Evolution, Biological

Biological evolution encompasses three issues: (1) the fact of evolution; that is, that organisms are related by common descent with modification; (2) evolutionary history; that is, when lineages split from one another and the changes that occur in each lineage; and (3) the mechanisms or processes by which evolutionary change occurs.

The fact of evolution is the most fundamental issue and the one established with utmost certainty. During the nineteenth century, Charles Darwin (1809–1882) gathered much evidence in its support, but the evidence has accumulated continuously ever since, derived from all biological disciplines. The evolutionary origin of organisms is today a scientific conclusion established with the kind of certainty attributable to such scientific concepts as the roundness of the Earth, the motions of the planets, and the molecular composition of matter. This degree of certainty beyond reasonable doubt is what is implied when biologists say that evolution is a fact ; the evolutionary origin of organisms is accepted by virtually every biologist.

The theory of evolution seeks to ascertain the evolutionary relationships between particular organisms and the events of evolutionary history (the second issue above). Many conclusions of evolutionary history are well established; for example, that the chimpanzee and gorilla are more closely related to humans than is any of those three species to the baboon or other monkeys. Other matters are less certain and still others—such as precisely when life originated on earth or when multicellular animals, plants, and fungi first appeared—remain largely unresolved. This entry will not review the history of evolution, but rather focus on the processes of evolutionary change (the third issue above), after a brief review of the evidence for the fact of evolution.

The evidence for common descent with modification

Evidence that organisms are related by common descent with modification has been obtained by paleontology, comparative anatomy, biogeography, embryology, biochemistry, molecular genetics, and other biological disciplines. The idea first emerged from observations of systematic changes in the succession of fossil remains found in a sequence of layered rocks. Such layers have a cumulative thickness of tens of kilometers that represent at least 3.5 billion years of geological time. The general sequence of fossils from bottom upward in layered rocks had been recognized before Darwin proposed that the succession of biological forms strongly implied evolution. The farther back into the past one looked, the less the fossils resembled recent forms, the more the various lineages merged, and the broader the implications of a common ancestry.

Although gaps in the paleontological record remain, many have been filled by the researches of paleontologists since Darwin's time. Millions of fossil organisms found in well-dated rock sequences represent a succession of forms through time and manifest many evolutionary transitions. Microbial life of the simplest type (i.e., procaryotes, which are cells whose nuclear matter is not bound by a nuclear membrane) was already in existence more than three billion years ago. The oldest evidence of more complex organisms (i.e., eukaryotic cells with a true nucleus) has been discovered in flinty rocks approximately 1.4 billion years old. More advanced forms like algae, fungi, higher plants, and animals have been found only in younger geological strata. The following list presents the order in which increasingly complex forms of life appeared:

The sequence of observed forms and the fact that all (except the procaryotes) are constructed from the same basic cellular type strongly imply that all these major categories of life (including plants, algae, and fungi) have a common ancestry in the first eukaryotic cell. Moreover, there have been so many discoveries of intermediate forms between fish and amphibians, between amphibians and reptiles, between reptiles and mammals that it is often difficult to identify categorically along the line when the transition occurs from one to another particular genus or from one to another particular species. Nearly all fossils can be regarded as intermediates in some sense; they are life forms that come between ancestral forms that preceded them and those that followed.

Inferences about common descent derived from paleontology have been reinforced by comparative anatomy. The skeletons of humans, dogs, whales, and bats are strikingly similar, despite the different ways of life led by these animals and the diversity of environments in which they have flourished. The correspondence, bone by bone, can be observed in every part of the body, including the limbs: Yet a person writes, a dog runs, a whale swims, and a bat flies with structures built of the same bones. Such structures, called homologous, are best explained by common descent. Comparative anatomists investigate such homologies, not only in bone structure but also in other parts of the body as well, working out relationships from degrees of similarity.

The mammalian ear and jaw offer an example in which paleontology and comparative anatomy combine to show common ancestry through transitional stages. The lower jaws of mammals contain only one bone, whereas those of reptiles have several. The other bones in the reptile jaw are homologous with bones now found in the mammalian ear. What function could these bones have had during intermediate stages? Paleontologists have discovered intermediate forms of mammal-like reptiles (Therapsida ) with a double jaw joint—one composed of the bones that persist in mammalian jaws, the other consisting of bones that eventually became the hammer and anvil of the mammalian ear. Similar examples are numerous.

Biogeography also has contributed evidence for common descent. The diversity of life is stupendous. Approximately 250,000 species of living plants, 100,000 species of fungi, and 1.5 million species of animals and microorganisms have been described and named, and the census is far from complete. Some species, such as human beings and our companion the dog, can live under a wide range of environmental conditions. Others are amazingly specialized. One species of the fungus Laboulbenia grows exclusively on the rear portion of the covering wings of a single species of beetle (Aphaenops cronei ) found only in some caves of southern France. The larvae of the fly Drosophila carcinophila can develop only in specialized grooves beneath the flaps of the third pair of oral appendages of the land crab Gecarcinus ruricola, which is found only on certain Caribbean islands.

How can one make intelligible the colossal diversity of living beings and the existence of such extraordinary, seemingly whimsical creatures as Laboulbenia, Drosophila carcinophila, and others? Why are island groups like the Galápagos inhabited by forms similar to those on the nearest mainland but belonging to different species? Why is the indigenous life so different on different continents? The explanation is that biological diversity results from an evolutionary process whereby the descendants of local or migrant predecessors became adapted to diverse environments. For example, approximately two thousand species of flies belonging to the genus Drosophila are now found throughout the world. About one-quarter of them live only in Hawaii. More than a thousand species of snails and other land mollusks are also only found in Hawaii. The explanation for the occurrence of such great diversity among closely similar forms is that the differences resulted from adaptive colonization of isolated environments by animals with a common ancestry. The Hawaiian Islands are far from, and were never attached to, any mainland or other islands, and thus they have had few colonizers. No mammals other than one bat species lived on the Hawaiian Islands when the first human settlers arrived; very many other kinds of plants and animals were also absent. The explanation is that these kinds of organisms never reached the islands because of their great geographic isolation, while those that reached there multiplied in kind, because of the absence of related organisms that would compete for resources.

Embryology, the study of biological development from the time of conception, is another source of independent evidence for common descent. Barnacles, for instance, are sedentary crustaceans with little apparent similarity to such other crustaceans as lobsters, shrimps, or copepods. Yet barnacles pass through a free-swimming larval stage, in which they look unmistakably like other crustacean larvae. The similarity of larval stages supports the conclusion that all crustaceans have homologous parts and a common ancestry. Human and other mammalian embryos pass through a stage during which they have unmistakable but useless grooves similar to gill slits found in fishes—evidence that they and the other vertebrates shared remote ancestors that respired with the aid of gills.

The substantiation of common descent that emerges from all the foregoing lines of evidence is being validated and reinforced by the discoveries of modern biochemistry and molecular biology, a biological discipline that has emerged in the mid twentieth century. This new discipline has unveiled the nature of hereditary material and the workings of organisms at the level of enzymes and other molecules. Molecular biology provides very detailed and convincing evidence for biological evolution.

The genetic basis of evolution

The central argument of Darwin's theory of evolution starts from the existence of hereditary variation. Experience with animal and plant breeding demonstrates that variations can be developed that are "useful to man." So, reasoned Darwin, variations must occur in nature that are favorable or useful in some way to the organism itself in the struggle for existence. Favorable variations are ones that increase chances for survival and procreation. Those advantageous variations are preserved and multiplied from generation to generation at the expense of less advantageous ones. This is the process known as natural selection. The outcome of the process is an organism that is well adapted to its environment, and evolution occurs as a consequence.

Biological evolution is the process of change and diversification of organisms over time, and it affects all aspects of their lives—morphology, physiology, behavior, and ecology. Underlying these changes are changes in the hereditary material (DNA). Hence, in genetic terms, evolution consists of changes in the organism's hereditary makeup. Natural selection, then, can be defined as the differential reproduction of alternative hereditary variants, determined by the fact that some variants increase the likelihood that the organisms having them will survive and reproduce more successfully than will organisms carrying alternative variants. Selection may be due to differences in survival, in fertility, in rate of development, in mating success, or in any other aspect of the life cycle. All these differences can be incorporated under the term differential reproduction because all result in natural selection to the extent that they affect the number of progeny an organism leaves.

Evolution can be seen as a two-step process. First, hereditary variation takes place; second, selection occurs of those genetic variants that are passed on most effectively to the following generations. Hereditary variation also entails two mechanisms: the spontaneous mutation of one variant to another, and the sexual process that recombines those variants to form a multitude of variations.

The information encoded in the nucleotide sequence of DNA is, as a rule, faithfully reproduced during replication, so that each replication results in two DNA molecules that are identical to each other and to the parent molecule. But occasionally "mistakes," or mutations, occur in the DNA molecule during replication, so that daughter molecules differ from the parent molecules in at least one of the letters in the DNA sequence. Mutations can be classified into two categories: gene, or point, mutations, which affect one or only a few letters (nucleotides) within a gene; and chromosomal mutations, which either change the number of chromosomes or change the number or arrangement of genes on a chromosome. Chromosomes are the elongated structures that store the DNA of each cell.

Newly arisen mutations are more likely to be harmful than beneficial to their carriers, because mutations are random events with respect to adaptation; that is, their occurrence is independent of any possible consequences. Harmful mutations are eliminated or kept in check by natural selection. Occasionally, however, a new mutation may increase the organism's adaptation. The probability of such an event's happening is greater when organisms colonize a new territory or when environmental changes confront a population with new challenges. In these cases there is greater opportunity for new mutations to be better adaptive. The consequences of mutations depend on the environment. Increased melanin pigmentation may be advantageous to inhabitants of tropical Africa, where dark skin protects them from the Sun's ultraviolet radiation; but it is not beneficial in Scandinavia, where the intensity of sunlight is low and light skin facilitates the synthesis of vitamin D.

Mutation rates are low, but new mutants appear continuously in nature because there are many individuals in every species and many genes in every individual. More important is the storage of variation, arisen by past mutations. Thus, it is not surprising to see that when new environmental challenges arise, species are able to adapt to them. More than two hundred insect species, for example, have developed resistance to the pesticide DDT in different parts of the world where spraying has been intense. Although the insects had never before encountered this synthetic compound, they adapted to it rapidly by means of mutations that allowed them to survive in its presence. Similarly, many species of moths and butterflies in industrialized regions have shown an increase in the frequency of individuals with dark wings in response to environmental pollution, an adaptation known as industrial melanism. The examples can be multiplied at will.

Dynamics of genetic change

The genetic variation present in natural populations of organisms is sorted out in new ways in each generation by the process of sexual reproduction. But heredity by itself does not change gene frequencies. This principle is formally stated by the Hardy-Weinberg law, an algebraic equation that describes the genetic equilibrium in a population.

The Hardy-Weinberg law plays in evolutionary studies a role similar to that of Isaac Newton's First Law of Motion in mechanics. Newton's First Law says that a body not acted upon by a net external force remains at rest or maintains a constant velocity. In fact, there are always external forces acting upon physical objects (gravity, for example), but the first law provides the starting point for the application of other laws. Similarly, organisms are subject to mutation, selection, and other processes that change gene frequencies, and the effects of these processes are calculated by using the Hardy-Weinberg law as the starting point. There are four processes of gene frequency change: mutation, migration, drift, and natural selection.

Mutations change gene frequencies very slowly, since mutation rates are low. Migration, or gene flow, takes place when individuals migrate from one population to another and interbreed with its members. The genetic make-up of populations changes locally whenever different populations intermingle. In general, the greater the difference in gene frequencies between the resident and the migrant individuals, and the larger the number of migrants, the greater effect the migrants have in changing the genetic constitution of the resident population.

Moreover, gene frequencies can change from one generation to another by a process of pure chance known as genetic drift. This occurs because populations are finite in numbers, and thus the frequency of a gene may change in the following generation by accidents of sampling, just as it is possible to get more or less than fifty "heads" in one hundred throws of a coin simply by chance. The magnitude of the gene frequency changes due to genetic drift is inversely related to the size of the population; the larger the number of reproducing individuals, the smaller the effects of genetic drift. The effects of genetic drift from one generation to the next are quite small in most natural populations, which generally consist of thousands of reproducing individuals. The effects over many generations are more important. Genetic drift can have important evolutionary consequences when a new population becomes established by only a few individuals, as in the colonization of islands and lakes. This is one reason why species in neighboring islands, such as those in the Hawaiian archipelago, are often more heterogeneous than species in comparable continental areas adjacent to one another.

Natural selection

Darwin proposed that natural selection promotes the adaptation of organisms to their environments because the organisms carrying useful variants leave more descendants than those lacking them. The modern concept of natural selection is defined in mathematical terms as a statistical bias favoring some genetic variants over their alternates. The measure to quantify natural selection is called fitness.

If mutation, migration, and drift were the only processes of evolutionary change, the organization of living things would gradually disintegrate because they are random processes with respect to adaptation. Those three processes change gene frequencies without regard for the consequences that such changes may have in the welfare of the organisms. The effects of such processes alone would be analogous to those of a mechanic who changed parts in a motorcar engine at random, with no regard for the role of the parts in the engine. Natural selection keeps the disorganizing effects of mutation and other processes in check because it multiplies beneficial mutations and eliminates harmful ones. Natural selection accounts not only for the preservation and improvement of the organization of living beings but also for their diversity. In different localities or in different circumstances, natural selection favors different traits, precisely those that make the organisms well adapted to the particular circumstances.

The origin of species

In everyday experience we identify different kinds of organisms by their appearance. Everyone knows that people belong to the human species and are different from cats and dogs, which in turn are different from each other. There are differences among people, as well as among cats and dogs; but individuals of the same species are considerably more similar among themselves than they are to individuals of other species. But there is more to it than that; a bulldog, a terrier, and a golden retriever are very different in appearance, but they are all dogs because they can interbreed. People can also interbreed with one another, and so can cats, but people cannot interbreed with dogs or cats, nor can these breed with each other. Although species are usually identified by appearance, there is something basic, of great biological significance, behind similarity of appearance; namely, that individuals of a species are able to interbreed with one another but not with members of other species. This is expressed in the following definition: Species are groups of interbreeding natural populations that are reproductively isolated from other such groups.

The ability to interbreed is of great evolutionary importance, because it determines that species are independent evolutionary units. Genetic changes originate in single individuals; they can spread by natural selection to all members of the species but not to individuals of other species. Thus, individuals of a species share a common gene pool that is not shared by individuals of other species, because they are reproductively isolated.

Adaptive radiation is a form of speciation that occurs when colonizers reach geographically remote areas, such as islands, where they find an opportunity to diverge as they become adapted to the new environment. Sometimes a multiplicity of new environments becomes available to the colonizers, giving rise to several different lineages and species. This process of rapid divergence of multiple species from a single ancestral lineage is called adaptive radiation.

Examples of speciation by adaptive radiation in archipelagos removed from the mainland have already been mentioned. The Galápagos Islands are about six hundred miles off the west coast of South America. When Darwin arrived there in 1835, he discovered many species not found anywhere else in the world—for example, fourteen species of finch (known as Darwin's finches). These passerine birds have adapted to a diversity of habitats and diets, some feeding mostly on plants, others exclusively on insects. The various shapes of their bills are clearly adapted to probing, grasping, biting, or crushing—the diverse ways in which these different Galápagos species obtain their food. The explanation for such diversity (which is not found in finches from the continental mainland) is that the ancestor of Galápagos finches arrived in the islands before other kinds of birds and encountered an abundance of unoccupied ecological opportunities. The finches underwent adaptive radiation, evolving a variety of species with ways of life capable of exploiting niches that in continental faunas are exploited by different kinds of birds. Some striking examples of adaptive radiation that occur in the Hawaiian Islands were mentioned earlier.

Rapid modes of speciation are known by a variety of names, such as quantum, rapid, and saltational speciation, all suggesting the short time involved. An important form of quantum speciation is polyploidy, which occurs by the multiplication of entire sets of chromosomes. A typical (diploid) organism carries in the nucleus of each cell two sets of chromosomes, one inherited from each parent; a polyploid organism has several sets of chromosomes. Many cultivated plants are polyploid: bananas have three sets of chromosomes, potatoes have four, bread wheat has six, some strawberries have eight. All major groups of plants have natural polyploid species, but they are most common among flowering plants (angiosperms) of which about forty-seven percent are polyploids.

In animals, polyploidy is relatively rare because it disrupts the balance between chromosomes involved in the determination of sex. But polyploid species are found in hermaphroditic animals (individuals having both male and female organs), which include snails and earthworms, as well as in forms with parthenogenetic females (which produce viable progeny without fertilization), such as some beetles, sow bugs, goldfish, and salamanders.

Gradual and punctuational evolution

Morphological evolution is by and large a gradual process, as shown by the fossil record. Major evolutionary changes are usually due to a building up over the ages of relatively small changes. But the fossil record is discontinuous. Fossil strata are separated by sharp boundaries; accumulation of fossils within a geologic deposit (stratum) is fairly constant over time, but the transition from one stratum to another may involve gaps of tens of thousands of years. Different species, characterized by small but discontinuous morphological changes, typically appear at the boundaries between strata, whereas the fossils within a stratum exhibit little morphological variation. That is not to say that the transition from one stratum to another always involves sudden changes in morphology; on the contrary, fossil forms often persist virtually unchanged through several geologic strata, each representing millions of years.

According to some paleontologists the frequent discontinuities of the fossil record are not artifacts created by gaps in the record, but rather reflect the true nature of morphological evolution, which happens in sudden bursts associated with the formation of new species. This proposition is known as the punctuated equilibrium model of morphological evolution. The question whether morphological evolution in the fossil record is predominantly punctuational or gradual is a subject of active investigation and debate. The argument is not about whether only one or the other pattern ever occurs; it is about their relative frequency. Some paleontologists argue that morphological evolution is in most cases gradual and only rarely jerky, whereas others think the opposite is true. Much of the problem is that gradualness or jerkiness is in the eye of the beholder.

DNA and protein evolution

The advances of molecular biology have made possible the comparative study of proteins and the nucleic acid DNA, which is the repository of hereditary (evolutionary and developmental) information. Nucleic acids and proteins are linear molecules made up of sequences of units—nucleotides in the case of nucleic acids, amino acids in the case of proteins—which retain considerable amounts of evolutionary information. Comparing macromolecules from two different species establishes the number of their units that are different. Because evolution usually occurs by changing one unit at a time, the number of differences is an indication of the recency of common ancestry. Changes in evolutionary rates may create difficulties, but macromolecular studies have two notable advantages over comparative anatomy and other classical disciplines. One is that the information is more readily quantifiable. The number of units that are different is precisely established when the sequence of units is known for a given macromolecule in different organisms. The other advantage is that comparisons can be made even between very different sorts of organisms. There is very little that comparative anatomy can say when organisms as diverse as yeasts, pine trees, and human beings are compared; but there are homologous DNA and protein molecules that can be compared in all three.

Informational macromolecules provide information not only about the topology of evolutionary history, but also about the amount of genetic change that has occurred in any given branch. Studies of molecular evolution rates have led to the proposition that macromolecules evolve at fairly constant rates and, thus, that they can be used as evolutionary clocks, in order to determine the time when the various branching events occurred. The molecular evolutionary clock is not a metronomic clock, like a watch or other timepiece that measures time exactly, but a stochastic clock like radioactive decay. In a stochastic clock, the probability of a certain amount of change is constant, although some variation occurs in the actual amount of change. Over fairly long periods of time, a stochastic clock is quite accurate. The enormous potential of the molecular evolutionary clock lies in the fact that each gene or protein is a separate clock. Each clock "ticks" at a different rate—the rate of evolution characteristic of a particular gene or protein—but each of the thousands of genes or proteins provides an independent measure of the same evolutionary events.

Evolutionists have found that the amount of variation observed in the evolution of DNA and proteins is greater than is expected from a stochastic clock; in other words, the clock is inaccurate. The discrepancies in evolutionary rates along different lineages are not excessively large, however. It turns out that it is possible to time phylogenetic events with accuracy, but more genes or proteins must be examined than would be required if the clock were stochastically accurate. The average rates obtained for several DNA sequences or proteins taken together provide a fairly precise clock, particularly when many species are investigated.

See also Adaptation; Darwin, Charles; Ecology; Fitness; Genetics; Life, Origins of; Life Sciences; Mutation; Selection, Levels of; Sociobiology

Bibliography

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ayala, francisco j., and fitch, walter m., eds. genetics and the origin of species: from darwin to molecular biology 60 years after dobzhansky. washington, d.c.: national academy press, 1997.

ayala, francisco j.; fitch, walter m.; and clegg, michael t., eds. variation and evolution in plants and microorganisms: toward a new synthesis 50 years after stebbins. washington, d.c.: national academy press, 2000

dawkins, richard. climbing mount improbable. new york and london: norton, 1996.

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francisco j. ayala

Biological Evolution

Biological evolution is the change in the allele frequency of a gene in a population over time. That is to say some genetic change has happened in the population between generations. Only populations can evolve, not individuals. Individuals can not change their genetic makeup. Only between generations, is there the possibility for genetic changes due to the forces of evolution. These forces are natural selection, mutation, gene flow, nonrandom mating, and genetic drift. Evolution is a measure of a population, not of an individual. Genetic variation, genetic differences between individuals, must exist for evolution to occur.

Charles Darwin defined evolution as descent with modification. However, Darwin did not understand the genetic basis to evolution. Not until Gregor Mendel's work was rediscovered in 1900 could modification with descent be understood in terms of maintaining genetic variation. The mathematical proofs of Godfrey Harold Hardy and Wilhelm Weinberg, known as the Hardy-Weinberg theorem, started the field of population genetics, the integration of Darwinian selection and Mendelian genetics. Their proof showed how variation can be maintained because each individual had two alleles for each gene. This is in contrast to Darwin, who specified a kind of blending inheritance in which offspring were intermediate to the parents. Just as importantly, their work specified the forces (causes) of evolution. Population genetics is the foundation for modern evolutionary biology. Other population geneticists, such as Ronald A. Fisher, John B. S. Haldane, and Sewall Wright, contributed to the foundations of the theory of population genetics from the 1920s to the 1940s.

All animals are the descendants of a single common ancestor. Biological evolution has created the diversity of organisms we see today, as well as extinct animals such as dinosaurs for which we have the fossil record . The diversifying action of evolution to create new species is called speciation. Speciation is the splitting of one former species into two species that are reproductively isolated from each other such that they no longer successfully reproduce and exchange genes. Speciation is the result of a combination of biogeography , natural selection, adaptations, and the other evolutionary forces.

There are two main modes of speciation: allopatric and sympatric. Allopatric speciation is the division of one population into two populations because of some geographical barrier. While separated, each population evolves differently from the other population. When contact is restored between the two populations, they cannot reproduce, and so are unable to exchange genes because of the differences they acquired while separated. Sympatric speciation is when one population splits into two without any geographical barrier. While this mode of speciation was doubted for years, in the 1960s Guy Bush conducted experiments on fruit flies that supported this mode of speciation. In the early 1980s, Bill Rice conducted laboratory experiments in which he was able to cause sympatric speciation. Even though sympatric speciation is possible, it is not as common as allopatric speciation.

Causes of Biological Evolution

There are five forces that cause evolution: natural selection, mutation, gene flow, nonrandom mating, and genetic drift. All five depend on the existence of genetic variation, which is necessary for any evolutionary change. Natural selection is the differences in the survival and reproduction rates of individuals with different phenotypes . When phenotypes can be genetically inherited, natural selection produces adaptations as the population evolves. Natural selection can remove variation from a population if it is stabilizing selection. Diversifying selection can increase the amount of variation in a population. Directional selection changes the average trait in the population.

Genetic mutations occur when errors are made in replicating (copying) and dividing DNA. Mutation is the ultimate source of genetic variation. Most of the time, mutations either have no effect on the phenotype, and therefore are neutral, or have a harmful effect. Rarely, a mutation will create a phenotype that is better, and so natural selection will favor this beneficial mutation. Mutations happen naturally at low levels of frequency. These levels can be much higher under some conditions. For example, exposure to radiation and to some toxic chemicals produces higher mutation rates.

Gene flow is the exchange of genes among populations or species. The exchange of genes between species is called hybridization. Introducing new genes into a population changes the gene frequency and causes evolution. Gene flow can be positive or negative for a population. Lots of gene flow can prevent local adaptation because any evolution produced by natural selection is swamped by the invading genes. On the other hand, gene flow can introduce a new beneficial gene into a population. Natural selection can favor this new adaptation, and it can spread through the population.

Nonrandom mating changes what combinations of genes are mixed together in sexual reproduction. Sexual reproduction creates new individuals, half of whose genetic information comes from the mother and half from the father. If individuals within a population who have a particular genotype pair off and mate at a rate different from the occurrence of that gentotype in the population, then nonrandom mating is occurring. Nonrandom mating can be caused by mating among close relatives, or inbreeding, which can result from population subdivision. Nonrandom mating also happens when individuals choose mates based on particular phenotypes. In some animal species, a few males get most of the matings because they have some highly desirable phenotype. Assortative mating also produces nonrandom mating, which is the mating of males and females of the same phenotype. For example, large male frogs mate with large female frogs and small males mate with small females.

Genetic drift causes evolution by random changes in the allele frequencies. One way for genetic drift to happen is for some of the alleles to be left after some kind of fluctuations in population size. For example, if disease wipes out most of a population, only some alleles will be left in the population. Also, only some combinations of alleles for different genes will be left. If natural selection is not acting on a gene, then random genetic drift can be a stronger force than if selection is present. The impact of drift depends on the population size. Drift is stronger in smaller populations; they are more susceptible to random changes in allele frequencies since there are not as many alleles present.

Limits to Biological Evolution

What can limit evolution? Three main factors restrict the amount of change evolution can make in a population: the degree of genetic variation is limited; natural selection produces adaptations that are a compromise in form and function; and most forces of evolution are not adaptive.

First, genetic variation is the ultimate barrier to evolution. If there is no genetic variation, no evolution can happen. Genetic variation is limited to the history of the organism. A bear will not suddenly gain wings in a few generations of evolution. No bear has ever had wings, and it is unlikely that any bear will evolve them. An organism contains only so much DNA and the amount of existing genetic variation, the raw material for evolution, is restricted by the past history of the species. A bear does not have the underlying genetic variation necessary for a mutation to produce wings from the existing variation.

Second, adaptations are usually compromises and therefore limit evolution. Natural selection works on a whole organism rather than just single traits, so it is the combination of traits that natural selection favors. A cheetah is a fast runner but a poor swimmer. Any cheetah with webbed feet would be a better swimmer but could certainly not run as fast. Adaptations are trade-offs.

Third, many forces of evolution are not adaptive. Natural selection is the force of evolution that produces adaptations but the other forces of evolution are not necessarily adaptive. Gene flow can introduce genes into a population that are better suited to another environment. Nonrandom mating can break up existing combinations of genes that work well together. Mutation is typically harmful. Random genetic drift is frequently not beneficial. Most forces of evolution are random and can be working counter to natural selection.

Rates of Evolution

Does evolution proceed at a fast pace or a slow pace? How much of evolution can we actually observe? In 1972 Niles Eldredge and Stephen J. Gould wrote an article that presented the idea of punctuated equilibrium . Some organisms for which there are good fossil records show long periods of no morphological evolution (evolution in the form and structure of organisms); the animals remain unchanged over thousands of years. But then there suddenly appears what looks like a morphologically similar new species. The theory of punctuated equilibrium is that long periods of no change are followed by short periods of rapid transition. This is in direct contrast to gradualism. Gradualism suggests slow but continuous change over geological time. How is one to know if the fossil record is incomplete, and that the seemingly rapid change is accounted for by missing intermediate stages?

This question has inspired research on the rate of evolutionary change. It is possible to calculate rates of morphological evolution from the fossil record. Evolutionary rates can be measured over several generations in natural and laboratory populations. It is also possible to measure the relative rate of change in molecules for which the gene sequence is known. The sequence of a gene is the order of nucleotides within it. Sexual reproduction can also increase the rate of evolution compared to asexual reproduction. This is due to increased genetic variation by recombination and independent assortment. Gene sequencing has made it possible to investigate how the rate of evolution changes with the degree of underlying genetic variation, also called genetic polymorphism . In 1991 the first important test of rates of molecular evolution and molecular polymorphism was conducted by J. H. McDonald and Martin Kreitman. As the entire genetic material (genomes) of more and more organisms are sequenced, we will understand more about the rate and mechanisms of evolution.

see also Adaptation; Genes; Genetics; Natural Selection.

Laura A. Higgins

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