Punctuated Equilibrium

BIBLIOGRAPHY

Punctuated equilibrium is a descriptive hypothesis in evolutionary biology concerned with macroevolutionary dynamics specifically at the level of speciation. Punctuated equilibrium holds that most species originate by the splitting of a population during brief geological periods (punctuations), and that subsequently species persist with only relatively moderate morphological change (stasis) for the remainder of their existences. “Evolution by jerks,” as punctuated equilibrium has been lucidly labeled, is usually contrasted with phyletic gradualism (“evolution by creeps”), which states that species evolve uniformly and slowly by the gradual transformation of large populations. Proposed jointly by paleontologists Niles Eldredge and Stephen Jay Gould (1941–2002) in 1972, punctuated equilibrium immediately lit a scientific controversy that has smoldered ever since.

Punctuated equilibrium was, however, hardly controversial at its inception. As a general characterization of macroevolutionary processes, it was largely presaged by Hugh Falconer (1808–1865) and Charles Darwin (1809–1882) in the mid-nineteenth century and later echoed by Hermann J. Muller (1890–1967) and George Gaylord Simpson (1902–1984) in the early twentieth century. Eldredge and Gould postulated their version of the hypothesis as a logical extension (into paleontology) of Ernst Mayr’s (1904–2005) ecological theory of allopatric speciation (1963), which was widely considered the dominant theory of speciation. Allopatric theory proposes that speciation occurs when a smaller subpopulation becomes geographically isolated from its parent population. Over time, this peripheral daughter population diverges in isolation until it can no longer interbreed with the parent. Speciation as such happens relatively quickly in a small population and in a limited geographic range. Due to the relative brevity of this localized speciation event, intermediate morphologies will be unlikely to fossilize and will be rare even if they do, producing an apparent paleontological pattern of stasis punctuated by discontinuous speciation events.

While born of an orthodox evolutionary theory, punctuated equilibrium has been associated throughout its existence with more radical evolutionary concepts. The scientific controversy over punctuated equilibrium is multi-faceted, but it has largely coalesced around three related issues: (1) whether punctuated equilibrium is “Darwinian”; (2) whether stasis truly is the predominant mode of evolution in the fossil record; and (3) whether the proposed mechanisms for both stasis and punctuations are valid.

One of the easiest means to draw attention to an evolutionary concept is to pronounce it “anti-Darwinian,” and punctuated equilibrium has been no exception. Both proponents and detractors of punctuated equilibrium often claim that Darwin explained the incompleteness of the paleontological record as solely the result of the imperfect geological preservation of fossils. However, as pointed out by Frank H. T. Rhodes (1983, 1987), Darwin in fact devoted the larger part of chapter 10 of On the Origin of Species (titled “On the Imperfection of the Geological Record”) to explaining how “gaps” in the fossil record are a direct consequence of speciation processes and the nature of natural selection. Like Eldredge and Gould, Darwin thought it likely that most species exist in a state of morphological stasis, only intermittently broken by bursts of localized change and speciation. Darwin found this point so important for understanding the paleontological record that he reiterated the argument in three separate chapters (Darwin [1872] 1993, chap. 4, “Natural Selection,” p. 152; chap. 10, p. 428; and chap. 15, “Recapitulation and Conclusion,” p. 619).

Furthermore, in debates over punctuated equilibrium, the term gradualism has been used in at least two different senses. Eldredge and Gould’s phyletic gradualism concerns the tempo of evolution, entailing evolutionary trajectories that are geologically slow, constant, and unidirectional—a concept pointedly contrasted with the rapid speciation described by punctuated equilibrium. For Darwin, however, gradual has little to do with rate ([1872] 1993, pp. 312–317). Rather, it means that evolution by natural selection advances in small “grades” that are dependent on a population’s normal genetic variation. Morphological change therefore can be genetically gradual and geologically rapid. While phyletic gradualism may have been a widespread evolutionary assumption in the twentieth century, it cannot be pinned on Darwin himself. Thus, given the significant overlap between the tenets of punctuationism and Darwin’s views, punctuated equilibrium is resolutely “Darwinian.”

Whether the fossil record truly displays a predominant pattern of stasis continues to be an active area of paleontological research. Punctuated equilibrium is most readily tested when geological strata are well resolved temporally with abundant fossil preservation. Empirical studies have resulted in mixed appraisals, with a roughly equal spread among those supporting phyletic gradualism, punctuated equilibrium, or a third “hybrid” process that may be described as punctuated gradualism (Erwin and Anstey 1995). Probably the most tangible contribution of the punctuated equilibrium controversy has been the widespread acceptance of Gould and Eldredge’s claim that “stasis is data.” Lack of morphological change is an evolutionary pattern that warrants an explanation.

The assortment of mechanisms that Gould and Eldredge have proposed to explain stasis and rapid speciation is the most contentious aspect of the punctuated equilibrium debate (Coyne and Charlesworth 1997). Eldredge and Gould initially implied that punctuated equilibrium is explained adequately by Mayr’s mainstream theory of allopatric speciation. However, they successively suggested numerous additional “non-Darwinian” mechanisms, including saltational mutations and species selection (Gould 1980, 2002), none of which have been broadly accepted. Emphasizing an antireductionist pluralism, Eldredge and Gould further claimed that each of these speculative mechanisms was significant for its potential to decouple lower-level genetic processes from upper-level macroevolutionary trends. Perhaps ironically, the punctuationist paradigm has been adopted more recently by molecular biologists, who have found within in vitro evolution experiments analogous patterns of stasis interleaved with rapid genetic change (Elena et al. 1996). Thus, punctuated equilibrium may ultimately find its raison d’être in the very reductionist realm so vigorously opposed by Gould: molecular evolution via the “selfish gene.”

SEE ALSO Anthropology, Biological; Archaeology; Darwin, Charles; Gould, Stephen Jay

BIBLIOGRAPHY

Coyne, Jerry A., and Brian Charlesworth. 1997. On Punctuated Equilibria. Science 276 (5311): 338–341.

Darwin, Charles. [1872] 1993. The Origin of Species by Means of Natural Selection. 6th ed. New York: Modern Library.

Eldredge, Niles, and Stephen Jay Gould. 1972. Punctuated Equilibria: An Alternative to Phyletic Gradualism. In Models in Paleobiology, ed. Thomas J. M. Schopf, 82–115. San Francisco: Freeman, Cooper.

Elena, Santiago F., Vaughn S. Cooper, and Richard E. Lenski. 1996. Punctuated Evolution Caused by Selection of Rare Beneficial Mutations. Science 272 (5269): 1802–1804.

Erwin, Douglas H., and Robert L. Anstey. 1995. Introduction. In New Approaches to Speciation in the Fossil Record, 11–38. New York: Columbia University Press.

Gould, Stephen Jay. 1980. Is a New and General Theory of Evolution Emerging? Paleobiology 6 (1): 119–130.

Gould, Stephen Jay. 2002. The Structure of Evolutionary Theory. Cambridge, MA: Belknap.

Gould, Stephen Jay, and Niles Eldredge. 1977. Punctuated Equilibria: The Tempo and Mode of Evolution Reconsidered. Paleobiology 3: 115–151.

Mayr, Ernst. 1963. Animal Species and Evolution. Cambridge, MA: Harvard University Press.

Rhodes, Frank H. T. 1983. Gradualism, Punctuated Equilibria, and the Origin of Species. Nature 305 (5932): 269–272.

Rhodes, Frank H. T. 1987. Darwinian Gradualism and Its Limits: The Development of Darwin’s Views on the Rate and Pattern of Evolutionary Change. Journal of the History of Biology 20 (2): 139–157.

Douglas L. Theobald

Punctuated Equilibrium

Resources

Punctuated equilibrium is a theory about how new species evolve that was first advanced by American paleontologists Niles Eldredge (1943–) and Stephen Jay Gould (1941–2002) in 1972. Although controversial, punctuated equilibrium has stimulated fruitful debate about speciation (the birth of new species) and the fossil record and has won at least partial acceptance among most evolutionary biologists.

Before punctuated equilibrium, most scientists assumed that evolutionary change occurs slowly and continuously in almost all species, and that new species originate either by slow divergence from parental stock of sub-populations or by slow evolutionary transformation of the parental stock itself. Punctuated equilibrium proposes that most species originate relatively suddenly (i.e., over tens of thousands or hundreds of thousands of years, rather than the millions of years assumed by traditional theory) and then do not evolve significantly for the rest of their time on Earth. Most species thus have a sudden or punctuated origin and then remain in stasis or equilibrium until extinction.

Eldredge and Gould proposed punctuated equilibrium to explain one of the most notable features of the fossil record: most species seem to appear suddenly, already clearly differentiated from the earlier, similar species from which they presumably evolved, and then remain unchanged until becoming extinct. (Most species become extinct a few million years after appearing; a few last for tens of millions of years or longer.) Traditional evolutionary theory, beginning with the Origin of Species (1859) by English naturalist Charles Darwin (1809–1882), proposed that gradual evolutionary changes are rarely observed in the fossil record because that record is radically incomplete. Fossils form only under certain special conditions, fossil-bearing rocks are eroded as well as deposited, and our knowledge even of those fossils that have been formed is fragmentary. It follows that in the fossil record we glimpse only a few isolated frames cut at long intervals from a long, slow-moving film. As Darwin himself put it, “I look at the natural geological record, as a history of the world imperfectly kept . . . of this history we possess the last volume alone, relating only to two or three countries. Of this volume, only here and there a short chapter has been preserved; and of each page, only here and there a few lines.”

Eldredge and Gould agree that the fossil record is incomplete, but contend that it could not be incomplete enough to account for the near-complete absence of gradualistic change from the fossil record. Rather, they propose, species normally originate too quickly for normal geological processes to record the event; a single bedding plane (minimal layer of sedimentary rock) often compresses tens of thousands of years into a thin slice. Furthermore, according to standard evolutionary theory of 1972, speciation usually occurs when small populations cut off from interbreeding with related groups—say, by loss of a watercourse connecting two lakes, or by colonization of an island—evolve rapidly in isolation. Because there are fewer individuals in such an isolated population, favorable mutations can spread more readily. A small, isolated, rapidly-evolving population may become extinct without leaving any trace at all in the fossil record. Eldredge and Gould argued that if it does eventually break out of its isolation and spread over a wider area, it is likely to be observed in the fossil record as making a sudden or punctuational appearance, fully formed. They also proposed that the appearance of stasis or unchanging form manifested by most species in the fossil record is not an artifact produced by gross imperfections in the fossil record, but a raw fact. Evolutionary change in living forms, the two scientists argued, occurs mostly during speciation events and hardly otherwise.

It is important to note that by the standards of recorded human history, which covers only about 7,000 years, speciation is still a very gradual process under punctuated equilibrium theory. Punctuated equilibrium argues for much faster speciation than traditional evolutionary theory, but does not involve the proposition that new species appear in a generation or two. It is an evolutionary theory according to which hundreds or thousands of generations are needed for speciation, and natural selection must favor (or at least permit) all changes at every step. The novelty of punctuated equilibrium lies in its two proposals about rates of evolutionary change: (1) change happens rapidly, by geological standards, during speciation, and (2) change happens slowly or not at all after speciation.

A growing body of evidence indicates that both gradualistic and punctuational speciation have often occurred in the history of life, and that morphological stasis (long-term stability of form)—the “equilibrium” of “punctuated equilibrium”—is, as Eldredge and Gould claimed, often real, rather than an artifact of dropout in the fossil record. Several unusually perfect series of fossils have been discovered that have allowed paleontologists (fossil specialists) to trace the detailed history of entire groups of related organisms. In most such cases, paleontologists have observed gradualistic speciation, punctuational speciation, and morphological stasis, all in a single series of rocks, with punctuational speciation occurring about 10 times more frequently than gradualistic speciation. Observing gradualistic speciation and punctuational speciation in a single series of fossils proves both gradualism by direct observation, and punctuated equilibrium by disproof of the alternative possibility that gaps are responsible for the relatively sudden appearance of species in this case.

Evolutionary biologists continue to debate the question of relative frequency, that is, which happens more frequently in the history of life: gradualistic evolution or punctuated equilibrium? Although scientists who support punctuated equilibrium claim that the evidence shows a much greater relative frequency for punctuated equilibrium, debate continues. In 2006, a group of researchers announced in the journal Science that they had discovered strong genetic evidence for a high frequency of punctuational speciation versus gradual speciation.

See also Biodiversity; Evolution, convergent; Evolution, divergent; Evolution, evidence of; Evolution, parallel; Evolutionary change, rate of; Evolutionary mechanisms.

Resources

BOOKS

Eldredge, Niles. The Pattern of Evolution. New York: W. H. Freeman, 1998.

Gould, Stephen Jay. Punctuated Equilibrium. Cambridge, MA: Belknap Press, 2007.

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

PERIODICALS

Gould, Stephen J. and Niles Eldredge. “Punctuated equilibria: The Tempo and Mode of Evolution Reconsidered.” Paleobiology 3 (1977): 115–51.

Pagel, Mark, et al. “Large Punctuational Contribution of Speciation to Evolutionary divergence at the Molecular Level.” Science. 314 (2006): 119-121.

OTHER

Eldridge, Niles “Species, Speciation, and the Environment.” Actionbioscience (October, 2002) <http://www.actionbioscience.org/evolution/eldredge.html> (accessed November 15, 2006).

Larry Gilman

Punctuated equilibrium

Punctuated equilibrium is a theory about how new species evolve that was first advanced by American paleontologists Niles Eldredge and Stephen Jay Gould (1941–2002) in 1972. Although controversial, punctuated equilibrium has stimulated fruitful debate about speciation (the birth of new species) and the fossil record and has, in recent years, won at least partial acceptance among most evolutionary biologists.

Before punctuated equilibrium, most scientists assumed that evolutionary change occurs slowly and continuously in almost all species, and that new species originate either by slow divergence from parental stock of sub-populations or by slow evolutionary transformation of the parental stock itself. Punctuated equilibrium proposes that most species originate relatively suddenly (i.e., over tens of thousands or hundreds of thousands of years, rather than the millions of years assumed by traditional theory) and then do not evolve significantly for the rest of their time on Earth . Most species thus have a sudden or punctuated origin and then remain in stasis or equilibrium until extinction .

Eldredge and Gould proposed punctuated equilibrium to explain one of the most notable features of the fossil record: most species seem to appear suddenly, already clearly differentiated from the earlier, similar species from which they presumably evolved, and then remain unchanged until becoming extinct. (Most species become extinct a few million years after appearing; a few last for tens of millions of years or longer.) Traditional evolutionary theory, beginning with the Origin of Species (1859) by English naturalist Charles Darwin (1809–1882), proposed that gradual evolutionary changes are rarely observed in the fossil record because that record is radically incomplete. Fossils form only under certain special conditions, fossil-bearing rocks are eroded as well as deposited, and our knowledge even of those fossils that have been formed is fragmentary. It follows that in the fossil record we glimpse only a few isolated frames cut at long intervals from a long, slow-moving film. As Darwin himself put it, "I look at the natural geological record, as a history of the world imperfectly kept ... of this history we possess the last volume alone, relating only to two or three countries. Of this volume, only here and there a short chapter has been preserved; and of each page, only here and there a few lines."

Eldredge and Gould agree that the fossil record is incomplete, but contend that it could not be incomplete enough to account for the near-complete absence of gradualistic change from the fossil record. Rather, they propose, species normally originate too quickly for normal geological processes to record the event; a single bedding plane (minimal layer of sedimentary rock ) often compresses tens of thousands of years into a thin slice. Furthermore, according to standard evolutionary theory of 1972, speciation usually occurs when small populations cut off from interbreeding with related gropus—say, by loss of a watercourse connecting two lakes, or by colonization of an island—evolve rapidly in isolation. Because there are fewer individuals in such an isolated population, favorable mutations can spread more readily. A small, isolated, rapidly-evolving population may become extinct without leaving any trace at all in the fossil record. Eldredge and Gould argued that if it does eventually break out of its isolation and spread over a wider area, it is likely to be observed in the fossil record as making a sudden or punctuational appearance, fully formed. They also proposed that the appearance of stasis or unchanging form manifested by most species in the fossil record is not an artifact produced by gross imperfections in the fossil record, but a raw fact. Evolutionary change in living forms, the two scientists argued, occurs mostly during speciation events and hardly otherwise.

It is important to note that by the standards of recorded human history, which covers only about 7,000 years, speciation is still a very gradual process under punctuated equilibrium theory. Punctuated equilibrium argues for much faster speciation than traditional evolutionary theory, but does not involve the proposition that new species appear in a generation or two. It is an evolutionary theory according to which hundreds or thousands of generations are needed for speciation, and natural selection must favor (or at least permit) all changes at every step. The novelty of punctuated equilibrium lies in its two proposals about rates of evolutionary change: (1) change happens rapidly, by geological standards, during speciation, and (2) change happens slowly or not at all after speciation.

A growing body of evidence indicates that both gradualistic and punctuational speciation have often occurred in the history of life, and that morphological stasis (long-term stability of form)—the "equilibrium" of "punctuated equilibrium"—is, as Eldredge and Gould claimed, often real, rather than an artifact of dropout in the fossil record. Several unusually perfect series of fossils have been discovered that have allowed paleontologists (fossil specialists) to trace the detailed history of entire groups of related organisms. In most such cases, paleontologists have observed gradualistic speciation, punctuational speciation, and morphological stasis, all in a single series of rocks, with punctuational speciation occurring about 10 times more frequently than gradualistic speciation. Observing gradualistic speciation and punctuational speciation in a single series of fossils proves both gradualism by direct observation, and punctuated equilibrium by disproof of the alternative possibility that gaps are responsible for the relatively sudden appearance of species in this case.

Evolutionary biologists continue to debate the question of relative frequency, that is, which happens more frequently in the history of life: gradualistic evolution or punctuated equilibrium? Although scientists who support punctuated equilibrium claim that the evidence shows a much greater relative frequency for punctuated equilibrium, debate continues.

See also Biodiversity; Evolution, convergent; Evolution, divergent; Evolution, evidence of; Evolution, parallel; Evolutionary change, rate of; Evolutionary mechanisms.

Resources

books

Eldredge, Niles. The Pattern of Evolution New York: W. H. Freeman, 1998.

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

periodicals

Gould, Stephen J., and Niles Eldredge. "Punctuated Equilibria: The Tempo and Mode of Evolution Reconsidered." Paleobiology 3 (1977): 115–51.

other

Eldridge, Niles "Species, Speciation, and the Environment." Actionbioscience. October 2002 [cited January 10, 2003]. <http://www.actionbioscience.org/evolution/eldredge.html>.

Larry Gilman

punctuated equilibrium A hypothesis, published in 1972 by N. Eldredge and Stephen J. Gould, proposing that in evolutionary history most change occurs very rapidly in short bursts lasting typically less than 100 000 years and is associated with speciation events. In between these speciation events are long periods (perhaps millions of years) of relative stasis, in which little evolutionary change occurs. This hypothesis, which contradicted the orthodox Darwinian view of evolution as a gradual and continuous process, prompted controversy and often heated debate. The authors based their hypothesis on studies of various fossil lineages (e.g. ammonite molluscs) in which forms intermediate between species are absent, citing this as evidence that speciation events are often so brief as not to be represented in the fossil record. Subsequent scrutiny of the evidence supports a pattern of punctuated equilibrium for some, but not all, lineages, so it cannot be regarded as universal. For example, the rodent lineage shows as much morphological change between speciation events as during speciation.

punctuated equilibrium Theory, expounded by US palaeontologists Stephen Jay Gould and Niles Eldredge in 1972, that is strongly sceptical of the notion of gradual change in the evolution of the natural world, as advocated by Charles Darwin. Fossil records rarely document the gradual development of a new species, rather showing its seemingly sudden appearance. Darwin argues that this is due to gaps in the fossil records. Punctuated equilibrium explains this by describing each species as in a steady state (equilibrium), which is punctuated by brief but intense periods of sudden change that give rise to new species.