Punctuated equilibrium

Punctuated equilibrium (pl. punctuated equilibria) is a part of evolutionary theory that states that evolution, particularly speciation, occurs relatively quickly with long periods of little change—equilibria—in between. This theory is one of the proposed explanations of the evolutionary patterns of species as observed in the fossil record, particularly the relatively sudden appearance of new species in a geologically short time period, and the perhaps typical lack of substantial change of species during their existence.

The theory

Punctuated equilibria relies heavily on Ernst Mayr's concept of peripatric speciation and was articulated as a distinct theory by Niles Eldredge and Stephen Jay Gould in 1972 as a criticism to the Darwinian Theory of Evolution. According to Gould, "the ideas came mostly from Niles, with yours truly acting as a sounding board and the term..." It has been summarized by Gould (1980, pp. 183-4) as follows:

Related Topics:
Ernst Mayr's - Peripatric speciation - Niles Eldredge - Stephen Jay Gould - 1972

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:"Large, stable central populations exert a strong homogenizing influence . New and favorable mutations are diluted by the sheer bulk of the population through which they must spread....But small, peripherally isolated groups are cut off from their parental stock ... selective pressures are usually intense because peripheries mark the edge of ecological tolerance for ancestral forms. Favorable variations spread quickly...

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:"What should the fossil record include if most evolution occurs by speciation in peripheral isolates? ... In any local area inhabited by ancestors, a descendant species should appear suddenly by migration from a peripheral region in which it evolved. In the peripheral region itself, we might find direct evidence of speciation, but such good fortune would be rare indeed because the event occurs so rapidly in such a small population."

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An unstated supposition is that, through competition, the descendant species eliminates the ancestral species. In the late 19th century, following Charles Darwin's publication of The Origin of Species, Moritz Wagner had similarly proposed that isolation is actually necessary for speciation.

Related Topics:
Charles Darwin - The Origin of Species - Moritz Wagner

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The theory is usually contrasted with phyletic gradualism, though critics, notably Richard Dawkins, have argued that phyletic gradualism is merely a straw man. Eldredge and Gould's advocacy of the theory brought punctuated equilibrium much attention, especially since they phrased it in terms that made it appear to be a radical re-thinking of evolutionary theory. This was used by some creationists to argue that the theory of evolution is based on questionable grounds. Some detractors among evolutionary biologists wryly termed punctuated equilibrium "evolution by jerks." (It is now sometimes referred to by the slang "punk eek," with no negative connotations implied.) The actual differences between the various evolution theorists were not as large as they were made to appear. Gould himself later said that the theory did not in fact refute Darwin's gradualism, it just added the ideas of catastrophism and stasis.

Related Topics:
Phyletic gradualism - Richard Dawkins - Straw man - Creationists

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