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