization of the marine polyp Flustra.

His second attempt at formal education shortly thereafter in theology at Cambridge University exposed him to the popular views of natural theology, with its focus on adaptation and design, especially the work of William Paley (1735–1805) and his Natural Theology (1802). Darwin's only formal scientific training had been in geology under the tutelage of Adam Sedgwick (1785–1873) when John Stevens Henslow (1796–1861), the professor of botany and his mentor, recommended him to the Admiralty for a geographic expedition to chart the coast of South America. Under the command of Captain Robert FitzRoy (1805–1865), the HMS Beagle set sail in 1831, with Darwin on board serving as gentleman-companion to the captain and increasingly playing the role of ship's naturalist. The five-year voyage, which charted the coastline of South America and then continued to the Galapagos Islands, Tahiti, New Zealand, and Australia, exposed Darwin to the variation and distribution of living organisms in both continental and island environments. Darwin collected extensive specimens of flora and fauna and made notable observations of the geological history of the locales he visited. Frequently referring to Charles Lyell's (1797–1875) recently published Principles of Geology (1830–1833), which was part of the personal library that he had taken with him, Darwin sought to understand the geographic distribution of plants and animals in terms of a uniformitarian geology. He was especially struck by the manner in which related forms appeared to replace each other as one traveled up and down the coast of eastern South America, by the resemblance of extinct fossil forms to extant life, by the similarity between island species in the Galapagos to nearby continental areas like western South America, and by the differences displayed between those island species on the Galapagos. All of these patterns suggested that some natural and gradual process that involved migration and adaptation to local environments had taken effect, rather than some act of unique or special creation.

Darwin increasingly sought a general explanation for his observations of the natural world during the five-year voyage, and after returning to England he devoted himself to this end by revising his journals, reexamining his specimens, consulting with noted experts, and reading extensively in the scientific literature available to him. Between 1837 and 1838—generally regarded as the crucial years in the formulation of his famous theory—Darwin read the Essay on the Principle of Population by Thomas Robert Malthus (1766–1834). More than any other work, the essay provided Darwin with the intellectual backdrop for his theory by suggesting that competition for natural resources was a fact of life and that populations remained stable as a result of processes that included checks and balances. In this competitive world where there was a struggle for existence, those organisms with the most favorable characteristics would be favored to survive and reproduce themselves. Given enough time, those with favorable traits and characters would diverge from the ancestral forms to give rise to new species. The new elements to transmutationism that Darwin introduced thus included the struggle for existence, but also the fact that heritable variation that was favorable and that conferred an advantage would be likely preserved in the process of reproduction.

Darwin recorded the development of his ideas at this time in a series of notebooks that reveal his attempts to understand the branching process for the origin of species, which had "descended" from some common ancestor. Although he had the major features of his theory at this time, Darwin did not make his work public until much later. There is much speculation in the scholarly literature about the delay in publishing his theory, but there is general agreement that Darwin spent the next interval of his life collecting evidence in support of what he knew would be a contentious theory. He wrote to experts collecting information that might be useful in support of his theory, he engaged in detailed taxonomic work on some little-known species of barnacles to familiarize himself with general problems in the taxonomy of a particular group, and most importantly he closely followed the practices of domestic breeders, especially by frequenting exhibitions and shows on popular or fancy breeds of pigeons. In 1842 he wrote a historical sketch outlining his theory and extended it into a longer historical essay in 1844, neither of which was made public, while all the while compiling the data he was amassing from experts all over the world and from his own research and reading. He was finally forced into joint publication of an abbreviated version of his theory in 1858 shortly after the English naturalist Alfred Russel Wallace independently formulated his own parallel theory. Up to that point, Darwin had been laboring for some twenty years to complete a comprehensive work he planned to title "Natural Selection."

Under pressure to complete a manuscript, Darwin took less than a year to outline his theory of species change, which he ultimately called "descent with modification" by means of natural selection. The full title of his famous book was On the Origin of Species by Means of Natural Selection; or, The Preservation of Favoured Races in the Struggle for Life. It appeared in bookstores on 24 November 1859 and sold out on the first day. It went through six editions as Darwin modified his theory in response to his many critics. Historians generally cite the first edition of this book for their scholarly attempts to understand Darwin's theory, since subsequent editions included such prolonged attempts to accommodate his critics that the text and scientific explication is considered unclear, if not actually inaccurate.

In addition to natural selection, Darwin included some four or five other ways that species change could take place, including the inheritance of acquired characters. Though he did not address human evolution in this book (only one sentence alludes to human evolution), Darwin's readers quickly made the connection between humans and primates thanks in part to the efforts of earlier transmutationists like Robert Chambers. Darwin turned to human evolution only later, in 1871, when he wrote the two-volume Descent of Man and Selection in Relation to Sex. In this book Darwin corrected earlier misconceptions of his work and made it clear that humans had not evolved from apes or monkeys but that both had shared a common ancestor. The book also included pronounced reflections on Darwin's views of human societies and the evolution of civilizations, some of which supports the present-day idea that Darwin himself was indeed a "Social Darwinist." The second part of this book, sexual selection, was an explication of Darwin's theory of sexual selection that was first articulated in 1859. Sexual selection was the process by which Darwin thought fanciful characters like male plumage had evolved, largely through the process of female mate choice. It was one point that led to a disagreement with Wallace, who did not support sexual selection because he thought it acted against natural selection.

Darwin's theory did face notable criticism in his day. One problem had to do with the absence of any viable theory of heredity in it. This led to the criticism—most closely ascribed to Fleeming Jenkin (1833–1885) in a famous review of 1867—that new or novel characters would be diluted or "swamped" out with subsequent generations. Darwin was aware of this criticism, and in his 1868 book The Variation of Animals and Plants under Domestication, he attempted to formulate his "provisional hypothesis of pangenesis," a theory postulating that organs of the body generate hereditary information in the form of gemmules which become combined in the gonads during reproduction. The hypothesis remained largely just an unproven hypothesis. This problem was eventually addressed, during the "rediscovery of Mendel" in 1900, by the discovery that heredity is particulate in nature.

Another problem was the age of the Earth, which, according to estimates made by William Thomson, later Lord Kelvin (1824–1907), was about one hundred million years old. This was an insufficient amount of time to account for the slow, gradual process that Darwin envisioned. The problem was solved after the late-nineteenth-century discovery of radioactivity, which, when accounted for in estimates of the age of the earth, increased it to nearly five billion years, an estimate of time long enough to account for evolution. Yet another problem came from objections to the randomness of the process and the apparent lack of rigor in Darwin's methodology. Leading this charge against Darwin's theory, the astronomer J. F. W. Herschel (1792–1871) described Darwinian natural selection as the "law of higgledypiggledy." Perhaps most problematic of all was the fact that Darwin had no direct proof for a process that took place over such a long stretch of time and that had not been easily detected in the fossil record (most of his evidence was indirect, based on evidence from biogeography, from analogies to artificial selection, or from "imaginary illustrations"). Darwin knew this, and he predicted that it would take some fifty years for evidence to support this theory. The first direct proof of evolution by means of natural selection in natural populations was finally provided beginning in the 1920s with the example of industrial melanism in the peppered moth, Biston betularia. Though the peppered moth example was later challenged by some who thought that the shift to the melanic form did not constitute a true speciation event, it remains a famous example of evolution in action since it demonstrates the rapid shift in allele frequencies following strong selection pressure. Since the case of industrial melanism became known, numerous studies in the wild or under laboratory conditions have provided definitive evidence in support of Darwinian natural selection. Some of the best examples are the morphological responses in beak shape and size to drought conditions on some of the Darwin finches in the Galapagos, and the evolution of antibiotic resistance in pathogenic strains of microorganisms that cause diseases like tuberculosis.

More difficult to resolve were the theological and philosophical questions that followed from the mechanistic theory of natural selection. Even though Darwin had only one line in his 1859 book on human evolution, the theory implied that humans were subject to the same mechanistic process as plants and animals. Natural selection challenged the argument for God's existence from design and led to a nonpurposive view of the world. To some, this echoed the fears raised earlier by the poet Alfred, Lord Tennyson (1809–1892), that such a competitive and nonpurposive view of nature implied that it was "red in tooth and claw." Darwin's own views of "nature," as embodied in works such as On the Origin of Species, appeared to be much more subtle. Nature, to Darwin, appeared to be benignly passive or indifferent to the drama playing itself out in the struggle for existence. Darwin's religious views became increasingly secular; there was no "death-bed conversion" to religious belief.

Despite considerable controversy over the mechanism, the fact of evolution was rapidly accepted by scientists and by popular

In Darwin's Shadow: Alfred Russel Wallace and the "Darwin-Wallace Theory of Evolution."

Though Darwin's name is virtually synonymous with the theory of evolution, the name of his codiscoverer Alfred Russel Wallace (1823–1913) is very often forgotten or relegated to the role of minor player. Yet Wallace himself was a gifted naturalist who distinguished himself for his scientific insights, especially in understanding island biogeography, and for his literary talents, especially excelling in the genre of the travel narrative.

Unlike Darwin, Wallace was born to a family of modest means in Usk, Monmouthshire, England. Lacking an extensive formal education, Wallace was an autodidact, frequenting lending libraries and attending public lectures on science. His first formal job at the age of fourteen was assisting his older brother at surveying counties in England, which gave him technical skills at observation and measurement, and he was later appointed as drawing master at the Collegiate School in Lancaster, where he fell in with Henry Walter Bates (1825–1892), a fellow teacher who shared a passion for the natural world.

In 1848 the two friends decided to embark on joint careers as commercial collectors of precious natural history specimens by traveling to the Amazon. They eventually took separate routes, collecting in increasingly unexplored regions, with Wallace traveling extensively up the Rio Negro, a tributary of the Amazon River. In 1852 Wallace returned to England, but nearly all of his magnificent collection was lost when his ship caught fire and sank. Wallace recovered from his setback rapidly, writing his account of the expedition in 1853, mostly from memory, as A Narrative of Travels on the Amazon and Rio Negro.

One year later he was once again in search of commercial specimens, this time setting off to the Malay Archipelago, where he traveled for nearly eight years. During this time he made one of the most notable observations in the history of island biogeography now associated with "Wallace's Line," an imaginary line that separated the two islands of Bali and Lombock; entirely different flora and fauna existed on either side of this geographic line. This disjuncture was only understood in the later twentieth century as the product of continental drift (the two areas are on separate plates).

Wallace's acute observations also led to his formulation, independently of Darwin, of a general theory of evolution that included a process closely resembling natural selection. In 1858, while suffering from malarial fever, Wallace connected the ideas of Malthus with his observations of the distribution of plants and animals and quickly drafted his celebrated paper "On the Tendency of Varieties to Depart Indefinitely from the Original Type." He proceeded to send the completed paper to Darwin, with whom he had had some previous correspondence. Darwin received the letter and immediately recognized the parallel views that Wallace had independently formulated. To avoid an unseemly priority dispute, Darwin turned over Wallace's paper along with his own historical sketch, essay, and correspondence with Asa Gray documenting the independence of the two efforts to Charles Lyell and Joseph Hooker and relied on them to negotiate the awkward situation. Both these men communicated these documents and a joint paper written by Darwin and Wallace to the Linnaean Society on 1 July 1858. Later that year the joint paper titled "On the Tendency of Species to form Varieties; and on the Perpetuation of Varieties and Species by Natural Means of Selection," by Charles Darwin and Alfred Wallace, was published in the Journal of the Proceedings of the Linnaean Society. Strangely enough, the paper failed to garner much negative attention or controversy. A year later, Darwin completed the famous "abstract" of his theory, published as On the Origin of Species, and from that point on Wallace's original contribution played at best only a supporting role.

Wallace himself was generally pleased to play this lesser part. He was a lifelong friend and supporter of Darwin, serving finally as pallbearer at Darwin's funeral with Huxley and Hooker. Darwin in turn supported Wallace through some especially difficult times, helping him secure a government pension. Wallace never properly attained financial security, and he also continued to uphold some radical ideas for his day that included land nationalization and an extreme opposition to vaccination campaigns in addition to more popular causes such as women's suffrage, labor reform, and universal education. Most disturbing to many establishment Victorian scientists generally, and Darwin in particular, was Wallace's growing support of spiritualist movements: Wallace's own support of the materialistic and mechanistic natural selection made an exception of the evolution of the human mind; he believed the "human spirit" could continue to progress even after the process of death.

His scientific work continued, earning him a permanent place in the history of science. Especially noteworthy were his contributions to island biogeography, which included the publication of The Geographical Distribution of Animals in 1876 and Island Life in 1880. In the late twentieth century Wallace's initial insights on the ecological dynamics of smaller island populations provided the intellectual backdrop to sciences such as conservation biology and to biodiversity studies.

audiences. Some of the greatest advocates and promoters of Darwin and his theory in fact disagreed with some rather major aspects of the theory. In the United States, the leading advocate of Darwin's theory was the botanist Asa Gray (1810–1888) at Harvard University. Though Gray found Darwin's theory useful to biogeography, he found the mechanistic implications of natural selection distasteful. It was Gray who brought Darwinian evolution to the attention of many American scientists and who defended Darwin against critical assaults by figures like his Harvard colleague in zoology Louis Agassiz (1807–1873). In Germany, Darwin and evolution found especially fertile ground in one of his greatest advocates, Ernst Haeckel. Beginning in 1866 with his Generelle Morphologie der Organismen (General morphology of organisms), Haeckel promoted Darwin and his evolutionary theory because of its materialistic flavor yet either misunderstood or disagreed with Darwinian natural selection. Rather than upholding an intricately branching and nonprogressivist view of evolution as Darwin had described in Origin, Haeckel retained a linear, progressive model only with some lateral branching. Haeckel continued to draw on embryological or developmental models for the evolutionary process and believed that evolution was guided by historical evolutionary forms that could still be seen in the process of individual development. In the process of development, the ontogeny (or developmental pattern of the organism) recapitulated the phylogeny (or the evolutionary history of the organism). Though German embryologists like Karl Ernst von Baer (1792–1876) argued against such a crude linear progressivist model of evolutionary development in favor of more complex branching models, the view that "ontogeny recapitulated phylogeny," also known as "the biogenetic law," continued to have mass appeal especially in Germany. Haeckel was a prolific and popular writer whose numerous attempts to reconstruct "phylogenetic trees" with the main trunk running upward to the human "race" as the pinnacle of development shaped popular understanding of evolution in the late nineteenth century. Haeckel's influence also had the unfortunate effect of linking Darwinian evolution with what he thought were materialist progressivist leanings toward the struggle for national development. His Generelle Morphologie and subsequent writings, which gained currency in the late nineteenth century, eventually provided pseudo-scientific justifications for nationalism and racism.

Darwin's most famous, indeed notorious, advocate was his close friend, the anatomist Thomas Henry Huxley. It was Huxley, along with the Kew Gardens botanist Joseph Dalton Hooker (1817–1879) and Charles Lyell, who formed the "inner circle" of friends and supporters who promoted and defended Darwin's name and his theory. Though Huxley earned himself the title of "Darwin's Bulldog" while supporting Darwin's theory—which stressed slow, gradual evolution—he preferred instead the view that evolution could take place more suddenly and rapidly. Like others of his time, Huxley closely linked views of evolutionary progress with social progress; he used the principles of evolution to support his reformist views of English society.

By the turn of the twentieth century, a staggering number of evolutionary theories—that added to, amended, or outright disagreed with Darwin's selection theory—were being actively entertained. These included a revitalization of the inheritance of acquired characters into movements associated with neo-Lamarckism; directed evolution, aristogenesis, and orthogenesis, all of which upheld the view that evolution was guided by an internal driving force; and "creative evolution," a quasi-mystical evolutionary theory endorsed by the French philosopher Henri Bergson (1859–1941), who postulated that living organisms were guided by an élan vital, or special living force.

One of the most popular alternatives to Darwinian selection theory was the "mutation theory" of the Dutch botanist Hugo de Vries (1848–1935). One of the "rediscoverers of Mendel," de Vries upheld a particulate theory of heredity that stressed the importance of what he termed "mutation pressure" in generating evolutionary novelty. Much of his theory was based on observations he made on the evening primrose plant, Oenothera lamarckiana, which appeared to throw off new varieties or species suddenly. De Vries erroneously interpreted these new forms as being entirely new species that had been generated by strong mutation pressure (these new forms of the primrose were subsequently shown to be regularly occurring varieties that resulted from its genetic structure). Natural selection was not ruled out, but it came into play only in selecting the most advantageous of these forms; it therefore played an eliminative role in evolution, while mutation pressure played the more active or creative role. Because it drew on the newer science of genetics, which appeared to be more rigorous because it was experimental, mutation theory was widely adopted by younger scientists at the turn of the century, who did not favor the natural-history-oriented approach associated with Darwin or his naturalist followers. The interval of time between approximately the rediscovery of Mendel and the late 1920s thus saw a period of dissonance between younger geneticists and older naturalists, all of whom sought a viable evolutionary theory that could be a rigorous experimental science that could also explain patterns of adaptation in natural populations. Others, who were strict followers of Darwinian selectionism and stressed the fact that Darwin endorsed a slow, gradual process that operated at the level of small, individual differences, turned to the newer science of statistics to create a new school known as "biometry." Francis Galton (1822–1911), Darwin's famous first cousin, was an exponent of the biometrical school, which tried to understand evolution in statistical terms. He was also the individual who coined the term eugenics in this attempt to formulate a viable theory of heredity and evolution that could then provide social reformers with the tools to "improve society" through selective breeding.

The turn of the century as a whole saw a series of proposed evolutionary theories that some have claimed were in fact "non-Darwinian" evolutionary theories. This was so much the case that Julian Huxley (1887–1975) the famous grandson of Thomas Henry Huxley, designated this interval of time "the eclipse of Darwin," a period of confusion and disagreement over the mechanism and mode of evolution. Only after the mechanism of heredity was understood and only after the science of genetics was integrated with natural history was the debate over the mechanism of natural selection extinguished. This did not take place until the interval of time between 1920 and 1950 and was part of the event called the "evolutionary synthesis." The synthesis brought together Darwinian selection theory with Mendelian genetics in a populational view of evolution to account for the origin of biological diversity. It first drew primarily on the work of three mathematical population geneticists, R. A. Fisher (1890–1962) and J. B. S. Haldane (1892–1964) in England and Sewall Wright (1889–1988) in the United States, all of whom offered models demonstrating the efficacy of natural selection under a range of different parameters. The theoretical work of these modelers was tested in field conditions with natural populations of organisms in the mid-1930s. This then led to the writing of a series of synthetic accounts that integrated Darwinian selection theory with Mendelian genetics.

The new science called evolutionary genetics was most closely associated with Theodosius Dobzhansky (1900–1975) and his synthetic book Genetics and the Origin of Species (1937). In addition to genetics, the "evolutionary synthesis" drew on systematics, botany, paleontology, cytology, and morphology to create what is now called the "synthetic theory of evolution" or the "neo-Darwinian theory of evolution." In addition to drawing on the work of Dobzhansky, it drew on the work of twentieth-century biologists like Ernst Mayr (b. 1904), George Ledyard Stebbins (1906–2000), George Gaylord Simpson (1902–1984), and Julian Huxley. It endorses the view that natural selection is the dominant mechanism that drives evolutionary change. Within such a modern, populational frame of reference, evolution itself became redefined as "any relative change in gene frequencies." This definition has been debated extensively, especially by naturalists-systematists who prefer a more inclusive consideration of the process of speciation.

Accompanying the emergence of the "modern synthesis" (Julian Huxley's exact phrase for the new science of evolution), the first international Society for the Study of Evolution (SSE) was formed in 1946 and sponsored the journal Evolution, the first international journal for the dissemination of scientific knowledge of evolution. With the "evolutionary synthesis," the varied sciences that informed evolution thus became reorganized into the new science of evolutionary biology. Because it drew on so many scientific disciplines, encompassing the breadth of the biological sciences and many of the social sciences, like psychology and anthropology, evolutionary biology began increasingly to play a central, integrative role in the biological sciences, especially beginning in the late 1950s and early 1960s. In 1975 Dobzhansky stated the important fact that "nothing in biology makes sense except in the light of evolution." In stating this, he was stressing the fact that evolution by means of natural selection serves as the central, unifying principle of the modern science of biology.

Though there have been varied attempts to amend or alter the synthetic theory as it was formulated during the "evolutionary synthesis" in the latter half of the twentieth century, the theory remains fundamentally intact. Among the varied points of agreement are included the primacy of natural selection, the continuation between microevolutionary and macroevolutionary processes, and the fact that evolution takes place at the level of small individual differences—all pretty much formulated in the 1930s–1950s. New techniques and methods from molecular biology have led to a virtual revolution in understanding evolution at the molecular level, while more traditional evolutionary biologists continue to mine the fossil record and to explore developmental biology, areas such as behavior and functional morphology as well as biochemistry to give a more detailed account of the evolutionary history of life on earth.

See also Biology ; Creationism ; Development ; Eugenics ; Genetics ; Lysenkoism ; Natural History ; Natural Theology ; Nature ; Naturphilosophie ; Progress, Idea of ; Social Darwinism .

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Vassiliki Betty Smocovitis

New Dictionary of the History of Ideas Smocovitis, Vassiliki

Evolution

views updated May 23 2018

EVOLUTION

CONCEPT

Among the dominant concepts of the modern world in general, and biology in particular, few are as powerful—or as misunderstood—as evolution. Even the name is something of a misnomer, since it almost implies some sort of striving to reach a goal, as though the "purpose" of evolution were to produce the most intelligent species, human beings. In fact, what drives evolution is not a quest for biological greatness but something much more down to earth: the need for organisms to survive in their environments. Closely tied to evolution are two processes, mutation and natural selection. Natural selection is a process whereby survival is related directly to the ability of an organism to fit in with its environment, while mutation involves changes in the genetic instructions encoded in organisms.

Although the English naturalist Charles Darwin (1809-1882) often is regarded as the father of evolutionary theory, he was not the first thinker to suggest the idea of evolution as such; however, by positing natural selection as a mechanism for evolution, he provided by far the most convincing theory of evolutionary biological change up to his time. In the years since Darwin, evolutionary theory has evolved, but the essential idea remains a sound one, and it is a "theory" only in the sense that it is impossible to subject it to all possible tests. The idea that evolution is somehow still open to question is another pervasive misconception, and it often appears hand in hand with the most pervasive misconception of all—that evolution is in some way anti-Christian, anti-religion, or anti-God.

HOW IT WORKS

The "Watch Analogy" and What It (Unintentionally) Teaches About Evolution

Throughout this essay, we discuss misconceptions relating to evolution. Such misconceptions have had such a strong impact on modern civilization that it is important to begin by setting aside a few misguided ideas that strike at the very heart of the evolutionary process. Many of these misconceptions are embodied in a popular "argument" against evolution that goes something like this: Suppose you took a watch apart and laid the pieces on the ground. If you came back in a billion years, would you really expect the watch to have assembled itself?

This argument is a virtual museum of all the fallacies associated with evolution. First of all, a watch (or any of the other variations used in similar arguments) is mechanical, not organic or biological, which is the class of objects under discussion within the framework of evolution. In that sense, the answer to this question is easy enough: No, a watch probably never would assemble itself, because it is not made of living material and it has no need for survival.

Another problem with the watch argument is that it starts with impossibly large pieces. Let us assume that the watch is a living being; even so, one would not expect its dials and gears to assemble themselves. But evolution does not make such claims: there is nothing in the theory of evolution to lead one to believe that a collection of organs lying around on a beach eventually would piece themselves together to make a whale.

According to what paleontologists (see Paleontology) and other scientists can deduce, over the course of three billion years life-forms evolved from extremely simple self-replicating carbon-based molecules to single-cell organisms. This is hardly what one would call breakneck speed. The more visible or "exciting" part of evolution, with the proliferation of species that produced the dinosaurs and (much later) humans, took place in the past billion years. In fact, the pace of change was still very, very slow until about half a billion years ago, and it has been accelerating ever since. For the vast majority of evolutionary history, however, change has been so slow that, by contrast, watching paint dry would be like playing a high-speed video game.

Ironically, for the watch scenario to be truly analogous to anything in evolution, one would have to start with atoms and molecules not whole gears and dials. Opponents of evolutionary theory might take this fact as being favorable to their cause, but if the watch were made of living, organic material rather than metal, it is possible that the molecules would have some reason to join in the formation of organelles and, later, cells. Or perhaps they would not. Therein lies another problem with the watch analogy and, indeed, with many of the attempts to argue against evolution on a religious basis. This might be called the "fallacy of intention," or the idea that evolution is driven by some overall purpose.

THE "FALLACY OF INTENTION."

Hidden in the watch analogy is the idea of the watch itself, the finished product, as a "goal." By the same analogy, the single-cell eukaryotes of a billion or two billion years ago were forming themselves for the purpose of later becoming pine trees or raccoons or people. This is not a valid supposition, as can be illustrated by analogies to human history.

The history of human beings, of course, has taken place over a much, much shorter span than evolutionary history. (The Paleontology essay contains several comparisons between the span of human life on Earth and Earth's entire existence.) Moreover, unlike cells, people do form goals and act on intentions, so if there were any good example of change with a goal in mind, it would have to come from human beings. Yet even in the few thousand years that humans have existed in organized societies, most trends have occurred not as part of a major plan but as a means of adapting to conditions.

Consider the situation of a group of nomads who lived in what is now southern Russia about 5,000 years ago. At some point, this vast collection of tribes began to migrate outward, some moving into an area that is now central Asia and the Indian subcontinent and others migrating westward. No sane person would argue that the westward-traveling members of this group knew that in moving to the geographically advantageous territory of Europe, they were putting in place conditions that would help give their descendants dominance over most of the planet some 4,500 years later. Rather, they were probably just trying to find better land for grazing their horses.

We cannot say what the Indo-Europeans, as they are known to history, were looking for. Our only evidence that they existed is the similarities between the languages of Europe, India, and Iran, first noted by the German philologist and folklorist Jacob Grimm (1785-1863) at about the same time that Darwin was formulating his theory of evolution. Grimm, in fact, used methods not unlike those of Darwin, but instead of fossils he studied words and linguistic structures. Along the way, he found remarkable links, such as the Sanskrit word agni, cousin to the Latin term ignis and such modern English words as ignite.

In contrast to the Indo-Europeans, we know a great deal about another group of westward-moving nomads, the Huns of around a.d. 300, who were indeed looking for better grazing lands. Dislocated from their native areas by the building of China's Great Wall, the Huns crossed the Danube River, displacing the Ostrogoths. The Ostrogoths, in turn, moved westward, and this migration set in motion a domino effect that would bring an end to the Western Roman Empire in a.d. 476.

Did the Huns intend to destroy the Roman Empire and bring about the Middle Ages? No reasonable person would adopt such a conspiratorial view of history. Even more absurd, did the Chinese build the Great Wall with the idea of precipitating this entire chain of events? Again, no one would assert such a premise. If those trends in the evolution of societies were not goal-directed, why would we assume that cells and organisms would have to be striving toward a particular end to obtain certain results?

CONFUSING EVOLUTION WITH GOD.

In fact, there is no driving "purpose" to evolution—no scientifically based substitute for God operating from behind the scenes and manipulating the evolutionary process to achieve its ultimate aims. Evolution is not guided by any one large aim but by a million or a billion small aims—the need for a particular species of mollusk to survive, for instance.

As we discuss in the course of this essay, the idea of an underlying conflict between evolution and Christianity (or any other religion, for that matter) is almost entirely without merit. On the other hand, it is theoretically possible that all the processes of evolution took place without a creator—but this still should not pose a threat to anyone's idea of God.

There is nothing in evolution that would lead to the conclusion that there is no God, that the universe is not God's handiwork, or that God does not continue to engage in a personal relationship with each human. Neither is there anything in evolution that would lead to the conclusion that God does exist. Rather, the matter of God is simply not relevant to the questions addressed by evolution. In other words, evolution leaves spiritual belief where it should be (at least, according to Christianity): in the realm of individual choice.

Natural Selection

As we noted earlier, one of the principal mechanisms of evolutionary processes is natural selection. This in itself illustrates the lack of intention, or "goal orientation," in evolution. Like the name evolution itself, the term natural selection can be deceptive, implying that nature selects certain organisms to survive and condemns others to extinction. In fact, something quite different is at work.

Species tend to overproduce, meaning that the number of field mice, for instance, born in any year is so large that this entire population cannot possibly survive. The reason is that there is never enough of everything—food, water, or living space—for all members of the population to receive what they need. Therefore, only those best adapted to the environment are likely to survive.

FASTER, FURRIER MICE.

Suppose, for instance, that the climate in the area where two field mice live is very cold, and suppose that some of the field mice have more protective fur than others; obviously, they are more likely to live. If there are many speedy predators around, judging purely on the basis of that factor alone, it would be easy to predict that the swiftest of the field mice would survive. Thus, faster-running, furrier mice would be "selected" over the slower or less furry mice.

Natural selection is not simply a matter of one particular mouse surviving in an environment. Instead, it involves the survival of specific strains, or lines of descent, that are more suited to the environment in question. Individuals adapted to an environment are more likely to live and reproduce and then pass on their genes to the next generation, while those less adapted are less likely to reproduce and pass on their traits. The genetic strains that survive are not "better" than those that do not—they are only better adapted.

The process of natural selection is ongoing. For example, in generation A, the furrier field mice survive and pass on their "furriness" gene to their offspring. Some of the offspring may still not be furry, and these mice will be less likely to survive and reproduce. In addition, since there are almost always several survival factors affecting natural selection, it is likely that other traits also will determine the survivability of certain individual mice and their genes.

For instance, there may be furry but slow mice in generation B, which despite their adaptation to temperature conditions are simply not fast enough to get away from predators. Therefore, the mice in generation C are likely to be furrier and faster than their ancestors. Additional survival factors may come into the picture, to ensure that the average member of generation D has sharper teeth in addition to swifter feet and a furrier body.

Although this illustration depicts evolutionary changes as taking place over the course of four generations, they are more likely to occur over the span of 400 or 4,000 or four million generations. In addition, the process is vastly more complicated than it has been portrayed here, because numerous factors are likely to play a part. The essential mechanism outlined here, however, prevails: certain traits are "naturally selected" because individuals possessing those traits are more capable of survival.

THE "SURVIVAL OF THE FITTEST."

The concept of natural selection sometimes is rendered popularly as the "survival of the fittest." Scientists are less likely to use this phrase for several reasons, including the fact that it has been associated with distasteful social philosophies or murderous political ideologies—for example, Nazism. Additionally, the word fittest is a bit confusing, because it implies "fitness," or the quality of being physically fit.

This implication, in turn, might lead a person to believe that natural selection entails the survival of the strongest, which is not the case. Yet this is precisely what proponents of a loosely defined philosophy known as social Darwinism claimed. Popular among a wide range of groups and people in the late nineteenth and early twentieth centuries, social Darwinism could be used in the service of almost any belief. Industrialists and men of wealth asserted that those who succeeded financially did so because they were the fittest, while Marxists claimed that the working class ultimately would triumph for the same reason. Across the political spectrum, social Darwinism confused the meaning of "fittest" with that of other concepts: "strongest," "most advanced," or even "most moral." All of this, it need hardly be said, is misguided, not least because evolutionary theory has nothing to do with race, ethnicity, or social class.

In fact, "survival of the fittest," in a more accurate interpretation, means that individuals that "fit," or "fit in with," their environments are those most likely to survive. This is a far cry from any implication of strength or superiority. Imagine a group of soldiers in combat: Which type of soldier is most likely to survive? Is it the one who scores highest on physical training tests, looks the finest in a uniform, comes from a more socially upper-class home, and has the most advanced education? Or is it the one who keeps his head low, acts prudently, does not rush into dangerous situations without proper reconnaissance, and obeys instruction from qualified leaders?

Clearly, the second set of characteristics has much more to do with survival, even though these qualities may seem less "noble" than the first set. Yet it is by adapting, or proving his or her adaptability, to the environment of war that a soldier survives—not by displays of strength or other types of "fitness" that simply appear impressive. In the same way, the fitness of a species does not necessarily have anything to do with strength: after all, the lion, the "king of beasts," would die out in a polar climate or a desert or an aquatic environment.

Mutation

Although natural selection is of principal importance in evolution, mutation also plays a pivotal role. Mutation is the process whereby changes take place in the genetic blueprint for an organism as a result of alterations in the physical structure of an organism's DNA (deoxyribonucleic acid). DNA is a molecule in all cells and in many viruses that contains genetic codes for inheritance. DNA carries genetic information that is transmitted from parent to offspring; when a mutation occurs, this new genetic information—often quite different from the genetic code received by the parent from the grandparent—is passed on instead.

Under normal conditions of reproduction, a copy of the DNA from the parent is replicated and transmitted to the offspring. The DNA from the parent normally is copied exactly, but every once in a while errors arise during replication. These errors usually originate in noncoding regions of the DNA and therefore have little effect on the observable traits of the offspring. On the other hand, some mutations may be lethal, and thus the offspring does not survive for the mutation to become apparent. In a very few cases, however, offspring with a slightly modified genetic makeup manage to survive.

CONTRAST WITH ACQUIRED CHARACTERISTICS.

Mutation is not to be confused with the inheritance of acquired characteristics, a fallacious doctrine that had its adherents when Darwin was a young man. If acquired characteristics were taken to an extreme, a lumberjack who loses his arm cutting down a tree and later conceives a child with his wife would most likely father a child who is missing an arm. This notion is absurd, and attempts to put forward a workable theory of acquired characteristics in the late eighteenth and early nineteenth centuries involved much greater subtlety. Still, the idea is misguided.

The French natural philosopher Jean Baptiste de Lamarck (1744-1829), one of the leading proponents of acquired characteristics, maintained that giraffes had gained their long necks from the need to stretch and reach leaves at the top of tall trees. In other words, if a giraffe parent had to stretch its neck, a giraffe baby would be born with a stretched neck as well. Later, Darwin's natural selection provided a much more plausible explanation for how the giraffe might have acquired its long neck: assuming that the nutrients it needed were at the highest levels of the local trees, the traits of tallness, long necks, and the ability to stretch would be selected naturally among the giraffe population.

MUTATIONS AND SURVIVAL.

Unlike the idea of acquired characteristics, mutation does not entail the inheritance of anatomical traits acquired in the course of an organism's life; rather, it is changes in the DNA that are passed on. For example, when mind-altering drugs became popular among young people in the 1960s, concerns were raised that the offspring of drug takers might suffer birth defects as a result of alterations in their DNA. For the most part, this did not happen. Conditions such as Huntington disease and cystic fibrosis, however, are the result of mutations in DNA; so, too, is albinism, which eliminates skin pigment.

Although mutations often are regarded as undesirable because they can affect the health of individuals adversely, they also can have positive effects for the population in question. Suppose a group of bacteria is exposed to an antibiotic, which rapidly kills off the vast majority of the bacteria. In a fraction of those who survive, however, a mutation may develop that makes them resistant to the medication. Eventually, these mutant bacteria will reproduce, creating more mutants and in time yielding an entire population resistant to the antibiotic.

This is the reason why antibiotics can lose their effectiveness over time: bacteria with mutant genes will render every antibiotic useless eventually. The same often can happen with insect sprays, as roaches and other pests develop into mutant strains that are capable of surviving exposure to these pesticides. Such species, with their short cycles of birth, reproduction, and death, are extremely well equipped for survival as a group, which explains why many an unpleasant "bug" (whether a bacterium or an insect) has long been with us. (See Mutation for more on this subject.)

REAL-LIFE APPLICATIONS

"Proving" Evolution

Later in this essay, we look at examples of evolution in action and other phenomena that support the ideas of evolutionary theory. But before examining these many "proofs" of evolution, a few words should be said about the very fact that evolution seems to require so much more proof than most other scientific theories.

All scientific ideas must be capable of being proved or disproved, of course, but the demand for proof in the case of evolution goes far beyond the usual rigors of science. In fact, at this point, the people demanding proof are not scientists but certain sectors of the population as a whole—in particular, religious groups or individuals who fear evolution as a challenge to their beliefs.

QUANTUM MECHANICS: A MUCH MORE DIFFICULT IDEA.

By contrast, quantum mechanics, though it encompasses ideas completely opposed to common sense, has not sustained anything approaching the same challenge or the demand for proof that evolution has encountered from nonscientists. A theory in physics and chemistry that details the characteristics of energy and matter at a subatomic level, quantum mechanics goes against such common assumptions as the idea that we can know both the location and the speed of an object. It is as though science had proved that down was up and up was down. If there were ever a "dangerous" theory, inasmuch as it undermines all our assumptions about the world, it is quantum mechanics not evolution, which is a fairly straightforward idea by comparison.

Quantum mechanics has gone virtually unchallenged (at least on a social or moral, as opposed to a scientific, basis), whereas even today there are many people who refuse to accept the idea of evolution. Granted, quantum mechanics is a much younger idea, having originated only in the 1920s, and it is vastly more difficult to understand. But the real reason why evolution has come under so much more challenge, of course, has to do with the fact that it is perceived (mistakenly) as challenging the primacy of God.

JUST A THEORY?

One of the aspects of evolution often cited by opponents is the fact that it is, after all, the theory of evolution. The implication is that if it is still just a theory, it must be open to question. In a sense, this is accurate: for scientific progress to continue, ideas should never be accepted as absolute, unassailable truths. But this is not what opponents of evolution are getting at when they cite its status as a "mere" theory. In fact, their use of this point as a basis for attack only serves to illustrate a misunderstanding with regard to the nature of scientific knowledge.

The word theory in "theory of evolution" simply means that evolutionary ideas have not been and, indeed, cannot be tested in every possible circumstance. Most ideas in science are simply theories rather than laws because in few cases is it possible to say with absolute certainty that something always will be the case. One of the few actual scientific laws is the conservation of energy, which holds that for all natural systems the total amount of energy remains the same, though transformations of energy from one form to another take place. This has been tested in such a wide variety of settings and circumstances that there is no reason to believe that would it ever not be the case.

By contrast, there probably never will be enough tests on evolution to advance it to the status of a law. The reason is quite simply that evolution takes a long time. Some examples, such as the instances of industrial melanism that we discuss later, unfold within a short enough period of time that humans can observe them. In general, however, evolutionary processes take place over such extraordinarily long spans of time that it would be impossible to subject them to direct observation.

None of this, however, does anything to discredit evolutionary theory. For that matter, the idea that the entire physical world is made of atoms is still technically a theory, though there is no significant movement of people attempting to discredit it. The reason, of course, is that atomic theory does not seem to contradict anyone's idea of God. (This was not always the case, however. Almost 2,500 years ago, a Greek philosopher named Democritus developed the first atomic theory, but because his ideas were associated with atheism, atomic theory was largely rejected for more than two millennia.)

FACING THE FACTS.

If people really understood the word theory, they would give it a great deal more respect. Unfortunately, the word so often is misused and applied to anything that has not been proved that it has begun to seem almost like an insult to call evolution a theory. After all, in the present essay, we refer to acquired characteristics as a theory, and in everyday life one often hears much less respectable ideas given the status of theory. For this reason, it is worth taking note of the process, from observation to hypothesis to the formulation of general statements, that goes into the development of a truly scientific theory.

In forming his theory of evolution, Darwin began with several observations about the natural world. Among the things he observed is the fact, which we noted earlier, that for a particular species, more individuals are born than can possibly survive with available resources. On the basis of this observation, he formed a hypothesis, or inference. His inference was that because populations are greater than resources, the members of a population must compete for resources.

A theory is made up of many hypotheses, but to proceed from a collection of hypotheses to a true theory, these inferences must be subjected to rigorous testing. Thus, Darwin, in effect, said to himself, "Is what I have said true? Are there more individuals of a species than there are available resources?" Then he began looking for examples, and like a true scientist, he did so with the attitude that if he found examples that contradicted his hypothesis, he would reject the hypothesis and not the facts.

As it turns out, of course, there are always more members of a population than there are resources. This can be illustrated in a small way by observing a litter of puppies or piglets struggling to obtain milk from their mother. Chances are that the mother will not have enough teats for all her babies, and the "runt," unless it is able to force its way through the others to the milk source, may die. Only after testing this hypothesis and other hypotheses, such as that of natural selection, did Darwin formulate his theory.

Evolution and Religion

The fact that some puppies or piglets die for lack of milk is not a nice or pleasant thought, but it is the truth. Again, like a true scientist, Darwin accepted reality, without attempting to mold it to fit his personal beliefs about how things should be.

As a great thinker from the generation that preceded Darwin's, the Scottish philosopher David Hume (1711-1776), wrote in his Enquiry Concerning Human Understanding: "There is no method of reasoning more common, and yet more blamable, than, in philosophical disputes, to endeavor the refutation of a hypothesis, by a pretense of its dangerous consequences to religion and morality." In other words, there is an understandable, but nonetheless inexcusable, human tendency to evaluate ideas not on the basis of whether they are true but rather on the basis of whether they fit with our ideas about the world.

A scientist may be a Christian, or an adherent of some other religion, and still approach the topic of evolution scientifically—as long as he or she does not allow religious convictions to influence acceptance or nonacceptance of facts. The scientist should start with no preconceived notions and no allegiance to anything other than the truth. If that person's religious conviction is strong enough, it can weather any new scientific idea.

CONFUSING ATHEISM WITH SCIENCE.

This brings up an important point regarding the alleged conflict between religion and science. Not all the blame for this belongs with religious groups or individuals who shut their minds to scientific knowledge. Many scientists over the years likewise have adopted the fallacy of maintaining that religion and science are somehow linked, in this case using scientific facts as a basis for rejecting religion.

One such scientist was Darwin himself, who embraced agnosticism because his own findings had proved that the biblical account of creation cannot be literally true. In this religious choice, he was following in a family tradition: his grandfather, the physiologist Erasmus Darwin (1731-1802), belonged to the mechanist school, a muddle of atheism, bad theory, and genuine science.

The mechanists claimed that humans were mere machines whose activities could be understood purely in terms of physical and chemical processes. Claims such as these ultimately led to the discrediting of their movement, whose ideas failed to explain such biological processes as growth. At the same time, such mechanist philosophers as the French physician and philosopher Julien de La Mettrie (1709-1751) went far beyond the territory of science, teaching that atheism was the only road to happiness and that the purpose of human life was to experience pleasure.

The thinker who perhaps did the most to confuse science and atheism was one of Darwin's most significant early followers, the German natural scientist and philosopher Ernst Haeckel (1834-1919). It was Haeckel, not Darwin, who first proposed an evolutionary explanation for the origin of human beings, which, of course, was a major step beyond even Darwin's claim that all of life had evolved over millions of years.

In the course of developing this idea, Haeckel, who was a practicing Christian until he read Darwin's On the Origin of Species by Means of Natural Selection, renounced his faith and adopted a belief system he called monism, which is based on the idea that there is only a physical realm and no spiritual one. Technically, Haeckel was not an atheist but a pantheist, since his philosophy included the idea of a single spirit that lives in all things, both living and nonliving. Whatever the case, Haeckel's monism is no more scientific than Christianity.

HUMANS AND "MONKEYS."

It is interesting that the man who put forward the notorious idea that humans and apes are related also would attempt to turn evolution into a sort of "proof" of atheism. In fact, the evolutionary connection between humans and lower primates, or "monkeys," has long been the most powerful point of contention between religion and evolution.

This, in fact, remains one of the most challenging aspects of evolutionary theory—not because it is hard to see how the human body is similar to an ape's body but because there is such a vast difference between a human mind and that of an ape. Whereas our physical similarity to primates is easy to establish, the fact is that no other animal—ape, dolphin, pig, or dog—comes close to humans in terms of reasoning ability. Nor is it reasoning ability alone that separates humans from other animals. Humans possesses a propensity for conceptualization and a level of self-awareness that sets them completely apart from other creatures, so much so that the brains of apes, cats, birds, and even frogs seem more or less alike compared with that of a human.

Animals are concerned with a few things: eating, sleeping, eliminating waste, and procreating. Some mammals have the ability to engage in play, but there is still no comparison between even the most advanced mammalian brains and that of a human. Other primates have the ability to use sticks or stones as tools, but only humans—practically from the beginning of the species 2.5 million years ago—have the ability to fashion tools. Only humans are gifted, or cursed, with restless minds ever in search of new knowledge.

Does any of this disprove evolution? It does not. Does it pose a significant challenge to the idea that humans and other primates evolved from a common ancestor? Not as it has been stated here. All that has been said in the preceding paragraphs is simply a matter of everyday observation, but it is not a scientific hypothesis, let alone a theory. Clearly, there are some questions still to be answered as to why and how humans developed brains so radically different from those of other primates, but the place for such questioning is within the realm of science not outside it.

CREATIONISM.

Another thing we can say about the human mind is that it has a tendency to mold ideas toward its own preconceptions as to how things should be. As Hume observed, there is a great temptation, in the minds of all people, to demand that scientific facts conform to a particular set of religious or political beliefs. Such is the case with creationism and "intelligent design theory," two scientific belief systems whose adherents have attempted to challenge evolutionary theory.

Creationism, which sometimes goes by the name of creation science, is based on the belief that God created the universe and did so in a very short period of time. This claim, creationists maintain, can be supported by scientific evidence. Scientific evidence, however, is not really what drives creationism, which is based on a literal reading of the first two chapters of the Book of Genesis. Taken to an extreme, this means that God created the universe about 6,000 years ago in six days of 24 hours each.

Adherents of creationism begin with the premise of a six-day Creation (or at least, a very young Earth) and then look for facts to support the premise—exactly the opposite of the approach taken by true science. The findings of creationists do not change much over the years, unlike evolutionary science, which has continued to develop with new discoveries.

Sometimes creationists attempt to use the findings of evolutionary science against it. For instance, they may interpret industrial melanism (the adaptation of moths to discoloration in the environment caused by pollution, discussed later in this essay) as proof that organisms can change very quickly. This, of course, does not take into account the fact that moths have very short life spans compared with humans, for whom evolutionary change takes much longer. Creationists also point to areas of evolutionary theory where all scientists are not in agreement, citing these as "proof" that the whole theory is unsound.

INTELLIGENT DESIGN THEORY AND THE COURT BATTLE.

In contrast to creationism, intelligent design theory is not based on any particular religious position. Instead, it begins with an observation that would find a great deal of agreement among many people, including those who support evolutionary theory. The idea is that evolution alone does not explain fully how life on Earth came to exist as it does, with all its complexity and order. According to intelligent design theory, there must have been some intelligence behind the formation of the universe.

There is another contrast between intelligent design theory and creationism. Whereas it is hard to imagine a genuine scientist embracing creationism, it is not difficult at all to picture a scientific thinker adopting the viewpoint of intelligent design. In fact, this has happened, though long before the "movement" had a name.

Darwin's contemporary, the English naturalist Alfred Russel Wallace (1823-1913), who published his own theory of evolution at about the same time as Darwin's Origin of Species, parted ways with Darwin because he maintained that there must be a spiritual force guiding evolution. Only such a force, he maintained, could explain the human soul. From a philosophical and theological standpoint, this idea has a great deal of merit, but because it cannot be tested, it cannot truly be regarded as science.

Neither creationism nor intelligent design has received any support in the scientific community—nor, during court battles over the teaching of creationism in the public schools during the 1980s, did that idea receive the support of the United States justice system. Creationism, the courts ruled, is a religious and not a scientific doctrine. Evolutionary theory is based on an ever increasing body of evidence that is both observable and reproducible. To teach these other doctrines alongside evolution in the public schools would convey the impression that creationism and intelligent design had been subjected to the same kinds of rigorous tests that have been applied to evolution, and this is clearly not the case.

Evidence for Evolution

A great deal of evidence for evolution appeared in the seminal text of evolutionary theory (mentioned previously), On the Origin of Species by Means of Natural Selection, which Darwin published in 1859. In fact, he had collected much of the evidence he discusses in this volume nearly three decades earlier, from 1831 to 1836, aboard a scientific research vessel off the coast of South America. (He delayed publication because he rightly feared the controversy that would ensue and resolved to present his ideas only when he learned that Wallace had developed his own theory of evolution.)

Just 22 years old, Darwin traveled on the HMS Beagle, from which he collected samples of marine life. His most significant work was done on the Galápagos Islands some 563 mi. (900 km) west of Ecuador. As he studied organisms there, Darwin found that they resembled species in other parts of the world, but they were also unique and incapable of interbreeding with similar species on the mainland. He began to suspect that for any particular environment, certain traits came to the forefront, favored for survival by nature.

Back in England, he already had seen such a mechanism at work in the artificial breeding of pigeons, whereby breeders favored certain gene pools—for instance, white-tailed birds—over others. (Breeders of dogs and other animals today still employ artificial-selection techniques to produce desirable strains.) Darwin posited a similar process of selection in nature, only this one was not artificial, directed by a goal-oriented human intelligence, but natural and guided by the need for survival.

THE SPREAD OF SPECIES.

Among the phenomena Darwin observed in the Galápagos was the differentiation among the 13 varieties of finch (a type of bird) on the islands as well as the contrasts among these finches and their counterparts on the mainland. As Darwin began to discover, they shared many characteristics, but each variety had its own specific traits (for instance, the ability to crack tough seeds for food) that allowed it to fill a particular niche in its own environment.

From the beginning Darwin was influenced by the recent findings in geology, a newly emerging science whose leading figures maintained that Earth was very, very old. (These scientists included the Scottish geologist Charles Lyell [1797-1875], whose Principles of Geology, published between 1830 and 1833, Darwin read aboard the Beagle ) The relationship between geology and evolution has persisted, and findings in the earth sciences continue to support evolutionary theory.

Among the leading ideas in geology and other geosciences since the mid-twentieth century is plate tectonics, which indicates (among other things) that the continents of Earth are constantly moving. (See Paleontology for further discussion of this topic.) This idea of continental drift provided a mechanism for species differentiation of the kind Darwin had observed.

It appears that in the past, when the land-masses were joined, organisms spread over all available land. Later, this land moved apart, and the organisms became isolated. Eventually, different forms evolved, and in time these distinct organisms became incapable of interbreeding. This is what occurred, for instance, when the Colorado River cut open the Grand Canyon, separating groups of squirrels who lived in the high-altitude pine forest. Eventually, populations ceased to interbreed, and today the Kaibab squirrel of the northern rim and the Abert squirrel of the south are separate species.

COMMON ANCESTRY.

Darwin recognized that some of the best evidence for evolution lies hidden within the bodies of living creatures. If organisms have a history, he reasoned, then vestiges of that history will linger in their bodies—as studies in comparative anatomy show. An example is a phenomenon that sounds as if it is made up, but it is very real: snake hips. Though their ancestors ceased to walk on four legs many millions of years ago, snakes still possess vestigial hind limbs as well as reduced hip and thigh bones.

In some cases widely divergent organisms possess a common structure, adapted to their individual needs over countless generations yet reflective of a shared ancestor. A fascinating example of this is the pentadactyl limb, a five-digit appendage common to mammals and found, in modified form, among birds. The cat's paw, the dolphin's flipper, the bat's wing, and the human hand are all versions of the same original, an indication of a common four-footed ancestor that likewise had limbs with five digits at the end.

The embryonic forms of animals also reflect common traits and shared evolutionary forebears. This is why most mammals look remarkably similar in early stages of development. In some cases animals in fetal form will manifest vestigial features reflective of what were once functional traits of their ancestors. Thus, fetal whales, while still in their mothers' wombs, produce teeth after the manner of all vertebrates (creatures with an internal framework of bones), only to reabsorb those teeth, which they will not need in a lifetime spent filtering plankton through their jaws.

The molecular "language" of DNA also provides evidence of shared evolutionary lineage. When one studies the DNA of humans and chimpanzees, very close similarities rapidly become apparent. Likewise, there are common structures in the hemoglobin, or red blood cells, of different types of organisms. Comparisons of hemoglobin make it possible to pinpoint the date of the last common ancestor of differing species. For example, hemoglobin analysis reveals an ancestor common to humans and frogs dating back 330 million years, whereas the common human and mouse ancestor lived 80 million years ago, and the ancestor we share with the rhesus monkey walked the earth "only" 26 million years ago.

THE FOSSIL RECORD.

The fossil record also provides an amazing amount of evidence concerning common ancestors. Fossilized remains of invertebrates (animals without an internal skeleton), vertebrates, and plants appear in the strata or layers of Earth's surface in the same order that the complexities of their anatomy suggest. The more evolutionarily distant organisms lie deeper, in the older layers, beneath the remains of the more recent organisms. Geologists are able to date rock strata with reasonable accuracy, and the age of a layer always correlates with the fossils discovered there. In other words, there would never be a stratum dating back 400 million years that contained fossils of mastodons, which evolved much later.

A fossil is the remains of any prehistoric life-form, especially those preserved in rock before the end of the last ice age, about 10,000 years ago. The process by which a once living thing becomes a fossil is known as fossilization. Generally, fossilization involves changes in the hard portions, including bones, teeth, and shells. This series of changes, in which minerals are replaced by different minerals, is known as mineralization.

Fossilized remains of single-cell organisms have been found in rock samples as old as 3.5 billion years, and animal fossils have been located in rocks that date to the latter part of Precambrian time, as long ago as one billion years. Certain fossil types, known as index fossils or indicator species, have been associated strongly with particular intervals of geologic time. An example is the ammonoid, a mollusk that proliferated for about 350 million years, from the late Devonian to the early Cretaceous periods, before experiencing mass extinction.

The fossil record is far from an open book, however, and interpreting fossil evidence requires a great deal of judgment. All manner of natural phenomena such as earthquakes can destroy fossil beds, rendering the evidence unreadable or at least unreliable. Nor is it a foregone conclusion that the animals who left behind fossils are fully representative of the species existing at a given time. Fossils are far more likely to be preserved in certain kinds of protected aquatic environments, for instance, than on land (particularly at higher elevations, where erosion is a significant factor), and therefore paleontologists' knowledge of life forms in the distant past is heavily weighted toward marine creatures.

FAUNAL SUCCESSION AND OTHER FORMS OF DATING.

Key to the demonstration of evolution is the age of samples and the idea that many of the processes described took place a long, long time ago. This raises the question of how scientists know the age of things. In fact, they have at their disposal several techniques, both relative and absolute, for dating objects.

One of the earliest ideas of dating in geology was faunal dating, or the use of bones from animals (fauna) to determine age. This was the brainchild of the English engineer and geologist William Smith (1769-1839), whose work is an example of the fact that evolutionary ideas were "in the air" long before Darwin. While excavating land for a set of canals near London, Smith discovered that any given stratum contains the same types of fossils, and therefore strata in two different areas can be correlated. Smith stated this in what became known as the law of faunal succession: all samples of any given fossil species were deposited on Earth, regardless of location, at more or less the same time. As a result, if a geologist finds a stratum in one area that contains a particular fossil and another in a distant area having the same fossil, it is possible to conclude that the strata are the same.

Faunal succession is relative, meaning that it does not provide clues as to the actual age in years of a particular sample. Since the mid-twentieth century, however, scientists have had at their disposal several means for absolute dating, which make it possible to determine the rough age of samples in years. Most of these mechanisms for dating are based on the fact that over time, a particular substance converts to another, mirror substance. By comparing the ratios between them, it is possible to arrive at an estimate as to the amount of time that has elapsed since the organism died.

Chief among the techniques for absolute dating is radiometric dating, which uses ratios between two different kinds of atoms for a given element: stable and radioactive isotopes. Isotopes are atoms that differ in their number of neutrons, or neutrally charged subatomic particles, and radioactive isotopes are ones that spontaneously eject various high-energy particles over time. Because chemists know how long it takes for half the isotopes in a given sample to stabilize (a half-life), they can judge the age of such a sample by examining the ratio of stable to radioactive isotopes. In the case of uranium, one isotopic form, uranium 238, has a half-life of 4,470 million years, which is very close to the age of Earth itself.

Evolution at Work

Every creature that exists today is the result of an incredibly complex, lengthy series of changes brought about by mutation and natural selection, changes that influenced the evolution of that life-form. Take for instance the horse, whose evolutionary background is as well-documented as that of any creature.

The horse family, or Equidae, had its origins at the beginnings of the Eocene epoch about 54 million years ago. This first ancestor, known as Hyracotherium or eohippus ("dawn horse") was extremely small—only about the size of a dog. In addition, it had four hooves on its front feet and three on each rear foot, with all of its feet being padded, which is quite a contrast with the four unpadded, single-hoofed feet of the modern horse. These and other features, such as head size and shape, constitute such a marked difference from what we know about horses today that many scientists have questioned the status of eohippus as an equine ancestor. However, comparison with fossils from later, also extinct, horses shows a clear line of descent marked by an increase in body size, a decrease in the number of hooves, an elimination of foot pads, lengthening of the legs and fusion of the bones within, development of new teeth suited for eating grass, an increase in the length of the muzzle, and a growth in both the size and development of the brain.

Of course, this was not a clear-cut, neat, and steadily unfolding process, and some features appeared abruptly; still, the progression is there to be observed in the fossil record. Over the course of the many millions of years since eohippus, species have emerged that were distinguished by a particular feature—for example, teeth size and shape—only to disappear if conditions favored species with other traits. Evolutionary lines have branched off, with some dead-ending, and others continuing.

Thus, during the Miocene epoch, which lasted from about 26 million to 7 million years ago, various evolutionary branches competed for a time until the emergence of Parahippus. This species had teeth adapted for eating grass, in contrast to those of earlier horse ancestors, which grazed on leaves and other types of vegetation that did not require strong teeth. After Parahippus came Merychippus, which resembled a modern pony, and from which came numerous late-Miocene evolutionary lines. Most of these were three-toed, but Pliohippus had one toe per foot, and it was from this form that the genus Equus (which today includes horses, donkeys, and zebras) began to emerge in the late Pliocene epoch about 3 million years ago.

INDUSTRIAL MELANISM AND THE PEPPER MOTH.

Despite the staggering spans of time involved in evolution, one need not look back billions of years to see evolution at work. Both natural selection and mutation play a role in industrial melanism, a phenomenon whereby the processes of evolution can be witnessed within the scale of a human lifetime. Industrial melanism is the high level of occurrence of dark, or melanic, individuals from a particular species (usually insects) within a geographic region noted for its high levels of dark-colored industrial pollution.

With so much pollution in the air, trees tend to be darkened, and thus a dark moth stands a much greater chance of surviving, because predators will be less able to see it. At the same time, there is a mutation that produces dark-colored moths, and in this particular situation, these melanic varieties are selected naturally. On the other hand, in a relatively unpolluted region, the lighter-colored individuals of the same species tend to have the advantage, and therefore natural selection does not favor the mutation.

The best-known example of industrial melanism occurred in a species known as the pepper moth, or Biston betularia, which usually lives on trees covered with lichen. (An example of a lichen is reindeer "moss"; see Symbiosis.) Prior to the beginnings of the Industrial Revolution in England during the late eighteenth century, the proportion of light-colored pepper moths was much higher than that of dark-colored ones, both of which were members of the same species differentiated only by appearance.

As the Industrial Revolution got into full swing during the 1800s, factory smokestacks put so much soot into the air in some parts of England that it killed the lichen on the trees, and by the 1950s, most pepper moths were dark-colored. It was at that point that Bernard Kettlewell (1907-1979), a British geneticist and entomologist (a scientist who studies insects), formed the hypothesis that the pepper moths' coloration protected them from predators, namely birds.

Kettlewell therefore reasoned that, before pollution appeared in mass quantities, light-colored moths had been the ones best equipped to protect themselves because they were camouflaged against the lichen on the trees. After the beginnings of the Industrial Revolution, however, the presence of soot on the trees meant that light-colored moths would stand out, and therefore it was best for a moth to be dark in color. This in turn meant that natural selection had favored the dark moths.

In making his hypothesis, Kettlewell predicted that he would find more dark moths than light moths in polluted areas, and more light than dark ones in places that were unpolluted by factory soot. As it turned out, dark moths outnumbered light moths two-to-one in industrialized areas, while the ratios were reversed in unpolluted regions, confirming his predictions. To further test his hypothesis, Kettlewell set up hidden cameras pointed at trees in both polluted and unpolluted areas. The resulting films showed birds preying on light moths in the polluted region, and dark moths in the unpolluted one—again, fitting Kettlewell's predictions.

ANGIOSPERMS AND GYMNOSPERMS.

A final interesting example of natural selection at work lies in the comparative success rates of angiosperms and gymnosperms. An angiosperm is a type of plant that produces flowers during sexual reproduction, whereas a gymnosperm reproduces sexually through the use of seeds that are exposed, for instance in a cone. Angiosperms are a beautiful example of how a particular group of organisms can adapt to its environment and do so in a much more efficient way than that of its evolutionary forebears. On the other hand, gymnosperms, with their much less efficient form of reproduction, perhaps one day will go the way of the dinosaur.

Flowering plants evolved only about 130 million years ago, by which time Earth long since had been dominated by another variety of seed-producing plant, the gymnosperm, of which pines and firs are an example. Yet in a relatively short period of time, geologically speaking, angiosperms have become the dominant plants in the world. In fact, about 80% of all living plant species are flowering plants. Why did this happen? It happened because angiosperms developed a means whereby they coexist more favorably than gymnosperms with the insect and animal life in their environments.

Gymnosperms produce their seeds on the surface of leaflike structures, and this makes the seeds vulnerable to physical damage and drying as the wind whips the branches back and forth. Furthermore, insects and other animals view gymnosperm seeds as a source of nutrition. In an angiosperm, by contrast, the seeds are tucked safely away inside the ovary. Furthermore, the evolution of the flower not only has added a great deal of beauty to the world but also has provided a highly successful mechanism for sexual reproduction. This sexual reproduction makes it possible for new genetic variations to develop, as genetic material from two individuals of differing ancestry come together to produce new offspring. (For more about angiosperms and gymnosperms, see Ecosystems and Ecology.)

WHERE TO LEARN MORE

Campbell, Neil A., Lawrence G. Mitchell, and Jane B. Reece. Biology: Concepts and Connections. 2nd ed. Menlo Park, CA: Benjamin/Cummings, 1997.

Darwin, Charles, and Richard E. Leakey. The Illustrated Origin of Species. New York: Hill and Wang, 1979.

Dennett, Daniel Clement. Darwin's Dangerous Idea: Evolution and the Meanings of Life. New York: Simon and Schuster, 1996.

Evolution and Natural Selection (Web site). <http://www.sprl.umich.edu/GCL/paper_to_html/selection.html>.

Evolution. British Broadcasting Corporation (Web site). <http://www.bbc.co.uk/education/darwin/index.shtml>.

"Evolution FAQs." Talk Origins (Web site). <http://www.talkorigins.org/origins/faqs-evolution.html>.

Evolution. Public Broadcasting System (Web site). <http://www.pbs.org/wgbh/evolution/>.

Evolution. University of California, Berkeley, Museum of Paleontology (Web site). <http://www.ucmp.berkeley.edu/history/evolution.html>.

Levy, Charles K. Evolutionary Wars: A Three-Billion-Year Arms Race. The Battle of Species on Land, at Sea, and in the Air. New York: W. H. Freeman, 1999.

Starr, Cecie, and Ralph Taggart. Biology: The Unity and Diversity of Life. 7th ed. Belmont, CA: Wadsworth, 1995.

KEY TERMS

ACQUIRED CHARACTERISTICS:

Sometimes known as acquired characters or Lamarckism after one of its leading proponents, the French natural philosopher Jean Baptiste de Lamarck (1744-1829), acquired characteristics is a fallacy that should not be confused with mutation. Acquired characteristics theory maintains that changes that occur in an organism's overall anatomy (as opposed to changes in its DNA) can be passed on to offspring.

DATING:

Any effort directed toward finding the age of a particular item or phenomenon. Methods of dating are either relative (that is, comparative and usually based on rock strata, or layers) or absolute. The latter, based on methods such as the study of radioactive isotopes, typically is rendered in terms of actual years or millions of years.

DNA:

Deoxyribonucleic acid, a molecule in all cells and in many viruses that contains genetic codes for inheritance.

FOSSIL:

The mineralized remains of any prehistoric life-form, especially those preserved in rock before the end of the lastice age.

FOSSILIZATION:

The process by which a once living organism becomes a fossil. Generally, fossilization involves mineralization of the organism's hard portions, such as bones, teeth, and shells.

GENE:

A unit of information about a particular heritable (capable of being inherited) trait that is passed from parent to offspring, stored in DNA molecules called chromosomes.

GEOLOGIC TIME:

The vast stretch of time over which Earth's geologic development has occurred. This span (about 4.6 billion years) dwarfs the history of human existence, which is only about 2.5 million years. Much smaller still is the span of human civilization, only about 5,500 years.

HYPOTHESIS:

An unproven statement regarding an observed phenomenon.

INDUSTRIAL MELANISM:

The high level of occurrence of dark, or melanic, individuals from a particular species (usually insects) within a geographic region noted for its high levels of dark-colored industrial pollution.

INVERTEBRATE:

An animal without an internal skeleton.

LAW:

A scientific principle that is shown always to be the case and for which no exceptions are deemed possible.

MINERALIZATION:

A series of changes experienced by a once living organism during fossilization. In mineralization, minerals in the organism are replaced or augmented by different minerals or the hard portions of the organism dissolve completely.

MUTATION:

Alteration in the physical structure of an organism's DNA, resulting in a genetic change that can be inherited.

NATURAL SELECTION:

The process whereby some organisms thrive and others perish, depending on their degree of adaptation to a particular environment.

PALEONTOLOGY:

The study of life-forms from the distant past, primarily as revealed through the fossilized remains of plants and animals.

SCIENTIFIC METHOD:

A set of principles and procedures for systematic study that includes observation; the formation of hypotheses, theories, and laws; and continual testing and reexamination.

THEORY:

A general statement derived from a hypothesis that has withstood sufficient testing.

VERTEBRATE:

An animal with an internal skeleton.

Science of Everyday Things

Evolution

views updated May 08 2018

Evolution

Historical background

The modern synthesis

Evidence for evolution

Evolutionary mechanisms

Species diversity and speciation

Phylogenetic systematics: Reconstructing evolutionary history

Resources

Evolution is the natural process by which all living things have descended with modifications from shared ancestors, shaped largely by natural selection. It involves change in the genetic constitution of populations over time due to the systematic filtering by natural selection of randomly occurring changes in DNA. These changes are passed on from parents to their offspring.

Evolution does not involve individual organisms. Individuals develop, but they do not evolve. The study of evolution encompasses two major facets: deducing the history of descent among living things (phylogenetic systematics) and investigating evolutionary

mechanisms as revealed by laboratory and field studies of living organisms and their fossilized remains. Evolutionary change occurs as a result of beneficial mutation, migration, genetic drift and natural selection, and it is ultimately a passive process, devoid of any purpose or goal. As a scientific theory, it is an interconnected series of hypotheses, corroborated by a large body of evidence. Thus, biologists accept the historical reality of evolution, even though many details that are unclear continue to be investigated.

Historical background

The birth of modern evolutionary theory can be traced to the mid-nineteenth century and the publication of Charles Darwin’s On the Origin of Species by Means of Natural Selection in 1859. The book is considered by some to be the most influential in all of biology and by others an unsubstantiated theory. It was not, however, a new concept. Even in the late eighteenth century, the French scientist Maupertuis, the philosopher Diderot, and others entertained the notion that new life-forms might arise by natural means, including spontaneous generation. Their definition of evolution more closely resembles our modern concept of development, since the course of evolution is unpredictable. Interestingly, Darwin did not use the word “evolution” in The Origin of Species, except in the form “evolved,” the last word of the book. He preferred the bulkier but more precise phrase “descent with modification.”

French naturalist Jean-Baptiste Lamarck (1744–1829) was the first to clearly articulate the theory that species could change over time into new species, a process later called speciation. In his 1809 book Philosophie zoologique, he argued that living things progress inevitably toward greater perfection and complexity, through a process guided by the natural environment. A changing environment would alter the needs of living creatures, which would respond by using certain organs or body parts more or less. The use or disuse of these parts would consequently alter their size or shape, a change that would be inherited by the creatures offspring. Lamarck, although not the first, discussed this notion of “acquired characters,” which his contemporaries—including Darwin, to some extent—accepted. It is unfortunate that Lamarck is famous today primarily because he was wrong in this belief, in that it implies that bodily changes acquired during an individuals lifetime can affect the inherited material (now known as DNA) and be inherited. Although contemporary biologists have learned that changes to cell chemistry associated with DNA can, to some extent, be passed on from one generation to the next—a phenomenon called epigenetic change—Lamarck’s belief that this effect could be the primary driving engine of evolution was incorrect. Darwin accepted the inheritence of acquired characters, but argued (correctly) that natural selection was the main source of evolutionary novelty, not Lamarckian change.

Charles Darwin (1809–1882) and his contemporary Alfred Russell Wallace (1823–1913) are credited with independently providing the first plausible theory for a mechanism to explain evolutionary change, which they called natural selection. However, Wallace did not present so great a synthesis, or consider all the ramifications of evolution, as Darwin did in The Origin of Species and his later works. One major difference between the two men was that Wallace did not believe that natural selection could have produced the human brain; Darwin rejected his colleague’s contention that human intellect could only have been created directly by a higher power.

In The Origin of Species, Darwin concluded that some individuals in a species are better equipped to find food, survive disease, and escape predators than others. He reasoned that if the differences between the survivors and the doomed are passed from parents to their offspring, these differences will be inherited and gradually will characterize the whole population. Darwin knew that the greatest weakness in his theory was his lack of knowledge about the mechanics of inheritance; his attempt at a theory of inheritance, known as pangenesis, was later refuted. The nature of genes and DNA were unknown in Darwin’s time. However, he did clarify the distinction between genotype and phenotype. The genotype is a set of genetic instructions for creating an organism; the phenotype is the observable traits manifested by those instructions. The issue that explains the mechanism of inheritance, however, went unsolved for several decades later.

The modern synthesis

By the 1920s, Mendel’s theory of heredity had been rediscovered, but the early Mendelians, such as William Bateson and Hugo de Vries, opposed Darwin’s theory of gradual evolutionary change. These geneticists favored the position that evolution proceeds in large jumps, or macro-mutations, rather than the gradual, incremental changes proposed by Darwin. The dispute between the Mendelians and the Darwinians was reconciled by important theoretical work published in the early 1930s by independent population geneticists Ronald A. Fisher (1890–1962), J.B.S. Haldane (1892–1964), and Sewall Wright (1889–1988). Their classic work showed that natural selection was compatible with the laws of Mendelian inheritance.

This reconciliation gave rise to a period of prolific research in genetics as the basis of evolution. Theodosius Dobzhansky (1900–1975), a Russian emigrant to the United States, began his classic investigations of evolution in fruit fly populations, and in 1937 published a book entitled Genetics and the Origin of Species, which has been among the most influential books in modern genetics. Others, like E. B. Ford (1901–1988) and H. B. D. Kettlewell (1901–1979), helped pioneer the subject Ford called “ecological genetics.” This was a fundamental concept because it brought together two formerly separate areas of investigation: ecological variation (the influence of different environmental conditions) and genetic variation (differences in genetic make-up) in natural populations.

Evidence for evolution

The evidence for evolution is vast and of many kinds. Scientists continue to wrangle happily over the details of how evolution happened, but the fact that it did happen (and is still happening) is as well-established as the nature of the solar system or any other major component of the scientific world-story.

Darwin recognized that some of the best evidence of evolution is hidden within the bodies of living creatures. He reasoned that, if organisms have a history, then their history can be deciphered from their remnants. In fact, virtually all living creatures possess vestigial or rudimentary features that were once functional traits of their ancestors. Fetal whales, still in their mothers’ womb, produce teeth like all vertebrates, only to reabsorb them in preparation for a life filtering plankton from their ocean habitat. Snakes, whose vertebrate ancestors ceased walking on four legs millions of years ago, still possess vestigial hind limbs, with reduced hip and thigh bones.

In some cases the same structures may be adapted for new uses. The cat’s paw, the dolphin’s flipper, the bat’s wing, and a human hand all contain counterparts of the same five bones forming the digits (fingers in humans). There is no known environmental or functional reason why there should be five digits. In theory, there could just as easily be four, or seven. The point is that the ancestor to all tetrapods (vertebrates with four legs) had five digits, and thus all living tetrapods have that number, although in a modified form. Such traits, when they reflect shared ancestry, are known as homologous structures or characters. Traits that have functional but not truly ancestral similarity, such as the wings of insects and the wings of birds, are known as analogous characteristics.

Further evidence of evolution involves the most basic homology of all, the genetic code, or the molecular “language” that is shared by all living things. Although the genes of each kind of organism may be represented by different “words,” coding for each species unique structures, the molecular alphabet forming these words is the same for all living things on Earth. This is interpreted as evidence that the genetic code arose in a way that a single organism was the ancestor to all organisms that exist today.

Finally, there is the evidence from the fossil records that the remains of invertebrates, plants, and animals appear in the rocky layers of Earth’s crust in the same order that their anatomical complexity suggests they should, with the more primitive organisms in the older layers and the more complex organisms in the more recent deposits. No one has ever found a flowering plant or a mammal in deposits from the Devonian age, for instance, because those organisms did not appear on Earth until much later.

This is the merest sketch of the evidence for evolution. In an important sense, all of modern biology is evidence for evolution, because evolution is the organizing principle of modern biology, the concept by which biologists make sense of millions of diverse phenomena. Evolution is used to understand the behavior of disease germs, insects developing resistance to insecticides, the origins of genetic disease, DNA, and literally everything else studied by the life sciencese. In the famous words of Theodosius Dobzhansky, evolutionary biologist (and Orthodox Christian), “Nothing in biology makes sense except in the light of evolution.”

Evolutionary mechanisms

To an evolutionary biologist, evolution is a change in the proportion of genes present in an existing population. Each individual in a breeding population possesses a unique genotype, the set of paired genetic alternatives that determines its physical attributes, or phenotype. According to a principle known as the Hardy-Weinberg equilibrium, if certain conditions prevail, the frequency of each genotype in the population will be constant over generations, unless evolution has occurred to cause a shift in the gene frequencies. What factors can cause the proportion of genotypes in a population to change?

Natural selection is one example. An organisms environment—including its habitat, diseases, predators, and others of its own kind—present numerous challenges to its survival and to its ability to reproduce. Individual organisms of the same species have slightly different phenotypes (observable attributes), and some of these individuals may be slightly better equipped to survive the hazards of existence than others. The survivors produce proportionally more offspring than do the others of their species, and so their traits will be better represented in subsequent generations. These traits lead to an outnumbered or skewed representation of the offspring compared to less successful individuals (in terms of reproductive capacities) with alternative genetic complements. The genotype that is better adapted to the prevailing conditions will spread throughout the population—at least until conditions change and new genotypes are favored.

Another means by which gene frequencies may change is genetic mutation. When organisms reproduce, a copy of the DNA from each parent is transmitted to the offspring. Normally, the parental DNA is copied exactly; but occasionally, errors occur during replication in their sex cells, the egg and the sperm. Many of these errors occur in non-coding regions of the DNA, and so may have no known effect on the offspring’s phenotype; others may be lethal. In some cases, however, an offspring with a slightly modified genetic makeup survives. Mutation is thought to be an important source of new variation, particularly in rare cases where the mutation confers a selective survival advantage or a gain of function. After all, not all mutations are disadvantageous. In fact, nature’s method for ensuring the survival of a species is to rely on gene mutations that might enhance the function of the protein it encodes in such a way that there is a survival advantage. The determination of this advantage does not only rely on the type of mutation but the environment circumstances that prevail.

Evolution may also occur by genetic drift, a random process in which gene frequencies fluctuate due to chance alone. For example, if a certain genetic background is over-represented in one generation, perhaps because the region happened to be colonized by a small, homogeneous population, these genetic complements are more likely to remain high in the future generations. Genetic drift has more potent effects in small, isolated populations where some characteristic are not necessarily the result of natural selection and therefore do not represent a selective survival advantage.

Finally, the proportions of genotypes in a population may change as a result of migration, resulting in gene flow, or the movement of individuals (and their sex cells) into and out of the population.

Species diversity and speciation

Biologists have estimated that there are as many as 50 million species living on Earth today. Remarkably, it is believed that this figure is only 1% of the species that have ever lived. The great diversity of organisms, living and extinct, is the result of speciation, the splitting and divergence of biological lineages to create new species.

The term “species” is derived from the Latin word for kind, and species are considered to be the fundamental natural units of life which can and will evolve. Species have been defined in various ways over the years, but the most widely used definition at present is the biological species concept. Most often associated with the name of Ernst Mayr (1904-), it defines a species as a group of interbreeding populations that is reproductively separate from other such groups.

The crucial event for the origin of new species is reproductive separation. How do barriers to reproduction emerge? Perhaps a homogeneous population becomes separated into two distinct populations due to environmental factors. For example, when a river divides the species into two geographically distinct populations. Once they are separated, one or more evolutionary mechanisms may act on each in different ways over time, with the result that the two will diverge so they can no longer interbreed. As another example, perhaps a genetic variant arises in a population of interbreeding individuals by mutation or immigration; if the new variant spreads, bearers of the new variant may begin to breed preferentially with other bearers, preferring the new type. Along the way, further ecological, behavioral, and phenotypic changes may occur. Over many years, the organisms can diverge into two distinct species based on a dually but mutually exclusive preferential mating selection process.

Phylogenetic systematics: Reconstructing evolutionary history

The process of classifying and reconstructing the evolutionary history, or phylogeny, of organisms is known as phylogenetic systematics. Its goal is to group species in ways that reflect a common ancestry. The members of each group, or taxon, share uniquely derived characteristics that have arisen only once. For instance, the taxon Amniota includes amphibians, reptiles, birds and mammals, all of which arose from a single common ancestor that possessed an amnion in the egg stage. Classifying species involves only one apect of phylogenetic systematics. Understanding the evolutionary interrelationships of organisms by investigating the mechanisms leading to diversification of life and the changes that take place over time encompasses phylogenetic systematics. Systematics exceeds taxonomy or naming groups within species by attempting to develop new theories to describe the possible mechanisms of evolution. Historical remnants leave residual clues that allow phylogeneticists to piece together using hypotheses and models to describe history and how organisms evolve.

KEY TERMS

Genetic drift— Random change in gene frequencies in a population.

Genotype— The full set of paired genetic elements carried by each individual, representing the its genetic blueprint.

Hardy-Weinberg equilibrium— The principle that, if certain conditions prevail, the relative proportions of each genotype in a breeding population will be constant across generations.

Macromutation— Mutation having a pronounced phenotypic effect, one that produces an individual that is well outside the norm for the species as it existed previously.

Mutation— Alteration in the physical structure of the DNA, resulting in a genetic change that can be inherited.

Natural selection— The process of differential reproduction, in which some phenotypes are better suited to life in the current environment.

Nucleotide— Molecular unit that is the building block of DNA.

Phenotype— The outward manifestation of the genotype, including an organism’s morphological, physiological, and many behavioral attributes.

Phylogeny— Branching tree diagram representing the evolutionary history of an organism, and its relationship to others via common ancestry.

Speciation— The divergence of evolutionary lineages, and creation of new species.

Systematics— The classification of organisms according to their evolutionary relationships, and shared genetic and phenotypic characteristics.

Taxon (taxa)— Group of related organisms at one of several levels, such as the family Hominidae, the genus Homo, or the species Homo sapiens.

Tetrapod— The group of vertebrates having four legs, including amphibians, reptiles, mammals and birds.

of one or more genes in the species and use this to contribute to determining the appropriate phylogeny. Comparison of the differences in these DNA sequences provides an estimate of the time elapsed since the divergence of the two lineages. With the ability to sequence an entire organisms genome, the remnants of various species DNA, extinct (depending on the quality of the DNA) and living, can be compared and contrasted. This has exciting implications in the analysis of evolutionary modeling and applications to phylogenetic systematics.

Resources

BOOKS

Carroll, Sean B. The Making of the Fitteset: DNA and the Ultimate Forensic Record of Evolution. New York: W.W. Norton, 2006.

Gould, Stephen J. The Structure of Evolutionary Theory. Cambridge, MA: Harvard University Press, 2002.

Mindell, David P. The Evolving World: Evolution in Everyday Life. Cambridge, MA: Harvard University Press, 2006.

Ridley, Mark. Evolution. New York: Oxford University Press, USA, 2004.

Scientific American, ed. Evolution: A Scientific American Reader. Chicago: University of Chicago Press, 2006.

Shermer, Michael. Why Darwin Matters: The Case Against Intelligent Design. New York: Times Books, 2006.

OTHER

National Geographic. “Was Darwin Wrong?” 2005. <http://magma.nationalgeographic.com/ngm/0411/feature1/index.html?fs=www7.nationalgeographic.com> (accessed October 31, 2006).

PBS. “Evolution Library.” 2001. <http://www.pbs.org/wgbh/evolution/library/index.html> (accessed October 31, 2006).

TalkOrigins.org. “29 Evidences for Macroevolution.” August 9, 2006. <http://www.talkorigins.org/faqs/comdesc/> (accessed October 31, 2006).

Susan Andrew

The Gale Encyclopedia of Science Andrew, Susan

Evolution

views updated Jun 27 2018

Evolution

The remains of stadiums, temples, and aqueducts indicate as clearly as any ancient document that the Roman Empire once existed. Likewise, fossils speak eloquently of a time when dinosaurs and not humans dominated Earth. Even without ancient ruins, similarities in appearance, language, customs, and genetic makeup show that the Italians, Spanish, English, and French all came from the same ancestral culture. Likewise, similarities in structure and genetic makeup persuade humankind that algae and plants, insects and crustaceans, chimpanzees and humans came from the same ancestral species.

Evolution, which can be defined as the natural change in the inherited characteristics of groups of organisms, is as well established as the Roman Empire or any other event that is accepted as fact. Unfortunately, the common phrase "theory of evolution" has misled many people into believing that evolution is "only" a theory. To biologists, "theory of evolution" refers to a proposal about how evolution occurs, not whether it occurs. There are, in fact, several theories of evolution. Like evolution itself, some of these theories are well supported by observations and experiments.

Development of Evolutionary Theory

Evolution is generally associated with Charles Darwin (1809–1882), but by the time he wrote about it in 1858, it had already been suggested by many people. In fact, Charles Darwin's grandfather, Erasmus Darwin (1731–1802), was one of many who suggested that living species had descended from different species that had lived in the past. His theory of how evolution occurred was similar to that of French biologist Jean-Baptiste Lamarck (1744–1829) and was based on the belief that characteristics that develop in an adult can be passed on to its offspring. Thus, for example, giraffes could have evolved because their short-necked ancestors stretched their necks to reach higher leaves and therefore had offspring with longer necks. Both Lamarck and Erasmus Darwin were ignored, scorned, and ridiculed for this idea.

Charles Darwin was well aware of the controversy over evolution. As a theology student at Cambridge University with a passion for biology, he heard his professors dismiss evolution as nonsense, and he saw no reason to doubt them. Between 1831 and 1836, however, while serving as naturalist on an around-the-world voyage of The Beagle, young Darwin made observations that convinced him that evolution had, in fact, occurred. He saw that the fossil animals in parts of South America were different from, but similar to, the animals still living there. This gave Darwin the idea that living organisms were descendants of extinct ones that had lived in the same place in the past.

Darwin also observed that regions isolated from each other often had different but similar species. He noted, for example, that each of the Galapagos Islands had distinct species of mockingbirds. This suggested that all were descendants of the same ancestral species, and each had taken its own evolutionary path after being separated from the others. Darwin was also influenced by reading Principles of Geology by Charles Lyell (1797–1875). Lyell argued convincingly that geological changes were not caused by sudden global catastrophes, as most geologists then thought, but by gradual processes like erosion. This made Darwin realize that evolution must also have been gradual, otherwise organisms could not have remained adapted to their changing environments.

While in the Galapagos, Darwin did not come up with any answers as to why the forms of life were so different in those remote islands from the rest of the world. But a couple of years later, in 1837, he wrote the following in his journal: "In July opened first notebook on transmutation of species. Had been greatly struck from about the month of previous March on character of South American fossils, and species on Galapagos Archipelago. These facts (especially the latter) are the origin of all my views."

Darwin eventually returned to England convinced of the reality of evolution. He knew, however, that no one else would believe it unless he could find a better theory to explain it than his grandfather and Lamarck had proposed. Since some of his relatives owned estates on which they had successfully altered domesticated animals by selective breeding, it occurred to Darwin that something like this artificial selection might explain evolution. But how could unconscious nature select which individuals would breed and which would not? Darwin studied agricultural journals, conducted breeding experiments, and pondered the question for months. Then one day in 1838 he decided to read ("for amusement," he says in his autobiography) the famous piece Essay on the Principle of Population (1798) by Thomas Malthus (1766–1834).

Natural Selection

The essential idea of this essay is now called the Malthusian Principle. It proposes that human population has a tendency to increase much faster than the food supply. Consequently, there will always be competition between those who can get food and those who cannot. Darwin saw in a flash that the same principle applies to all organisms. Virtually all species have the natural ability to produce many more offspring than can survive with the available resources. Within any species there will be some individuals that are better able to compete for food, mates, and other resources, and they will be more likely than others to produce more offspring. Scientists would now say that they have a greater fitness. To the extent that their fitness is hereditary, their offspring will also be better able to compete, and so on, generation after generation. In this way the fitter individuals become increasingly numerous, and the species gradually evolves. Darwin gave his theoretical mechanism of evolution the name "natural selection."

Natural selection may be the simplest yet most powerful theory in science. With it one can immediately see that evolution is not only possible but, given enough time, inescapable. All that is required is that there be competition among individuals of the same species, and that individual organisms have inherited traits that make some better able than others to compete. Darwin must have realized the importance of his theory. Rather than risk his budding reputation with a hasty report to a scientific journal, however, he began to accumulate supporting evidence for a book. Twenty years later he was still at work on his book when a remarkable coincidence forced him to publish. In 1858 he received a manuscript from an English collector in the East Indies, Alfred Russel Wallace (1823–1913).

WALLACE, ALFRED RUSSEL (1823–1913)

English-born naturalist and explorer who helped formulate the principles of biological evolution and natural selection. Wallace traveled more than 14,000 miles in the area that is now known as Indonesia and Malaysia and catalogued more than 125,000 biological specimens.

As Darwin read the manuscript he was stunned to see that Wallace had hit upon the same theory of natural selection that he had been laboring over for two decades. Darwin reluctantly agreed to publish an outline of his ideas along with Wallace's paper. (It was discovered later that the basic concept of evolution by natural selection had already been proposed almost thirty years earlier by a little-known Scotsman named Patrick Matthew [1790–1874]. Matthew had also been ignored.) Ultimately, what finally made the words "evolution" and "Darwinism" well known was Darwin's book, On the Origin of Species by Means of Natural Selection, which was published in 1859. Its vast documentation and powerful arguments soon convinced the majority of biologists that evolution is a fact, and natural selection is one of the reasons why it occurs.

HARDY, GODFREY HAROLD (1877–1947)

Professor of mathematics at Trinity College and the University of Oxford and a leading mathematician who recognized, shortly after Weinberg, the relevance of Mendel's laws of inheritance to the study of population genetics.

Since the publication of On the Origin of Species, few biologists have doubted that evolution occurs. By the early twentieth century, however, natural selection appeared to be heading toward extinction. One criticism of natural selection was that any adaptation that made an individual only slightly more fit would be diluted when the individual mated. For example, if a giraffe ancestor with a slightly longer neck mated with a normal member of its species, their offspring would have necks with lengths between that of the two parents. This reduction in neck length would continue with each generation. Thus any adaptation would be blended out of the species before natural selection would have a chance to favor it. In addition, beginning in 1900, genetic mutation seemed to provide an alternative theory that was better than natural selection. The discovery of the work of Gregor Mendel and further research on genetics suggested that new species resulted from large mutations occurring within a single generation instead of small mutations being selected over many generations.

Neo-Darwinism

By the middle of the twentieth century, however, biologists saw that Darwin's theory of natural selection was not really in conflict with genetics. They synthesized the two views, resulting in what is now called the neo-Darwinian or Synthetic Theory of Evolution. The neo-Darwinian theory was aided by a shift in thinking about the scale of evolution. Rather than conceiving of evolution as something that happened to entire species, biologists began to think of it as occurring within smaller groups of inter-breeding organisms, called populations. Most species comprise many populations.

The neo-Darwinian Theory was also made possible by a mathematical proof called the Hardy-Weinberg equilibrium. The Hardy-Weinberg equilibrium showed that adaptations would not be blended out of populations, and it also showed that natural selection was indeed a possible cause of evolution. This proof, which was proposed in 1908 independently by English mathematician G. H. Hardy (1877–1947) and German physician Wilhelm Weinberg (1862–1937), shows that under certain conditions even rare mutations persist indefinitely. In modern terms, scientists would say that the Hardy-Weinberg equilibrium shows that the gene frequency—the proportion of a particular type of gene in a population—will remain constant if certain conditions occur. These conditions are as follows:

The size of the population is practically infinite.
Individuals in the population mate at random.
All individuals in the population have the same fitness, regardless of their genes.
There is no gain or loss of genes due to immigration into or emigration out of the population.
There is no new mutation in the population.

Violating any one of these conditions can lead to a change in gene frequency. This is important because changes in gene frequency can result in evolution. In fact, many biologists now define evolution as any change in gene frequency. As an example, suppose a genetic mutation had caused an ancestor of giraffes to have a slightly longer neck. A departure from the Hardy-Weinberg conditions could continually increase the frequency of that mutated gene in the population. Gradually the entire population would have longer necks. This process repeated over thousands of generations could cause that population to evolve into the giraffe. The Hardy-Weinberg equilibrium therefore amounts to a list of conditions that, if absent, can cause evolution. The potential causes of evolution include small population size, nonrandom mating, natural selection, immigration and emigration, and mutation.

WEINBERG, WILHELM (1862–1937)

German physician, geneticist, medical statistician, and early founder of population genetics who demonstrated the importance of Mendel's laws to the genetic composition of populations.

Small Population Size. A change in gene frequency due to small population size is called genetic drift. Genetic drift is now recognized as one of the major causes of evolution, although its results are usually random rather than adaptive. Chance events operating in small populations can have huge effects on gene frequency. Imagine, for instance, an isolated population of a very rare, endangered species of mountain sheep, whose males have horns that are either curved or straight. If a severe snowstorm happened to kill the few sheep with genes for curved horns, the proportion of sheep with straight horns would increase greatly in future generations.

A related phenomenon, called a population bottleneck, occurs when a large population is decimated by disease, predation, or habitat destruction. The few surviving members constitute the "bottleneck" through which the species passes. The genes of those few members dominate the gene pool of future generations. Similarly, a population of organisms could differ from others simply because the few founders of the population happened to have a gene frequency different from that of the species as a whole. This is called the founder effect. The wide differences in blood group frequencies between the Old Order Amish of Pennsylvania and other U.S. populations of European ancestry is due to the founder effect operating in the Amish population. The role of genetic drift in species formation is an important area of research in evolution.

Nonrandom Mating. A second potential cause of evolution is nonrandom mating. Nonrandom mating usually occurs when individuals choose their mates. Animals often select mates on the basis of fitness, and the results of such sexual selection are indistinguishable from natural selection. On the other hand, mate selection can be based on characteristics that have nothing to do with fitness. For example, the tail feathers of the peacock or the bright coloration of the male pheasant are not thought to confer selective advantage in any arena other than mate selection. But because females choose the showier bird, the trait is selected for in males. This is called sexual selection.

Natural Selection. Natural selection, which is due to hereditary differences in fitness, is a third potential cause of evolution, as Charles Darwin argued. Natural selection is now considered to be the main, if not the only, cause of the evolution of adaptations that increase fitness. For example, the speed of the gazelle and the cheetah that chases it are both due to natural selection.

The Africanized honey bee was first found in the United States near Brownsville, Texas, in 1990. Since that time, the bees have spread throughout the state. They've also been found in Arizona, California and New Mexico.

Immigration and Emigration. Immigration and emigration can bring in or remove particular genes. The global travel of human beings has increased the importance of these forces not only in human populations, but in many other species that travel with humans, such as Africanized honey bees. The so-called killer bees from Africa are currently changing the gene frequencies of bee populations in the southern United States.

Mutation. Finally, mutation can obviously change the frequency of a gene. Mutation can be especially potent when combined with genetic drift in small populations.

Mutation

As noted earlier, many biologists once thought that mutation by itself was the major cause of evolution. In the 1920s, however, British biologist J. B. S. Haldane (1892–1964), British statistician Ronald A. Fisher (1890–1962), and American geneticist Sewall Wright (1889–1988) published three different mathematical proofs showing that mutation by itself is insufficient. They showed that a rate of mutation fast enough to cause evolution would also be fast enough to undo any evolution that had happened in the past. Scientists now know that mutations are too rare (about one per billionnucleotides per human lifetime) to account for most evolutionary change without the help of natural selection. Also, contrary to what Erasmus Darwin and Lamarck thought, scientists know of no way that the efforts or experience of an organism can induce specific, adaptive mutations in its offspring.

For a time, many biologists thought that natural selection was so rigorous that it would eliminate most mutations since most mutations were presumed to be harmful. Starting in the 1950s, however, it was found that genetic variations resulting from past mutations are quite abundant in most species. Most mutations have little effect on fitness, and they can accumulate generation after generation with little selection against them. With increased competition or some change in the environment, however, some of these mutations may result in differences in fitness. Natural selection can then bring about evolution by increasing the frequency of the beneficial mutations. Natural selection therefore seldom has to sit and wait for just the right mutation to come along and make an individual more fit. The mutations are usually already present in most populations.

Microevolution and Macroevolution

Changes in gene frequency that occur within a population without producing a new species are called microevolution. As microevolution continues, a population may become so different that it is no longer able to reproduce with members of other populations. At that point, the population becomes a new species. As the new species continues to evolve, biologists might eventually consider it to be a new genus, order, family, or higher level of classification. Such evolution at the level of species or higher is called macroevolution.

Microevolution can occur very quickly; indeed, it is probably always occurring. For example, in less than half a century after the discovery of antibiotics, many bacteria evolved resistance to them. Resistance to antibiotics evolves when antibiotics are used improperly, allowing the survival of a few bacteria with mutated genes that confer resistance. Natural selection then leads to the evolution of antibiotic-resistant strains. Pesticide-resistant insects and herbicide-resistant weeds are additional examples of rapid microevolution.

Macroevolution occurs over much longer periods and is seldom observed within the human life span. Occasionally, however, scientists do see evidence that new species have recently evolved. There are species of parasitic insects, for example, that are unable to reproduce except in domesticated plants that did not even exist a few centuries ago. The pace of evolution can be quite variable, with long periods in which there is little change being punctuated by relatively brief periods of tens of thousands of years in which most changes occur. This idea that the pace of evolution is not always slow and constant is referred to as punctuated equilibrium . It was first proposed by paleontologists Niles Eldredge and Stephen Jay Gould in 1979, and it is one of many examples of how scientists' views of evolution are continually changing.

Several possible mechanisms exist for rapid evolution. Chromosomal aberrations, such as breakages and rejoining of chromosomal parts, can introduce large changes in genes and the sequences that regulate them. This may lead to changes much larger than that brought about by simple point mutations.

Environmental catastrophes can set the stage for rapid evolution as well. It is thought that the extinction of the dinosaurs was triggered by a large comet impact. This rapid loss of the dominant fauna in many ecosystems opened up many new niches for mammals, which at the time were a small group of fairly unimportant creatures. The sudden appearance of many new opportunities led to rapid and widespread speciation, in a process called adaptive radiation .

Other areas of biology are also continually changing under the influence of evolution. For example, as Charles Darwin predicted in The Origin of Species, classification has become more than simply the grouping of organisms into species, genera, families, and so on based on how physically similar they are. Classification now aims to group species according to their evolutionary history. Thus two species that diverged recently from the same ancestor should be in the same genus, whereas species that shared a more distant common ancestor might be in different genera or higher taxonomic levels.

Until the 1980s, evolutionary history, or phylogeny, of organisms could only be inferred from anatomical similarities. Since that time, however, it has been possible to determine phylogeny from comparisons of molecules. Often this molecular phylogeny agrees with the phylogeny based on anatomy. For example, about 99 percent of the sequence of bases in the deoxyribonucleic acid (DNA) of chimpanzees and humans is identical. This finding confirms the conclusion from anatomy that chimpanzees and humans evolved from the same ancestor only a few million years ago. Such agreement between anatomical and molecular phylogeny would not be expected if each species were a totally different creation unrelated to other species, but it makes sense in light of evolution. It is one of many examples of the famous saying by the geneticist Theodosius Dobzhansky (1900–1975): "Nothing in biology makes sense except in light of evolution."

see also Adaptation; Buffon, Count (Georges-Louis Leclerc); Convergent Evolution; Darwin, Charles; Endangered Species; Evolution, Evidence for; Extinction; Hardy-Weinberg Equilibrium; Lamarck, Jean-Baptiste; Natural Selection; Speciation

C. Leon Harris

Bibliography

Dawkins, Richard, The Selfish Gene. Oxford: Oxford University Press, 1990.

Freeman, Scott, and Jon C. Herron. Evolutionary Analysis, 2nd ed. Upper Saddle River, NJ: Prentice Hall, 2001.

Futuyma, Douglas J. Evolutionary Biology, 3rd ed. Sunderland, MA: Sinauer Associates, 1998.

Stanhope, Judith. Hardy-Weinberg Equilibrium. <www.accessexcellence.org/AE/AEPC/WWC/1994/hwintro.html>.

Biology Harris, C. Leon

Evolution

views updated May 29 2018

EVOLUTION

EVOLUTION. Jean-Louis Flandrin, in his introduction to Food : A Culinary History, sets out many of the crucial questions basic to our understanding of the evolution of human diet:

When and how did the eating behavior of human beings diverge from that of other animal species? Did humans distinguish themselves by the type of variety of foods they ate? By the fact that they prepared their food before eating it? By the ceremonial forms with which they surrounded the act of eating? Or by the conviviality of dining and its characteristic social forms? (p. 14)

These questions, as they relate to the evolution of human foodways, remain unanswerable. A major reason is the vast gulf that separates the living from earlier ancestors. Today, virtually all humans subsist on the products of agricultural activities, which include the raising of domestic animals for food. However, this way of life developed very late in the course of human evolution, with the domestication of plants appearing in several locations around the world at some point after 12,000 years ago; the domestication of food animals followed somewhat later. The vast earlier time, during which humans evolved from more primitive beings, was marked by other forms of subsistence. This time span, more than six million years in duration, witnessed dramatic changes in human biology, behavior, and adaptation. Although we have a treasure trove of fossil bones and archaeological materials that document much of this development, there is little in the record that can inform us of the precise dietary items consumed by these remote ancestors of ours, or enable us to answer the questions posed by Flandrin. There are, however, tantalizing hints of the ways of life followed by these earliest members of the human family, and in this essay, this record will be described, and the available evidence for the evolution of human foodways evaluated.

The data at our disposal for this investigation include the fossil bones and teeth of our ancestors, testaments to their evolving biological structures. There are also the residues of their activities, in the very earliest deposits often preserved as parts of natural accumulations of organic and inorganic remains, jumbled in with the fossil bones of very early human ancestors. Later in time, we find the archaeological remains of the actual living areas, where our ancestors slept, made tools, prepared and ate their food, and often buried or left their dead. All this varied information provides important insights about our evolutionary past, but it is very incomplete data for reconstructing dietary patterns. For example, very little in the way of actual food remains is found during archaeological excavations, and only relatively durable items like animal bones are preserved. This may provide some indication of the presence of meat in the diet, but it is not clear just how much it represents the total subsistence pattern and how much was composed of other foods, like vegetables and insects, which leave no archaeological traces. Similarly, the bones and teeth of our ancestors may preserve chemical and other traces of the sorts of foods that were emphasized in their diets, but these signs are often complex and must be carefully evaluated.

Given the difficulties in deciphering the actual residues, other, more indirect, sources of information have come to play an important role in reconstructing the foodways of our ancestors. These data come from the study of our closest living primate relatives, the chimpanzees, and observations recorded from the anthropological studies of those few modern human groups, called gatherers and hunters, who did not practice agriculture, but subsisted on an assortment of gathered vegetable foods, the collection of small animals, such as insects and small vertebrates, and the occasional successful hunting of larger animals. Comparisons with these living examples are often used to furnish clues to what sorts of foods our ancestors consumed. However, correlations of this sort have numerous limitations, and they must be used with caution. Chimpanzees and humans have had separate evolutionary pathways for at least six million years, and it is possible that during this time, chimpanzees have changed as much as humans in their biology and adaptation, making comparisons of living chimpanzees with our earliest ancestors tenuous at best (we have no fossil record of the specific evolutionary history of chimpanzees). Further, those few living gatherers and hunters who have been studied exist in environments that may be dramatically different from the locales of our ancestors. Finally, and perhaps most importantly, our early ancestors were neither bipedal apes nor humans in fur suits, but a series of biologically and behaviorally unique species whose way of life and biology are now wholly extinct.

Both modern chimpanzees and those gatherers and hunters who have been studied, and do not live in very specialized environments (like the Arctic, for example), have somewhat similar diets. The field research by Jane Goodall and her associates on chimpanzees living in the Gombe National Park in western Tanzania, as well as observations from other chimpanzee living-sites in Africa, indicate that these animals are overwhelmingly vegetarians, with a broadly based diet composed, at the Gombe, of the fruits, leaves, stems, blossoms, and gums of more than eighty different plants. Chimpanzees, however, emphasize a variety of fruits as the major part of their diet. Chimpanzees have also been observed consuming insects, sometimes using twigs, specially broken off and trimmed as tools, to obtain termites. Chimpanzees (often males), behaving together in a cooperative fashion, also deliberately hunt, kill, and eat a variety of small vertebrates, including bush pigs, monkeys, and antelopes. Meat, however, makes up a very small percentage of their total diet.

Human gatherers and hunters in tropical or subtropical areas also subsist on a diet that emphasizes a broad array of vegetable food sources, with smaller amounts of insects and vertebrate animals. The exact percentage of each of these elements differs seasonally or yearly, as well as varying between specific groups.

Like living gatherers and hunters, until the advent of agriculture, our ancestors probably lived an unsettled existence, regularly shifting their encampments to new locales in search of resources. Food storage would have been very difficult, and consumption of collected and hunted foods was probably immediate. Groups would have been small, with the social organization flexible enough to allow group size to fluctuate with the seasonal availability of food and other resources.

These comparisons provide only a very limited insight, and for more information, it is necessary to examine the direct evidence from the archaeological and fossil records.

Diet and Human Evolution

A variety of comparative genetic studies document that chimpanzees are our closest living relative. It has been estimated, for example, that humans and chimpanzees share about 98.5 percent of their genetic material. Calculations of the rate of genetic change over time indicate that humans last shared a common ancestor with this African ape between five and eight million years ago. This is the period when the evolutionary line that eventually led to living humans split from the line that led to chimpanzees, representing the beginnings of human evolution. The living and extinct members of this human evolutionary lineage are traditionally grouped into a biological family, the Hominidae, members of which are known as hominids.

We have no fossil or other evidence of the earliest members of the hominid family, just after they split off from the lineage leading to chimpanzees. We do not know what sorts of environments they lived in or what sorts of foods they ate. Because chimpanzees are native to Africa, and the earliest known hominid fossils are limited to Africa, it seems reasonable to place the homeland of the human family on that continent.

The Earliest Hominids

The recognition of Africa as the human homeland first came in 1924, with the discovery of the fossilized skull and jaw of a young child at T'aung, in the Cape Province of South Africa. Named Australopithecus africanus by its discoverer, Raymond Dart, hundreds of additional fossil specimens of this group, known collectively as the australopithecines, have subsequently been uncovered in south, east, and central Africa. There are now at least eight species of australopithecines, sometimes placed in other genera, like Paranthropus or Kenyanthropus. The australopithecines lived in Africa from about four million to perhaps as late as one million years ago. Like all members of the hominid family, they walked upright, allowing them to efficiently carry objects and food. Chimpanzees habitually walk on all four legs. However, the australopithecines were apelike in many of their biological features, possessing small, chimpanzee-sized brains in an apelike skull with a large, projecting face positioned out in front of the braincase. Their teeth were human-like in form, but they possessed massive back chewing teeth, the premolars and molars, that were much larger than those of living humans. The australopithecines, like all hominids, possessed nonprojecting canine teeth. This is in marked contrast to the large, tusklike canines of the apes. Like gorillas, australopithecines also seem to have been sexually dimorphic in body size, with the males considerably larger than the females.

There are fossil bones found in East Africa of still earlier-in-time creatures, for example, Orrorin tugenensis, at six million years, possibly the earliest hominid yet discovered, and Ardipithecus ramidus, who lived about four and a half million years ago, but little is currently known about these creatures and their biology.

The fossil bones of the australopithecines are most often discovered in natural accumulations that are the result of various sorts of geological activities. These fossil bones may have been transported by water over long distances before they were deposited in their final location. They are only infrequently discovered in a context that represents the locale where they actually lived. Thus, little is known about the kinds of environments in which the australopithecines lived, or how the various australopithecine species may have differed in habitat usage or in food choice and general diet.

For many years after the initial discoveries of the australopithecines, there was a prevalent idea that these creatures lived on the open grasslands or savannas of eastern Africa. According to this theory, their habitat would have provided only a limited selection of foods, and was the selective factor responsible for the development of hunting and meat eating. More recent reconstructions, however, have revealed a much more complex environmental context for these early hominids, with evidence for the use of forests and woodlands. Just how important hunting and meat eating has been in human evolution continues to be debated, and its importance in the ultimate appearance of modern humans remains unclear.

Australopithecine fossil bones have been carefully examined in a number of ingenious ways, in order to learn more about their dietary patterns, but thus far with only limited success.

For example, on the basis of comparisons with the teeth of other mammals, it is clear that these early hominids were not specifically adapted to meat eating. As in modern humans, the chewing surfaces of the teeth are covered with thick layers of enamel. Some australopithecine species, known as the ''robust" australopithecines, possessed truly massive back teeth, along with very large jawbones to house them, and large chewing muscles, sometimes so large that they formed a crest on the top of the skull. These general biological features of australopithecine jaws and teeth suggest that they emphasized the chewing of coarse vegetable food sources, but not the consumption of grasses, whose high cellulose content would have been very difficult for these creatures to digest.

Other studies of the dentition have attempted to determine more specific aspects of the dietary patterns of the australopithecines. One series of studies utilized scanning electron microscopy to examine the minute scratches and pits left by food particles on the chewing surface of the teeth. The results of these observations suggest that some of the australopithecines ate a diet rich in fruits, while others were consuming a more varied, but basically vegetarian, diet. One problem with these sorts of studies is that they tend to focus on the final meals the creatures ate before they died, providing a somewhat limited view of their overall diet, especially if they were seasonally exploiting a variety of different habitats and foods.

Other studies have examined the chemical composition of australopithecine fossil bones. One study employed the ratio of calcium and strontium in the fossil bones to determine whether the australopithecines were generally herbivorous, carnivorous, or omnivorous.

Another chemical analysis, based on staple isotopes including ¹³C and ¹²C, has reached a conclusion similar to that from the calcium-strontium analyses: some australopithecines, at least, were consuming animal foods, though the identity of these animals, and whether they were vertebrates or invertebrates, has not been determined.

These studies continue to support a variety of opinions about the dietary patterns of these early hominids, with some anthropologists suggesting a diet based primarily on fleshy fruits, nuts, and seeds, while others advocate a more broadly based diet, including some animal foods.

There is no direct evidence that the australopithecines collected foods to be brought back to some central camp to be consumed as part of a group activity. Rather, like chimpanzees, it appears likely that they consumed food continuously as they foraged in their environment.

The Evolution of the Genus Homo

Good evidence of the evolution of members of our genus, Homo, begins to appear around two million years ago at sites in East Africa. There was a dramatic increase in brain size, from the 500 ml common in the australopithecines to brains as large as 800 ml in these early humans (though still about half the size of those of living people). They also possessed smaller back chewing teeth. Chipped stone tools, first used about two-and-a-half million years ago, now became more common. These durable tools, made from water-rounded pebbles, are known as Oldowan tools. They were made by striking two stones together, knocking off chips to produce a cutting edge or point. Though crudely made, their development represented a major advance in the ability of the early hominids to exploit a wider variety of food sources. Hominids lacked sharp and hardened claws, as well as projecting and pointed canine piercing teeth, making them inefficient in dealing with many potential food sources. For example, without a digging tool or claws, many subterranean foods like insects, small burrowing mammals, tubers, and rhizomes, would have been impossible to obtain. The australopithecines are only rarely found in association with these chipped pebble tools, and most anthropologists believe the first stone tool makers were early members of the human genus Homo.

Also found at this time are animal bones, mainly from antelopes, with butchery marks made by a sharp stone edge. Although isotopic studies have indicated that the earlier australopithecines may have consumed animal foods, these cut marks represent definitive evidence of early meat eating. What is still being debated is the origin of these bones. They may have been the result of hunting activities, which is entirely reasonable given our knowledge of the cooperative hunting patterns of chimpanzees, but some scientists have suggested that they may also have been the result of scavenging activities. A safe way, it is said, to obtain bones with scraps of meat still adhering to them would be to claim animal bones from a predator kill after primary scavengers, such as hyenas and jackals, have finished with them. Thus, the initial meat eating in human evolution, according to this view, was to utilize stone tools to scrape off bits of rotting tissue from the bones of predator kills. One major flaw with this notion is that no primate is equipped with digestive mechanisms to protect them from the serious consequences of eating spoiled meat.

By about 1.8 million years ago, there are a number of different species of early Homo coexisting in eastern Africa. In addition, several species of robust australopithecines were also living at this time. What the possible dietary differences, if any, between all these hominids is unknown.

Expansion Out of Africa

At some point after 1.8 million years ago, in one of the most momentous events in human evolution, the hominids begin to move out of Africa. One site along the Jordan River Valley in Israel, dated at about one and a half million years old, is located along what must have been a major route into Eurasia. Along with stone tools similar to those from Africa were found numerous bones of African mammals, suggesting that the hominids were not the only creatures moving out of that continent.

Hominid sites in the Republic of Georgia and on the island of Java also testify to this dramatic increase in range. Although the reasons the hominids left Africa at this moment are unclear, one reasonable explanation is that stone tools enabled hominids to expand the range of dietary items open for exploitation, allowing them to move into new habitats.

During the course of the next million years, hominid brain size increased, so that by about 300,000 to 400,000 years ago, the volume of the braincase reached 1,200 ml, within the range of living humans. It may be that there was an associated increase in body size during this period as well. Increasing brain size would have required greater intakes of oxygen, as well as nutrients. It has been suggested that this brain size expansion relied on increased amounts of dietary fats. Hunted animals could have supplied these fats, but gathered insects, many of which are richly endowed with this nutrient (especially the essential fatty acid, linoleic acid), are equally likely sources. Larger body size also necessitated a greater number of calories.

The occupation of the European subcontinent appears to have taken place later than human expansion into more hospitable habitats in Asia. This is no doubt related to the presence of glaciers, which, beginning about two million years ago, periodically covered major parts of Europe. The earliest occupation site in Europe, dating to about 800,000 years ago, is located in northern Spain, near the present city of Burgos. From that time onwards, hominid presence in Europe was closely tied to the advance and retreat of the glaciers, with the continent relatively uninhabited during times of maximal glacial activity.

By 500,000 years ago, hominids, placed in the category Homo erectus, were intermittently occupying a large cave on the outskirts of what is now the village of Zhoukoudian, about twenty-five miles from Beijing, in northern China. Although there was no glacial activity in this part of Asia, winter would have been severe (Zhoukoudian is about as far north as Philadelphia). While it remains unclear if hominids actually wintered this far north, the earliest well-documented evidence of fire has been found here. Fire allowed hominids to use food sources that would be uneatable, or actually toxic, without cooking. Burned deer bones, as well as those with cut marks, testify to the use of meat by the inhabitants of the cave, but whether the meat was obtained by hunting or scavenging remains unknown.

From about the same time, a hominid skull was found in Ethiopia with cut marks on its frontal bone, suggesting skinning or scalping. Cannibalism has been documented at a number of other, later-in-time hominid sites; was the flesh a part of the diet, or was eating a dead friend or relative part of a ritual?

Modern Human Origins

The last 200,000 years of human evolution are much richer in data because actual living places have been located and excavated. Prior to this time, only a very few sites, like Zhoukoudian, represented the remains of an encampment, where the evidence of hominid activities are directly preserved. By about 115,000 years ago, our ancestors had begun the practice of the deliberate burial of their dead, thereby reducing the risk that the body would be destroyed by scavengers. Burying the dead resulted in a vast increase of ancient skeletons that have been preserved for study.

There continues to be debate about the precise way by which living humans emerged from our earlier ancestry. Some anthropologists suggest that modern humans evolved from these earlier hominids and, thus, are the culmination of a very long evolutionary history in various geographic areas. For example, living Asians are the descendants of ancestors who reached Asia more than a million years ago.

Most anthropologists support another theory, that all modern humans originated in Africa some 100,000 to 300,000 years ago and, subsequently, spread out from there to populate the rest of the planet, replacing the earlier hominids who were already living in these areas, descendants of the much earlier initial expansion.

One extinct fossil group that has figured prominently in these theories is the Neanderthals, a group of hominids who lived in Europe and the Middle East from about 130,000 to about 30,000 years ago, when they disappeared from the scene. Because they lived in Europe, where the most intensive archaeological investigations have taken place over the last 150 years, we have much more evidence about these creatures than about any other fossil hominids. This has provided a rich data source, but it also has a number of serious limitations. The most important is that emphasizing the Neanderthals gives a very Eurocentric view of human origins. The final glaciation occurred during much of the time Neanderthals were in Europe; this made major portions of the continent uninhabitable. Those parts that could be occupied by humans represented marginal environments that would have limited population density to extremely low levels.

Given the harsh environments of Europe in which the Neanderthals were living, vegetable foods were probably relatively scarce through much of the year, and meat was almost certainly a major dietary resource. This is confirmed by chemical analyses of their bones, which indicate that for some Neanderthals, fully 80 percent of their diet came from meat. The bones of numerous large animals, such as deer, aurochs, wild boar, and horses are preserved at Neanderthal sites, along with smaller animals. At sites along the Mediterranean, shells testify to the consumption of seafoods. Our evidence for the diet of peoples contemporary with the Neanderthals, but living in Africa and southern Asia, remains limited. At one site, located on the very southern coast of Africa, Klasies River Mouth Cave, there is abundant evidence of the use of a variety of food resources, including land and sea animals and shellfish. Because much of our current evidence comes from humans, like the Neanderthals, who lived in a harsh environment, the emphasis on hunting and meat eating that has come to characterize the diets of earlier hominids may represent a very biased picture.

Although the precise evolutionary relationships of the Neanderthals to living humans remain shadowy, excavation of their sites has revealed a complex picture. Often, living areas with hearths and signs of social areas around them have been uncovered. The bones of selected parts of animals, often with butchery marks on them, are scattered about. Clearly, Neanderthals, like living human gatherers and hunters, were carrying back to a central camp chosen pieces of animals. They may also have brought back other dietary items from their foraging and hunting activities, but the relative absence of small animal bones suggests that they may have been consumed immediately where they were found. It is quite possible that they sat around a fire sharing and consuming food, perhaps engaging in the uniquely human dinnertime interactions of storytelling and discussions of the day's activities. It is unclear, however, if the Neanderthals were actually able to use language, so this reconstruction remains a tentative one.

Sometime after 40,000 years ago, modern human-like peoples appeared in Europe, perhaps migrating there from their origins in Africa, or developing from ancestors already living in Europe. These modern humans brought with them new sorts of tool-making technologies, based on a broader array of raw materials, such as ivory, bone, and wood, with a wider assortment of beautifully made stone tools that show far greater sophistication than those made by the Neanderthals. The first artistic expressions also made their appearance at this time, with plastic art in the form of ivory and bone carvings of animals and people. Deep inside caves, they produced engravings and painted images of animals, and occasionally humans, some of them of great genius.

The sites occupied by these modern humans are littered with the bones of the same sort of animals, the earlier Neanderthals hunted, but the concentrations of bones indicate greater skills in hunting and a corresponding larger number of captured animals. This is also the case with much larger accumulations of shellfish along the coast.

These early modern humans continued this sort of hunting activity to the end of the last glacial period, about 12,000 years ago. In Europe, the retreat of the glaciers resulted in the spread of forests and a major change in dietary habits, with peoples hunting forest animals, like deer and rabbit, and utilizing to a much greater extent the riches of the sea. By this time, however, peoples in the Middle East and along the Yangtze River Valley in southern China were beginning to experiment with the cultivation of plants, which represented the beginnings of the agricultural revolution, and formed the foundations of settled urban life and the origins of civilization.

Although this sketch brings together much of our current knowledge of the evolution of human foodways, much clearly remains to be learned. For one thing, it tells us little about how human diet changed from eating what was necessary for nutritional needs to consuming what was enjoyable and pleasant to eat. Perhaps our ancestors always selected those foods that were enjoyable to eat, bringing about the basis of the consumption of food as a central focus in the social life of humans.

See also Agriculture, History of ; Cannibalism ; Hunting and Gathering .

BIBLIOGRAPHY

Eaton, S. Boyd, and Melvin Konner. "Paleolithic Nutrition." The New England Journal of Medicine (1985) 312:283–289.

Flandrin, Jean-Louis, and Massimo Montanari, eds. Food: A Culinary History from Antiquity to the Present. New York: Columbia University Press, 1999. (English edition edited by Albert Sonnenfeld; first published as Histoire de l'alimentation ; Rome, 1996.)

Goodall, Jane. The Chimpanzees of Gombe: Patterns of Behavior. Cambridge, Mass.: Harvard University Press, 1986.

Hayden, Brian. "Cultural Capacities of Neandertals: A Review and a Re-evaluation." Journal of Human Evolution (1993) 24:113–146.

Kelly, Robert L. The Foraging Spectrum: Diversity in Hunter-Gatherer Lifeways. Washington, D.C.: Smithsonian Institution Press, 1994.

Klein, Richard. The Human Career. 2d ed. Chicago: University of Chicago Press, 2002.

Mann, Alan. "Diet and Human Evolution." In Omnivorous Primates: Gathering and Hunting in Human Evolution. Edited by R. Harding and G. Teleki. New York: Columbia University Press, 1981.

Somer, Elizabeth. The Origin Diet. New York: Henry Holt, 2001.

Stiner, Mary C. Honor Among Thieves: A Zooarchaeological Study of Neandertal Ecology. Princeton, N.J.: Princeton University Press, 1994.

Stringer, Chris, and Clive Gamble. In Search of Neanderthals. New York: Thames and Hudson, 1993.

Wolpoff, Milford H. Paleoanthropology. 2d. ed. New York: Mc-Graw Hill, 1999.

Alan Mann

Encyclopedia of Food and Culture Mann, Alan

Evolution

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EVOLUTION

The International Darwin Centennial Convention (Chicago 1959) defined evolution as an irreversible process of developmental change in time, which during its course generates novelty, diversity, and higher levels of organization. It operates in all sectors of the phenomenal universe but has been most fully described in the biological sector. The general statement of evolution explicitly asserts that the natural history of organisms has been and is being accomplished by materials and forces belonging to the organic world, and that no miraculous intrusion is needed to account for the proliferation of new forms.

Philosophical Evaluation

A key issue that contemporary evolutionary theory poses for the philosopher is how best to express both the extent and the limitations of the concept of evolution. Since the publication of Charles darwin's Origin of Species (1859), a tendency has developed to make of scientific evolution a philosophical principle for evolutionism. But to regard all things as in a state of flux and to make change the sole principle of all knowledge, the sole property of all activity and behavior, and the sole condition of all laws, art, morals, religion, and history is to extend evolution far beyond its documented limits.

Evolution is not a philosophical principle. The value of this theory is great, especially in the biological sciences, but it has important limitations. There is a sense in which all cosmological events have a history, a spatio-temporal context, but of equal importance is the stability of nature and of physical laws. When scientists speak of the evolution of life, of chemical elements, of planets, of the stars and galaxies, the concept of evolution is extended metaphorically. Even in the purely scientific sphere, evolution is not a single-valued term, equally tested in every area of science. For example, in speaking of the evolution of man, one must understand the limited sense in which the term is extended to man's unique psychosocial conditions. Man's spiritual endowments, his intelligence and freedom, necessitate new nonbiological principles of development. Man, in a real sense, fashions his own evolution, his own future niche, his destiny. Though subject to the conditions of time and place, man's work, his art, his language, his morals, his religion, and his history are within man's control and subject to his intelligence and freedom. Evolutionism as a philosophical principle—variously expressed in the ideologies of historicism, existentialism, and Marxism—cannot derive verification of its assumptions from biological evolution.

Nor is evolution an ultimate explanation of reality. It is not intrinsically bound up with any one metaphysical system. Thus it is not intrinsically materialistic, or mechanistic, or vitalistic; it is not intrinsically atheistic, or pantheistic, or theistic. Yet evolution, like all great scientific theories, needs a philosophical context, and this must be provided by disciplines other than biology and anthropology.

Stability and Change. Although change and its mechanisms have preoccupied evolutionists for many decades, evolutionary change would be meaningless without its corollary, stability. The organic world and its species are amazingly stable. Many representatives of vertebrate families have existed for millions of years unchanged. Although genetic mutation, natural selection, and isolation are natural phenomena, organisms strenuously resist change. And when a variation does come about, the general process is (1) to proliferate, (2) to adapt functionally, (3) to stabilize and find equilibrium, and (4) to become extinct. Extinction of species, so dominant a part of the evolutionary process, is the result of specialization and overstabilization.

The important philosophical insight here is that nature is to be seen in terms of two tendencies, two correlative principles both of being and of knowledge, namely, change and stability. The importance of knowledge of history for the understanding of the cosmos has been clarified by studies of the evolution of organisms. Indeterminism in cosmology and physics, brought to light by relativity and the quantum theory, has meant the death of physical determinism and of the view that the universe is fixed in all of its parts and its development. Nevertheless, science remains founded upon the stability of the cosmos. Hence, science of nature built upon stability alone or change alone must remain fragmentary and illusory. Realism is maintained by applying both correlative principles.

Natural Species. How best to define natural species has always been a problem for both the logician and the naturalist. The theory of evolution has forced the concept of species to refer to something more fluid and develop-mental. Traditional philosophies of nature, based upon the teachings of plato and aristotle, tended to define natural species like numbers, claiming that they are immutable, eternal, indivisible, and necessary. The Platonic tradition divided nature into kinds of essences with fixed properties that were, in turn, immutable, eternal, indivisible, and necessary. In his biological works, Aristotle repudiated dialectical classifications that attempted to place all of nature into sic et non categories. But until natural history accumulated real evidence for the transformation of organic species, the concept of natural species tended to be identified with that of an immutable, eternal essence.

Although Darwin, and others since, thought that a species was but an arbitrary category for individually distinct organisms, contemporary evolutionists insist that real specific discontinuity exists in nature. Yet, the classical morphological basis of taxonomy is giving way to a more dynamic biological concept of populations. Natural species are not fixed physiological or morphological types, as Linnaeus defined them. They are interbreeding populations isolated reproductively from other inter-breeding populations. Where this genetic and ecological definition cannot be applied (e.g., to nonsexual organisms), the old morphological concept is valuable. But the concept of fixed essences has been replaced in biology by a more dynamic concept of relatively stable populations.

This latter concept corresponds favorably with the Aristotelian concept of nature, the intrinsic principle of activity and behavior. The term "essence" designates the principle of the being of a thing; the term "nature" designates the principle of activity. The physical nature of an organism could be known and its essence remain unknown. Nature is no more fixed than is the relation between the generator and the generated, and, unlike essence, the nature admits of the developmental orientation implied in a true evolution of species. Organisms are not immutable, necessary, eternal, and indivisible. The type of concept that denoted essences of this description is not compatible with evolutionary development, whereas the concept of nature can fully embrace evolutionary theory and a true transformation of natural species.

Higher from Lower. Without reinstituting the debate between Darwin and Lamarck concerning innate tendencies toward perfection, can one say that there is evidence for progress and direction in the known course of evolution? Is there a real sense of evolutionary movement from the lower forms to the higher, from the less perfect to the more perfect? To the biologist, lower and higher or less and more perfect can be judged only in terms of the capacity of the organism to secure its survival. Purpose and finality in evolution are not a priori inferences; but a posteriori, the biologist can observe a direction in evolution in terms of relative survival value. Life has tended to occupy every available niche to ensure the perdurance of the biotic community on this planet. The biologist defines life in terms of an active capacity to transform environmental energies into living energy; in such transformation the organism attains some measure of independence from the environment. Biologically, life is higher and more perfect to the extent that it attains greater independence in any single ecological niche. In this sense, vertebrates are higher than invertebrates, vascular plants than nonvascular plants, reptiles than fishes, birds than reptiles, mammals than birds, and man than other primates.

Furthermore, greater freedom from the control of the environment is achieved by the greater complexity of parts and more integral subordination of parts to the whole organism. This direction to higher complexity and integration is observed also in the fossil record of plants and animals. The general course of evolution has been from the simple forms to the highly complex forms of life. The primary, universal goal of evolution is survival, and although the future of any species cannot be predicted a priori, its history in retrospect can be seen to have followed orderly laws. Those laws have brought forth higher forms from the lower forms in the sense defined. To this extent the natural philosopher may speak of a hierarchy within organic nature.

For the philosopher, the question naturally arises: how can higher organisms arise from lower organisms? The biologist describes the evolutionary process in terms of mutation and selection pressures acting upon an inter-breeding population, with isolating mechanisms moving parts of the population further and further from the original through races, varieties, and subspecies all the way to new species. This progress is entirely natural. But the natural philosopher asks for an account of the efficient causes that are sufficient to educe these higher effects. Does this process necessitate the intrusion of nonbiological causes?

The resolution of this problem can be found in the complex interplay of both univocal and equivocal agents acting upon organic matter to produce an effect slightly different from the univocal parent. Mutations are caused by many factors outside the univocal agent, such as cosmic rays and chemical changes caused by atmospheric conditions. Again, many parent-progeny endowments remain latent and are induced only by the requirements of adaptation. No single univocal agent is sufficient to explain the origin of the bird from reptile stock, but a unified complex of univocal and equivocal causes, some acting quite per accidens over a long period of gradual change, are sufficient reason for the origin of the new species. This unified convergence of univocal and equivocal causes would be sufficient to explain the natural origin of life by biopoesis, provided that the dynamic order of causes could be accounted for. Although the development of life manifests the presence of some opportunism and randomness, the dynamic order of the whole process must be accounted for.

Chance and Order. In all evolution, chance is an important factor. Environmental forces command adaptation on the part of the organism. The organism itself has very limited power to dominate its ecological niche; moreover, although the environment tends to be stable, it too is subject to chance alterations. Genetic mutations are apparently caused by chance concurrence of environmental events. Does this influence of chance destroy any basic order in the history of organisms? By no means. Evolutionary history cannot be predicted a priori, but neither is it random.

The relation of chance and order in evolution can be compared to the tossing of a coin. In the individual toss, the chance of heads occurring is 50–50, even though one cannot predict the next individual toss. But in 1,000 tosses, one can predict the chances of heads to be about 500 times. Actuarial tables are constructed upon this statistical principle. The same principle applies to evolution. The forces of nature that bring about change and variation are complex and unpredictable in the individual event or phylogeny, but the general process of speciation takes place according to verifiable physical and biological laws. Every biological law restricts evolution. And although the scientist cannot predict the course of evolution from these laws, he can explain the course of evolution that has ever been under the dominance of these laws. The application of the laws of statistics to organic populations has become a very useful tool in analyzing events of the group when cosmic influences upon the individual are too complex to yield orderly treatment. Events, then, that seem to occur quite by chance at the level of the individual are subject to regular laws at the level of the group. Thus, in the general course of evolution, a dynamic order prevails.

Order and God. It is this dynamic order of the organic world—so dependent upon the order of the microcosm on the one hand and upon the order of the megalacosm on the other—that raises for philosophers the question whether or not the ultimate explanation of evolution demands the existence of a transcendent cause as designer. The classical arguments for the existence of God obtain with as much force in a dynamic unfolding order as they do in a universe of static order alone, if not more. Thus, for many thinkers, organic evolution must necessarily be theistic to be complete. This is not to say that God in His intelligent providence and governance of this dynamic order is seen to make a miraculous intrusion upon the natural causes and laws with which He has endowed His creation. In a real sense the laws of nature are the operation of His governance. The dynamic order of nature, especially in the unfolding process of evolution, thus makes the inference of a sufficient coordinating and governing cause seem a necessity.

For other thinkers this cause is intrinsic to the cosmos, a conclusion that leads to pantheism. For one who subscribes to atheism, on the other hand, no such cause is needed. Philosophically, neither the pantheistic nor the atheistic position accounts sufficiently for the organization and the governance of the dynamic order of the cosmos, both of which demand the operation of intelligence and will. A being endowed with such faculties would of necessity be spiritual and transcendent to the cosmic order.

Bibliography: r. j. nogar, The Wisdom of Evolution (New York 1963). p. g. fothergill, Evolution and Christians (New York 1961). j. de bivort de la saudÉe, ed., God, Man and the Universe (New York 1953). f. c. bawden et al., Symposium on Evolution (Pittsburgh 1959). e. c. messenger, ed., Theology and Evolution (London 1952). c. hauret, Beginnings: Genesis and Modern Science, ed. e. p. emmans (Dubuque 1955). b. vawter, A Path through Genesis (New York 1956). j. levie, The Bible: Word of God in Words of Men, tr. s. h. treman (New York 1962). l. r. ward, God and World Order (St. Louis 1961). p. teilhard de chardin, The Phenomenon of Man, tr. b. wall (New York 1959). m. j. adler, ed., The Great Ideas: A Syntopicon of Great Books of the Western World, 2 v. (Chicago 1952) 1:451–467. a. m. dubarle, The Biblical Doctrine of Original Sin, tr. e. m. stewart (New York 1965).

[r. j. nogar]

CATHOLIC TEACHING

Man, intelligent, free, graced, and the recipient of revelation, has a relationship beyond other creatures to God, his Creator. Hence, there are theological dimensions to such evolutionary postulates as the historical origin of mankind from lower animal forms, the present state of human nature in a condition of dynamic flux, and future goals of the race to be achieved, perhaps, through man's conscious control of his own evolution.

Pope Pius XII confined the question of man's genesis to "the origin of the human body as coming from preexistent and living matter" (H. Denzinger, Enchiridion symbolorum, ed. A. Schönmetzer [32d ed. Freiburg 1963]3896). The human soul, spiritual and immortal, is not explained by purely material antecedents. Man may share biological continuity with lower animals, but there is at least some measure of psychic and moral discontinuity.

To the question "What is the origin of man?" Scripture gives a religious answer. The Book of genesis teaches that God is man's Creator and Father and that man is the most excellent of earthly creatures, a creature "made to the image and likeness of God" (Gn 1.3). The Genesis account is a "popular description of the origin of the human race … in simple and figurative language adapted to the mentality of a people" in a nontechnological culture (H. Denzinger, Enchiridion symbolorum 3898). In his hymn of praise of the Creator, the sacred writer may have taken something (under inspiration) from popular, nonbiblical narrations.

Traditional Idea. For most of Christian history the possibility of evolution in its modern connotation was imagined only as most improbable speculation. The traditional idea that God had created things as they are, fixed in species, did not have a serious rival. Evolution (transformism) came into prominence chiefly through the work of Charles Darwin (The Origin of Species, 1859) and Alfred R. Wallace. Early proponents of Darwinism (e.g., T.H. Huxley in England and Ernst Haeckel in Germany) were militant materialists, atheists, or agnostics (Huxley's term). Many churchmen responded to evolution as to an attack upon Christianity, and an unfortunate atmosphere of controversy was the result. In a famous debate, typical of the times, Anglican bishop Samuel Wilberforce confronted Huxley at Oxford in 1860. In Dayton, Tennessee, in 1925 the celebrated "monkey trial" of John T. Scopes saw Clarence Darrow ridicule the fundamentalist ideas of William Jennings Bryan.

Statements of the Fathers and theologians before Darwin are not strictly ad rem, since the issue had not even been raised. St. Gregory of Nyssa is sometimes cited as an evolutionist, but he also held that man was privileged above other creatures in being formed by God's own hands. St. John Chrysostom held the view, hardly evolutionary in the modern sense, that Adam's body was lifeless before it received a soul. St. Augustine taught that in the beginning God created rationes seminales, the seeds or germs of all things that would eventually develop in time. These seminal causes were postulated to account for the appearance of new things without contradicting the doctrine that God had created all things simultaneously. Augustine also proposed principles applied in 1893 by Pope Leo XIII to cases of apparent conflict between science and the Bible: "Whatever they [scientists] can really demonstrate to be true of physical nature, let us show to be capable of reconciliation with our Scriptures" (H. Denzinger, Enchiridion symbolorum 3287); and "the Holy Spirit [in the Scriptures] … did not intend to teach men these things in no way profitable unto salvation" (H. Denzinger, Enchiridion symbolorum 3288).

After Darwin. Although churchmen after Darwin regarded evolution as inopportune and dangerous, no official statement of condemnation issued from Rome, and no written work was placed in the index of forbidden books for that reason. The record of the attitudes of theologians, however, shows considerable change since mid-19th century. The provincial council of Cologne (not ecumenical) in 1860 declared that the theory of transformism whereby some lower form spontaneously became a human body was contrary to Scripture and to the faith. Vatican Council I in 1870 was content to repeat the commonsense advice: the same God gives revelation and reason; one truth cannot contradict the other (H. Denzinger, Enchiridion symbolorum 3017). In 1909 the Biblical Commission refused to call into question the literal and historical meaning of Genesis in cases "which touch the fundamental teachings of the Christian religion" but ruled that one is not bound to seek for scientific exactitude of expression in the first chapter of Genesis and that free discussion of the six days of creation is permitted (June 30, 1909; H. Denzinger, Enchiridion symbolorum 3512–19). In 1948 a letter to Cardinal E. Suhard from the secretary of the Biblical Commission noted that the replies of the Biblical Commission in 1909 are "in no way a hindrance to further truly scientific examination of the problems in accordance with the results acquired in these last 40 years" (H. Denzinger, Enchiridion symbolorum 3862).

In 1950 the encyclical humani generis marked the starting point of a new development. Materialism and pantheism were condemned, caution and moderation were counseled in reinterpreting Scripture, but evolution was expressly recognized as a valid hypothesis:

The teaching authority of the Church does not forbid that, in conformity with the present state of human sciences and sacred theology, research and discussions on the part of men, experienced in both fields take place with regard to the doctrine of evolution. [H. Denzinger, Enchiridion symbolorum 3896]

Pope John Paul II, in a discourse to the Pontifical Academy of Sciences (Oct. 22, 1996), reiterated Pius XII's teaching from Humani generis on the direct creation of the human soul by God. He cast the whole topic of evolution in the terms of a Christian anthropology, insisting that both theology and the sciences must be judged by what Christian faith knows about human nature: namely, that man is made in the image and likeness of God. The Christian has no objection to evolutionary theories in se, but only those that, because of their philosophical principles, do not do justice to who man is. "The theories of evolution which, because of the philosophies which inspire them, regard the spirit either as emerging from the forces of living matter, or as a simple epiphenomenon of that matter, are incompatible with the truth about man. They are therefore unable to serve as the basis for the dignity of the human person" ("Discourse," 5).

Bibliography: r. collin, Evolution: Hypotheses and Problems, tr. s. j. tester (New York 1959). d. l. lack, Evolutionary Theory and Christian Belief (London 1957). s. tax, ed., Evolution after Darwin, 3 v. (Chicago 1960).

[o. w. garrigan/eds.]

New Catholic Encyclopedia NOGAR, R. J.

Evolution

views updated May 18 2018

Evolution

The term evolution in general refers to the process of change. For example, one can describe the way in which a section of land evolves over time. Geologic evolution comes about as the result of forces such as earthquakes, volcanoes, land movements, rain, snow, wind, and other factors. In biology, the term evolution refers to changes that take place in organisms over long periods of time. For example, one can study the changes that take place in a population of fruit flies over many generations. The characteristics of that population after 100 generations is likely to be quite different from the characteristics of the first generation of flies.

Scientists and laypeople often refer to the theory of evolution. The term "theory" in this phrase does not refer to a scientific guess, as the term is sometimes used. Instead, the term refers in this case to a large collection of well-established laws and facts about the ways organisms change over time. The theory of evolution is not in any sense an idea whose truth needs to be tested. Instead, it is one of the most fundamental and most important general concepts in all of the biological sciences.

Historical background

The English naturalist Charles Darwin (1809–1882) is generally regarded as the father of modern evolutionary theory. However, evolutionary thought can be traced to much earlier periods. In the mid-eighteenth century, for example, the French mathematician Pierre-Louis Maupertuis (1698–1759; last name pronounced moe-per-TWEE) and the French encyclopedist Denis Diderot (1713–1784; name pronounced da-NEE deeduh-ROE) proposed evolutionary theories that contained ideas that reappeared in Darwin's own theory a century later.

The French naturalist Jean-Baptiste Lamarck (1744–1829) was the first to clearly explain the theory that species could change over time into new species. In his 1809 book Philosophie zoologique, he argued that living things progress inevitably toward greater perfection and complexity. The driving force behind that change, he said, was the natural environment. According to Lamarck's theory, changes in the environment altered the needs of living creatures. These creatures, in turn, responded by using certain organs or body parts more or less. As the body parts were either used or disused, they would change in size or shape. That change would then be inherited by the creatures' offspring, Lamarck said. Such changes could then be regarded as "acquired characters." According to this notion of the inheritance of acquired characteristics, also known as Lamarckism, giraffes would have "acquired" their long necks from stretching to reach leaves not available to other animals. Members of each succeeding generation would have stretched their necks to attain leaves at ever higher levels, leading to the modern giraffe. Although this theory was later discredited

and abandoned, Lamarck remains the first scientist to acknowledge the adaptability of organisms.

Darwin and his contemporary, Alfred Russell Wallace (1823–1913), are credited with independently providing the first logical theory for a mechanism to explain evolutionary change. Darwin and Wallace called that theory natural selection. However, Wallace did not develop his ideas as fully as did Darwin. As a result, it is Darwin who is generally given credit for having founded the modern theory of evolution. He outlined the fundamental ideas of that theory in his 1859 book The Origin of Species by Means of Natural Selection and his later works. One major difference between the two men was that Wallace did not believe that natural selection could have produced the human brain. He thought that human intellect could only have been created by a higher power (a god), a concept that Darwin rejected.

Words to Know

DNA (deoxyribonucleic acid): A large, complex chemical compound that makes up the core of a chromosome and whose segments consist of genes.

Fitness: The ability of an organism to survive in an environment as measured by the average number of offspring produced by individuals with a certain set of genes, relative to that of individuals with a different set of genes.

Gene: A section of a DNA molecule that carries instructions for the formation, functioning, and transmission of specific traits from one generation to another.

Mutation: A change in the physical structure of an organism's DNA (deoxyribonucleic acid), resulting in a genetic change that can be inherited.

Natural selection: Also referred to as "survival of the fittest," the process by which some organisms are better able to survive and reproduce in some present environment.

In The Origin of Species, Darwin concluded that some individuals in a species are better equipped to find food, survive disease, and escape predators than others. He reasoned that these individuals are more likely to survive, mate, and produce offspring. Individuals that are not as well-adapted to their environment are less likely to survive, mate, and produce offspring. As a result, each generation of a population will consist of individuals that are better and better adapted to their environment. The overall characteristics of the population will change to reflect this better adaptation.

The major problem with Darwin's theory—which he acknowledged—was that he didn't know the mechanism by which successful adaptations could be passed from one generation to the next. The solution to that problem lay in the research done by the Austrian monk and botanist Gregor Mendel (1822–1884). Mendel discovered that hereditary characteristics are transmitted from generation to generation in discrete units that he called "factors" and that we now call genes. Darwin's theory can be restated to say that individuals who are better adapted to their environment are more likely to pass their genes to the next generation than are other members of a population.

Evidence for evolution

Support for the theory of evolution comes from a number of sources. One of these sources is the science of embryology, the study of early forms of an organism. Darwin reasoned that organisms that have passed through a period of evolution will retain some reminders of that history within their bodies. As its turns out, virtually all living creatures possess vestigial features. A vestigial feature is a structure that once served some function in an ancestor and remains in an organism at some stage of its development. But the structure no longer serves any function in that organism.

As an example, the embryos of all vertebrates (animals with backbones) look remarkably alike at an early stage. They all contain, for example, a tail-like structure that may or may not be lost as the individual develops. Also, fetal whales, still in their mothers' wombs, produce teeth like all vertebrates. However, those teeth are later reabsorbed in preparation for a life of filtering plankton from their ocean habitat. Snakes, whose vertebrate ancestors ceased walking on four legs millions of years ago, still possess vestigial hind limbs with reduced hip and thigh bones.

In some cases, the same structures may be adapted for new uses. The cat's paw, the dolphin's flipper, the bat's wing, and a human hand all have a similar structure. They all contain counterparts of the same five bones forming the digits or fingers (in humans). There is no known environmental or functional reason why there should be five digits. In theory, there could just as easily be four or seven. The point is that the ancestor to all tetrapods (vertebrates with four legs) had five digits. Thus, all living tetrapods have that number, although in a modified form.

Another important source of evidence about evolution comes from the fossil record. In general, one would expect, if evolutionary theory is correct, that the older a fossil is the simpler and more primitive it is. Such, in fact, is the case. Fossil invertebrates (animals that lack backbones), plants, and animals appear in the rocky layers of Earth's crust in the same order that their anatomical complexity suggests they should: with the more primitive organisms in the older layers, beneath the increasingly complex organisms in the more recent deposits. No one has ever found a flowering plant or a mammal in deposits from 400 million years ago, for instance, because those organisms did not appear on Earth until much later.

In some cases, evolution can actually be observed. Organisms that reproduce rapidly can be exposed to environmental factors that would affect the make-up of the population. In one famous experiment, Joshua Lederberg (1925–) and Esther Lederberg (1922–) exposed bacterial colonies to an antibiotic. In the first stages of the experiment, most bacteria in the colony were killed off by the antibiotic; only a very few survived. As the colony reproduced, however, that pattern began to change. More and more individuals in the population were resistant to the antibiotic. Eventually, the antibiotic was no longer successful in killing off the new strain of bacteria that had evolved.

Finally, evidence for evolution can be found in the most fundamental part of living organisms: the structure of their DNA molecules. DNA (deoxyribonucleic acid) is the molecule in all living cells that controls the functions of those cells. When one studies the DNA of animals that appear to be related to each other on a superficial level, such as humans and chimpanzees, very close similarities in the DNA of these animals can also be observed.

Evolutionary mechanisms

One of the fundamental questions in evolutionary thought is how changes take place within a species. How does it happen that the organisms that make up a population today are different in important ways from the organisms that made up a similar population a thousand or a million years ago?

For biologists, that question can be rephrased in terms of changes in gene frequencies. Suppose one could make a list of all the genes in a population of muskrats in a particular geographical region. Since all the individuals in that population are muskrats, they will share a great many genes in common with each other. However, all muskrats in the population will not have identical genes. There will be some variability among those genes.

Natural selection. An important factor in the evolution of a species is that, as a whole, a species tends to overproduce. That is, under most circumstances, more muskrats (or any other organism) are born each year than can possibly survive. Environmental factors such as food, water, and living space limit the number of individuals that will survive in any one year.

Origin of Life

So, how did it all begin? Any discussion of evolution eventually leads to the most basic question of all: how did life begin on Earth?

Humans have wondered about that question for centuries. One of the oldest beliefs is that organisms are created by the process of spontaneous generation. According to this theory, organisms arise out of nonliving matter almost magically—from garbage, refuse, muddy water, and other places where dirt collects. In the 1860s, French chemist Louis Pasteur (1822–1895) showed that living organisms only come from other living organisms. So the question remained, where did the first organisms come from?

The most popular theory today is that the first living organisms probably grew out of the warm "chemical soup" that existed on Earth's surface three to four billion years ago. That soup consisted of compounds of nitrogen, oxygen, carbon, and hydrogen. With energy provided by sunlight, lightning, and the heat of volcanoes, those compounds apparently came together to form amino acids. Those amino acids, in turn, reacted with each other to form proteins, the building blocks of all forms of life. In 1953, American chemist Stanley Miller (1930–) showed in a laboratory experiment how such reactions might take place. Since that time, more and more experiments have strengthened the "chemical soup" theory of the origin of life.

Still, other theories remain. For example, some scientists believe that the seeds from which life on Earth began arrived on our planet millions of years ago from outer space, brought by meteors that fell to the planet's surface. Recent discoveries by astronomers of the presence of these "life-giving" chemicals (complex carbon molecules and water) in comets, in the dust and gas of distant stars, and in the emptiness of outer space lend some support to this theory.

For all living organisms, then, life can be seen as a struggle. A constant battle goes on among the individuals to determine which individuals survive and which will die. In determining the outcome of that battle, it should be obvious that those individuals best adapted to an environment will survive. For example, in a cold environment, individuals that are somewhat better able to live in cold temperatures are more likely to live than those that are adapted to a somewhat warmer environment.

More to the point, individuals that are adapted to an environment are more likely to live and to reproduce. Those individuals are more likely to survive, reproduce, and pass their genes on to the next generation. Individuals that are less well-adapted are less likely to reproduce and pass on their generations. Over a long period of time, the individuals that make up a population are better and better adapted to the environment in which they live.

This fact of life has been summarized in one of the most famous expressions in all of biology, the "survival of the fittest." The phrase simply means that individuals that "fit" the needs, demands, and opportunities of the environment are most likely to have offspring that will also "fit in" with the environment.

This process is described as natural selection. The term natural selection refers to the tendency of organisms that are better adapted to an environment to survive, reproduce, and pass on their genes to the next generation.

The effects of natural selection can be seen when an environment changes. As an environment changes, the characteristics needed to survive within it also change. Evolution continues, but the direction it takes may alter.

For example, suppose that global climate changes occur. Suppose that the annual average temperature in northern Canada begins to drop. Over a one-million year period, the region becomes much colder. In such a case, organisms that are better adapted to cold climates will survive, reproduce, and pass on their "cold weather" genes to their offspring. The individuals that make up a population will differ from the individuals in a population one million years earlier because the population has adapted to new environmental conditions. Natural conditions have "selected" those individuals (along with their genes) best adapted to the new environment.

Mutation. An important factor in evolution is mutation. Mutation is the process by which changes occur in an organism's genes that are transmitted to the organism's offspring. Mutations are often regarded as undesirable events because they often lead to genetic disorders that result in the death of individuals. But mutations also can be positive events.

For example, consider a group of disease-causing bacteria that are exposed to an antibiotic. The vast majority of those bacteria will be killed by the antibiotic. The use of antibiotics to cure certain diseases is an example of this fact. But suppose that a mutation has occurred in a small fraction of the bacteria—in even a single bacterium—treated with the antibiotic. And assume that that mutation provides the bacteria with an immunity (tolerance) to the antibiotic. In such a case, the vast majority of bacteria lacking the mutant gene will die off. The few bacteria that have the mutant gene will be able to survive, reproduce, and pass on their immunity to the antibiotic to their offspring.

In fact, this kind of evolution takes place all the time in the real world. Any antibiotic that has ever been discovered or invented eventually loses its effectiveness in treating disease. The reason is that bacteria with mutant genes become resistant to the antibiotic, reproduce, and eventually become the dominant forms of the bacteria.

The same kind of change occurs in every kind of organism. A mutant gene may provide a tree with the ability to grow taller, giving it an advantage over other trees lacking the mutant gene. A tiger may attain a mutant gene that makes it more aggressive, stronger, or able to out-hunt its brothers and sisters. A mutant gene in the human brain may provide a person with more intelligence, better perception, or some other trait that gives him or her a slight advantage over his or her peers. In each case, the mutant gene changes the course of evolution and gives one individual a survival advantage over other individuals in the same population.

[See also Adaptation; Genetics; Geologic time; Mendelian laws of heredity; Migration; Mutation; Nucleic acid ]

UXL Encyclopedia of Science

Evolution

views updated Jun 27 2018

Evolution

Evolution refers to biological change. Biological evolution involves change in the genetic constitution of populations over time such that complexity is achieved due to the formation of new genes or gene-encoded functions rather than harmful mutations. These changes are passed on from parents to their offspring, but biological evolution does not involve individual organisms. Individuals develop, but they do not evolve. The study of evolution encompasses two major facets: deducing the history of descent among living things (phylogenetic systematics), and investigating evolutionary mechanisms as revealed by laboratory and field studies of living organisms and their fossilized remains. Evolutionary change occurs as a result of beneficial mutation , migration , genetic drift and natural selection , and it is ultimately a passive process, devoid of any purpose or goal. As a scientific theory, it is an interconnected series of hypotheses, corroborated by a large body of evidence. Thus, biologists accept the historical reality of evolution, even though many details that are unclear continue to be investigated.

Historical background

The birth of modern evolutionary theory can be traced to the mid-nineteenth century, and the publication of Charles Darwin's book, The Origin of Species by Means of Natural Selection in 1859. The book is considered by some to be the most influential in all of biology and by others an unsubstantiated theory. It was not, however, a new concept. Even in the late eighteenth century, the French scientist Maupertuis, the philosopher Diderot, and others entertained the notion that new life forms might arise by natural means, including spontaneous generation . Their definition of evolution more closely resembles our modern concept of development, since the course of evolution is unpredictable. Interestingly, Darwin did not use the word "evolution" in The Origin of Species, except in the form "evolved" which is the last word of the book.

French naturalist Jean-Baptiste Lamarck (1744–1829) was the first to clearly articulate the theory that species could change over time into new species, a process later called speciation. In his 1809 book Philosophie zoologique, he argued that living things progress inevitably toward greater perfection and complexity, through a process guided by the natural environment. A changing environment would alter the needs of living creatures, which would respond by using certain organs or body parts more or less. The use or disuse of these parts would consequently alter their size or shape, a change that would be inherited by the creatures offspring. Lamarck, although not the first, discussed this notion of "acquired characters." His contemporaries accepted. It is therefore unfortunate that Lamarck is famous today primarily because he was wrong in this belief, which Darwin upheld, in that it implies that bodily changes acquired during an individuals lifetime can affect the inherited material (now known as DNA) and be inherited. This hypothesis is not true as depicted in the example that a blacksmith's large muscles (an acquired character) are not passed on to the blacksmith's children.

Charles Robert Darwin (1809–1882) and his contemporary Alfred Russell Wallace (1823–1913) are credited with independently providing the first plausible theory for a mechanism to explain evolutionary change, which they called natural selection. However, Wallace did not present so great a synthesis, or consider all the ramifications of evolution, as Darwin did in The Origin of Species and his later works. One major difference between the two men was that Wallace did not believe that natural selection could have produced the human brain ; Darwin rejected his colleague's contention that human intellect could only have been created by a higher power.

In The Origin of Species, Darwin concluded that some individuals in a species are better equipped to find food, survive disease , and escape predators than others. He reasoned that if the differences between the survivors and the doomed are passed from parents to their offspring, these differences will be inherited and gradually will characterize the whole population. Darwin knew that the greatest weakness in his theory was his lack of knowledge about the mechanics of inheritance; his attempt at a theory of inheritance, known as pangenesis, was later refuted. However, he is credited for his remarkable insight into the hidden variation in the form of traits that are genetically recessive and only manifest themselves when they occur as a genetic "double dose"—along. He also helped clarify the disctinction between genotype and phenotype . The genotype is a set of genetic instructions for creating an organism; the phenotype is the observable traits manifested by those instructions. The issue that explains the mechanism of inheritance, however, went unsolved for several decades later.

The modern synthesis

By the 1920s, Mendel's theory of heredity had been rediscovered, but the early Mendelians, such as William Bateson and Hugo de Vries, opposed Darwin's theory of gradual evolutionary change. These geneticists favored the position that evolution proceeds in large jumps, or macro-mutations, rather than the gradual, incremental changes proposed by Darwin. The dispute between the Mendelians and the Darwinians was reconciled by important theoretical work published in the early 1930s by independent population geneticists Ronald A. Fisher (1890–1962), J. B. S. Haldane (1892–1964), and Sewall Wright (1889–1988). Their classic work showed that natural selection was compatible with the laws of Mendelian inheritance.

This reconciliation gave rise to a period of prolific research in genetics as the basis of evolution. Theodosius Dobzhansky (1900–1975), a Russian emigrant to the United States, began his classic investigations of evolution in fruit fly populations, and in 1937 published a book entitled Genetics and the Origin of Species, which has been among the most influential books in modern genetics. Others, like E. B. Ford (1901–1988) and H. B. D. Kettlewell (1901–1979), helped pioneer the subject Ford called "ecological genetics." This was a fundamental concept because it brought together two formerly separate areas of investigation: ecological variation (the influence of different environmental conditions) and genetic variation (differences in genetic make-up) in natural populations.

Evidence of evolution

Darwin recognized that some of the best evidence of evolution remains hidden within the bodies of living creatures. He reasoned that, if organisms have a history, then their history can be deciphered from their remnants. In fact, virtually all living creatures possess vestigial or rudimentary features that were once functional traits of their ancestors. Fetal whales, still in their mothers' womb, produce teeth like all vertebrates , only to reabsorb them in preparation for a life filtering plankton from their ocean habitat . Snakes , whose vertebrate ancestors ceased walking on four legs millions of years ago, still possess vestigial hind limbs, with reduced hip and thigh bones.

In some cases the same structures may be adapted for new uses. The cat's paw, the dolphin's flipper, the bat's wing, and a human hand all contain counterparts of the same five bones forming the digits (fingers in humans). There is no known environmental or functional reason why there should be five digits. In theory, there could just as easily be four, or seven. The point is that the ancestor to all tetrapods (vertebrates with four legs) had five digits, and thus all living tetrapods have that number, although in a modified form. Such traits, when they reflect shared ancestry, are known as homologous structures or characters. Traits that have functional but not truly ancestral similarity, such as the wings of insects and the wings of birds ,are known as analogous characteristics.

Further evidence of evolution involves the most basic homology of all, the genetic code, or the molecular "lan guage" that is shared by all living things. Although the genes of each kind of organism may be represented by different "words," coding for each species unique structures, the molecular alphabet forming these words is the same for all living things on Earth . This is interpreted as evidence that the genetic code arose in a way that a single organism was the ancestor to all organisms that exist today.

Finally, there is the evidence from the fossil records that the remains of invertebrates , plants, and animals appear in the rocky layers of the earth's crust in the same order that their anatomical complexity suggests they should, with the more primitive organisms in the older layers and the more complex organisms in the more recent deposits. No one has ever found a flowering plant or a mammal in deposits from the Devonian age, for instance, because those organisms did not appear on Earth until much later.

Evolutionary mechanisms

To an evolutionary biologist, evolution is a change in the proportion of genes present in an existing population. Each individual in a breeding population possesses a unique genotype, the set of paired genetic alternatives that determines its physical attributes, or phenotype. According to a principle known as the Hardy-Weinberg equilibrium, if certain conditions prevail, the frequency of each genotype in the population will be constant over generations, unless evolution has occurred to cause a shift in the gene frequencies. What factors can cause the proportion of genotypes in a population to change?

Another means by which gene frequencies may change is genetic mutation. When organisms reproduce, a copy of the DNA from each parent is transmitted to the offspring. Normally, the parental DNA is copied exactly; but occasionally, errors occur during replication in their sex cells, the egg and the sperm. Many of these errors occur in non-coding regions of the DNA, and so may have no known effect on the offspring's phenotype; others may be lethal. In some cases, however, an offspring with a slightly modified genetic makeup survives. Mutation is thought to be an important source of new variation, particularly in rare cases where the mutation confers a selective survival advantage or a gain of function. After all, not all mutations are disadvantageous. In fact, natures method for ensuring the survival of a species is to rely on gene mutations that might enhance the function of the protein it encodes in such a way that there is a survival advantage. The determination of this advantage does not only rely on the type of mutation but the environment circumstances that prevail.

Evolution may also occur by genetic drift, a random process in which gene frequencies fluctuate due to chance alone. For example, if a certain genetic background is over-represented in one generation, perhaps because the region happened to be colonized by a small, homogeneous population, these genetic complements are more likely to remain high in the future generations. Genetic drift has more potent effects in small, isolated populations where some characteristic are not necessarily the result of natural selection and therefore do not represent a selective survival advantage.

Finally, the proportions of genotypes in a population may change as a result of migration, resulting in gene flow, or the movement of individuals (and their sex cells) into and out of the population.

Species diversity and speciation

The term "species" is derived from the Latin word for kind, and species are considered to be the fundamental natural units of life which can and will evolve. Species have been defined in various ways over the years, but the most widely used definition at present is the biological species concept. Most often associated with the name of Ernst Mayr (1904-), it defines a species as a group of interbreeding populations that is reproductively separate from other such groups.

Phylogenetic systematics: Reconstructing evolutionary history

The process of classifying and reconstructing the evolutionary history, or phylogeny , of organisms is known as phylogenetic systematics. Its goal is to group species in ways that reflect a common ancestry. The members of each group, or taxon, share uniquely derived characteristics that have arisen only once. For instance, the taxon Amniota includes amphibians , reptiles , birds and mammals , all of which arose from a single common ancestor that possessed an amnion in the egg stage. Classifying species involves only one apect of phylogenetic systematics. Understanding the evolutionary interrelationships of organisms by investigating the mechanisms leading to diversification of life and the changes that take place over time encompasses phylogenetic systematics. Systematics exceeds taxonomy or naming groups within species by attempting to develop new theories to describe the possible mechanisms of evolution. Historical remnants leave residual clues that allow phylogeneticists to piece together using hypotheses and models to describe history and how organisms evolve.

Systematists gather as much evidence as they can concerning the physical, developmental, ecological, and behavioral traits of the species they wish to group, and the results of their analysis are one (or more) branching "tree" diagrams, representing the hypothetical relationships of these taxa. Analysis of the genetic material itself has become an increasingly valuable tool in deducing phylogenetic relationships. The approach is to determine the nucleotide sequence of one or more genes in the species and use this to contribute to determining the appropriate phylogeny. Comparison of the differences in these DNA sequences provides an estimate of the time elapsed since the divergence of the two lineages. With the ability to sequence an entire organisms genome , the remnants of various species DNA, extinct (depending on the quality of the DNA) and living, can be compared and contrasted. This has exciting implications in the analysis of evolutionary modeling and applications to phylogenetic systematics.

Resources

books

Futuyma, Douglas J. Science on Trial: The Case for Evolution. New York: Random House, 1983.

Gould, Stephen J. Ever Since Darwin. New York: W. W. Norton, 1977.

Gould, Stephen J. The Structure of Evolutionary Theory. Cambridge, MA: Harvard University Press, 2002.

Ridley, Mark. Evolution. Cambridge, MA: Blackwell Scientific Publications, 1993.

other

PBS. "Evolutionary Thought." 2001 [cited January 13, 2003]. <http://www.pbs.org/wgbh/evolution/>.

University of California at Berkeley. "Welcome to the Evolution Wing." UCMP exhibit halls. November 15, 2002 [cited January 13, 2003]. <http://www.ucmp.berkeley.edu/history/evolution.html>.

Susan Andrew

KEY TERMS

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Genetic drift: —Random change in gene frequencies in a population.
Genotype: —The full set of paired genetic elements carried by each individual, representing the its genetic blueprint.
Hardy-Weinberg equilibrium: —The principle that, if certain conditions prevail, the relative proportions of each genotype in a breeding population will be constant across generations.
Macromutation: —Mutation having a pronounced phenotypic effect, one that produces an individual that is well outside the norm for the species as it existed previously.
Mutation: —Alteration in the physical structure of the DNA, resulting in a genetic change that can be inherited.
Natural selection: —The process of differential reproduction, in which some phenotypes are better suited to life in the current environment.
Nucleotide: —Molecular unit that is the building block of DNA.
Phenotype: —The outward manifestation of the genotype, including an organism's morphological, physiological, and many behavioral attributes.
Phylogeny: —Branching tree diagram representing the evolutionary history of an organism, and its relationship to others via common ancestry.
Speciation: —The divergence of evolutionary lineages, and creation of new species.
Systematics: —The classification of organisms according to their evolutionary relationships, and shared genetic and phenotypic characteristics.
Taxon (taxa): —Group of related organisms at one of several levels, such as the family Hominidae, the genus Homo, or the species Homo sapiens.
Tetrapod: —The group of vertebrates having four legs, including amphibians, reptiles, mammals and birds.

The Gale Encyclopedia of Science

Evolution

views updated May 21 2018

EVOLUTION

Although evolutionary ideas are very old, being found in the works of Greek philosophers and echoed in the aggadah and the Midrash, the main stimulus to evolutionary thought came from the theory developed at the end of the 18^th century, according to which life on earth has existed for millions of years, and not for less than 6,000, as held by the biblical tradition current in the civilized world. The theory of evolution produced a sharp reaction on the part of those intellectuals and scholars who subscribed to the assumption of the stability of the species from the six days of creation. Besides scientists who cast doubts on the validity of the theory, its chief opponents were (and still are) religious people who accept the creation story in Genesis literally. There are, however, religious thinkers who see in the principle of evolution a concept that accords with the idea of divine providence, some of them not only regarding the account of creation in Genesis as a simple explanation "in ordinary language" of the actual process of creation, but seeing in it a deeper significance. Some point to aggadic and midrashic statements which contain allusions to evolution and to the fact that life existed on earth in epochs preceding the accepted reckoning in Judaism. When discussing the Greek philosophers' views on "the eternity of the universe," a similar problem confronted Maimonides, who says (Guide, 2:25):

We do not reject the eternity of the universe because certain passages in Scripture confirm the creation; for such passages are not more numerous than those in which God is represented as a corporeal being. Nor is it impossible or difficult to find for them a suitable interpretation. We might have explained them in the same manner as we did in respect to the incorporeality of God and this might have been easier … However, we have not done so … for the eternity of the universe has not been proved and there is no need of scriptural passages to reject it … If we were to accept the eternity of the universe as taught by Plato, we should not be in opposition to the fundamental principles of our religion … The scriptural text might have been explained accordingly … But there is no necessity for this expedient, so long as the theory has not been proved. As there is no proof sufficient to convince us … we take the text of the Bible literally.

Applying this to the subject under discussion and stating it in contemporary terms, it may be said that if proofs were forthcoming for the theory of evolution (on the assumption that there exists One who directs creation), a way would be found of explaining the biblical passages accordingly.

Although Judah Halevi clearly recognized the need to accept the Scriptures literally, he nevertheless stated in his Kuzari (1:67): "If, after all, a believer in the Law finds himself compelled to admit an eternal matter and the existence of many worlds prior to this one, this would not impair his belief." In the latter part of this sentence Judah Halevi alludes to the statement of R. Abbahu (Gen. R. 3:7) that God "created worlds and destroyed them," while according to R. Judah b. Simon there was "a succession of times (days and nights) before that," that is, before the first day of creation (ibid.). To this province belong also such statements as: "'And there was evening and there was morning, the sixth day' (Gen. 1:31). R. Simon b. Marta said, 'Up to this point we count according to the reckoning of the world, after it according to another reckoning'" (Gen. R. 9:16), that is, time before the final creation of the world has a different meaning from that after it, which is the reckoning that we follow. The relativity of time in the term "day" is referred to in the statement (Gen. R. 19:8) which distinguishes between the human and the divine day, the latter being a thousand years in duration, as it is said (Ps. 90:4): "For a thousand years in Thy sight are but as yesterday." Alongside these notions there are homiletical interpretations of biblical passages according to which all organisms were fully created in the six days of creation, after which no changes have occurred in them (Hul. 60a). In contrast to those who extended the period of creation, a tanna curtailed it by stating that "on the first day the entire world was created," this being the view of R. Nehemiah, who disagreed with R. Judah's opinion that "the world was created in six days" (Tanḥ. B., Gen. 7). All these sages based their views on biblical verses, which could be interpreted either way. In this connection Rabbi A.I. *Kook has remarked that "everyone knows that the creation is one of the mysteries of the Torah, and if all the statements are merely to be taken literally, what mystery is there?" (Iggerot ha-Re'iyyah (1961²), no. 91). The literature of the sages is pluralistic in its world outlook, especially in the spheres of cosmology and biology. The tannaim and amoraim absorbed legends and "factual" stories, the views of Greco-Roman science, and the folklore of ancient peoples. Among these were ideas which have no basis in fact nor any support in biblical passages and are even in conflict with the creation story. There was, for example, the view about the development of living organisms from nonliving substances which, known as spontaneous generation and accepted until the 19^th century, penetrated into the halakhah. There was also the "assumption that a mouse does not breed" (Sifra, Shemini, Parashah 5; Ḥul. 127a), and hence the halakhic discussion on the question of the uncleanness of "a mouse which is half flesh and half earth" (Ḥul. 9:6. As late as in 1652 Helmont, a Dutch chemist, still suggested a method of producing mice by putting rags into a heap of grain). There was similarly the prevailing view that vermin originate from perspiration or from dirt. Thus the Talmud (Shab. 107b) declared that "vermin do not breed," against which an objection was raised from the reference made to "eggs of vermin" (see Ḥul. 9:6). The salamander, too, was thought to originate from a fire which burnt continuously for several years (see Ḥag. 27a). In the belief that some organisms develop on food itself, it was permitted to eat certain foods on which maggots develop.

The folklore of various nations tells of organisms, such as *mandrakes, that are half plant and half human. There was also the belief that some birds grow on trees in the form of fruit, and R. Tam (12^th century) was asked whether they require sheḥitah, to which he replied that they do (see Loew, Flora, 4 (1934), 348). The halakhah mentions an organism called אַבְנֵי הַשָּׂדֶה or אַדְנֵי הַשָּׂדֶה, whose corpse, like that of a human being, communicates uncleanness (Sifra, Shemini, ch. 6; Kil. 8:5). Explained by some as referring to the chimpanzee, it is said in the Jerusalem Talmud (Kil. 8:5, 31c) to be "a man of the mountains who lives from his navel; if the navel is severed, he does not live," the reference being to a manlike organism joined by its navel like a plant to the ground. Legends about such an organism were current among various nations (see R. Patai, Adam ve-Adamah, 1 (1942), 216ff.). Mermaids, the legendary half-human, half-fish beings, also figure in the halakhah, the unclean "living creatures that are in the waters" (Lev. 11:10) being interpreted as "including mermaids," which however, unlike a human corpse, do not communicate uncleanness when dead (Sifra, Shemini, Parashah 3). The sages who quoted these halakhot or statements were influenced by the leading scientists of the time, such as Aristotle, Galen, and others, who had confirmed these "facts" and to whom there undoubtedly applied the principle that "if someone tells you that there is wisdom among the non-Jews, believe him" (Lam. R. 2:13). As it deals with all spheres of life and with theoretical subjects, the halakhah also on occasion incorporated legendary, fictitious ideas. In the field of "science" the sages were ready to accept various views current among their contemporaries (but proved in our day to be without foundation) and did not hesitate to give expression to them even if they were contrary to their accepted views.

To this province belong halakhot relating to *mixed species (kilayim). Despite the assumption inherent in the Bible that in the six days of creation all organisms were fully formed, statements of the sages in the aggadah and the halakhah refer to the production of new species by hybridization and grafting. The Tosefta and the Jerusalem Talmud of tractate Kilayim cite many "facts" about the formation of a third species by grafting two species of flora, some systematically very remote from each other (see *Biology; it is now evident that no new species can be produced by grafting). Thus, for example, it is asserted that, by sowing together the seeds of an apple and a watermelon a third species, the *melon (called in Greek melopepon, the apple-melon), is obtained (tj, Kil. 1:2, 27a), even as a dangerous creature called arvad is produced by mating a snake with a species of *lizard (Ḥul. 127a). Another tradition holds that after Anah the son of Zibeon had produced a *mule, which is a dangerous animal, by crossing a stallion and a sheass (יֵמִם, a hemi-onos, i.e., a half-ass; cf. Gen. 36:24), "the Holy One blessed be He appointed a ḥakhina [a poisonous snake] which He mated with a ḥardon [a species of lizard] to produce a ḥavarbar," a species of noxious animal whose bite proved fatal to Anah (tj, Ber. 8:6, 12b). This story is mentioned in a discussion on whether mixed species originated during the six days of creation (ibid.; Tosef. Ber. 6:11). On this subject there is the view of the tanna R. Yose (Pes. 54a) that "two things God originally planned to create on the eve of the Sabbath [of the creation] but were not created until the termination of the Sabbath, and at the termination of the Sabbath the Holy One blessed be He granted Adam knowledge of a kind like the divine, whereupon he took two stones, rubbed them together, and fire issued from them [cf. the tale of Prometheus]; he also took two [heterogeneous] animals and crossed them, and from them came forth the mule." Thus R. Yose held that hybridization represents a remarkable wisdom granted to man, who is prone to produce new organisms, "like the divine creator." Another aggadah, which declares that God Himself "changes His world once every seven years," mentions various animals, one of which is replaced by the other (tj, Shab. 1:3, 3b). The reference here may be to seven years of God, one of whose days is a thousand years (see above; although there is a statement (bk 16b) that "the male hyena (צבוע) becomes a bat after seven years," etc.).

Proofs of Evolution

The existence in prehistoric times of gigantic animals, then extinct, is alluded to in biblical verses referring to the dragon, the *leviathan, the Rahab, and others. Having perhaps found traces of the footprints of primeval animals or remains of their skeletons (footprints of prehistoric reptiles have been discovered near Jerusalem in recent times), the ancients had their imaginations stirred to describe these huge animals and explain the reasons for their extinction.

One of the crucial problems confronting the evolutionists was the question of the transition from ape to man. In the literature of the sages there are allusions to a connection between man and ape. Thus the amoraim Rav and Samuel held divergent views on the nature of the rib from which woman was created, the one holding that it was a tail (Ber. 61a). In the opinion of R. Judah: "[God] made him [i.e., man] a tail like an animal and then removed it from him for his honor" (Gen. R. 14:12). Even Adam was not the first man, for "974 generations preceded the creation of the world and they were swept away in a trice because they were evil" (Mid. Ps. to 90:13; cf. Shab. 88b). Nor was Adam anatomically perfect, since he was a hermaphrodite (Gen. R. 8:1); the fingers of his hands were joined together, and it was only from Noah onward that people were born with separated fingers (Mid. Avkir to Gen. 5:29; and similarly in Tanh. to ibid.). In the days of Enosh there took place a moral degeneration; human beings changed, and "their faces became like apes" (Gen. R. 23:6; cf. Sanh. 109b). All these statements are based on a homiletical interpretation of biblical verses, but underlying them was probably the view of the tanna or amora which he expressed in this manner. Finally there is a statement that attests to an observation and a conclusion drawn from the realm of comparative anatomy: the amora R. Samuel of Cappadocia concluded from a common feature in fishes and birds that the latter, too, were created "out of alluvial mud": this can be proved "from the fact that birds have on their legs scales like those of fishes" (Ḥul. 27b).

At the beginning of the 20^th century the naturalist De Vries (1848–1935) drew attention to the fact that in some flora and fauna characteristics suddenly appear which, though not present in their progenitors, are transmitted by heredity to the progeny. These changes, known as mutations, for the most part small and fortuitous, are in the view of scholars the basis of the evolutionary processes. Through the accumulation of these mutations, organisms were separated during millions of years of evolution into strains, species, and higher systematic groups. According to Neo-Darwinism the fortuitous mutations and the operation of natural selection were responsible for evolution, whereas according to Neo-Lamarckism, development cannot be accounted for without assuming that there is something in the living substance which guides it toward development. In this sense there is a statement of the rabbis: "There is no herb which has not a guardian angel in heaven that strikes it and says, Grow!" (Gen. R. 10:6). Other scholars maintain that there are metaphysical factors that guide and direct the existence and development of the organism. This theory, known as teleology, approximates to the religious view of the Creator's providence over His creatures. Some leading evolutionists, although dissociating themselves from the teleological approach, nevertheless agreed that it was impossible to explain evolution on the basis only of known forces. Thus G.L. Stebbins, who made a study of evolution in flora, argued that evolution can be explained by mutations, hybridization, and natural selection directed by a certain force of unknown nature. Certain embryologists, too, assumed that in ontogeny – the development of the individual during the embryonic period – there is an unknown or nonrational force directing it toward its development, and in this there is a parallel between phylogeny, the development of the species, and ontogeny, the development of the individual.

However much these views fall out of fashion as molecular biology progresses and the fossil record is clarified, these assumptions have an indubitable religious significance, and in this connection mention should be made of the words of Rabbi Kook: "The theory of evolution, which is at present increasingly conquering the world, is more in harmony with the mysteries of Kabbalah than all other philosophical theories" (Orot ha-Kodesh, ii, 558). On the other hand there are many evolutionists who are not prepared to include in the scheme of creation and evolution a nonrational force and hold that these "unknown" forces, responsible for the evolutionary process, will be revealed and defined as known chemical or physical forces. There are numerous theories to explain the mechanism of evolution, but the doubts exceed the certainties. When Rabbi Kook was asked about the problem of evolution, he summed it up as follows: "Nothing in the Torah is contradicted by any knowledge in the world that emerges from research. But we must not accept hypotheses as certainties, even if there is a wide agreement about them" (Iggerot ha-Re'iyyah, no. 91).

bibliography:

S.B. Ulman, Madda'ei ha-Teva u-Veri' at ha-Olam (1944); M.M. Kasher, in: Sefer Yovel… Samuel K. Mirsky (1958), 256–84; idem, in: Sinai, 48 (1960), 21–33; J. Feliks, Kilei Zera'im ve-Harkavah (1967), 7–12, 112–5; idem, in: Teva va-Areẓ, 7 (1965), 330–7; O. Wolfsberg, in: L. Jung (ed.), Jewish Library, 2 (1968), 145–70.

[Jehuda Feliks]

Encyclopaedia Judaica Feliks, Jehuda