Punctuated equilibrium
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Punctuated equilibrium, or punctuated equilibria, is a theory of evolution which states that changes such as speciation can occur 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.
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The theory
Punctuated equilibria was proposed as a distinct theory by Niles Eldredge and Stephen Jay Gould in 1972. According to Gould, "the ideas came mostly from Niles, with yours truly acting as a sounding board and [coining] the term..." It has been summarized by Gould (1980, pp. 183-4) as follows:
- "Large, stable central populations exert a strong homogenizing influence [on the gene pool]. New and favorable mutations are diluted by the sheer bulk of the population through which they must spread....But [in] small, peripherally isolated groups [that] 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...
- "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."
An unstated supposition is that, through competition, the descendant species eliminates the ancestral species. The theory relies heavily on Ernst Mayr's concept of peripatric speciation. 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.
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.
Misconceptions
Punctuated equilibrium is often confused with saltationism and catastrophism, and thus mistakenly thought to oppose the concept of gradualism; it is actually more properly understood to be a form of gradualism. This is because even though the changes are considered to be occurring relatively quickly, they are still occurring gradually, with no great changes from one generation to the next. This can be understood by considering an example: Suppose the average length of a limb on a particular species grows 50 centimeters (a large amount) over 70000 years (a geologically short period of time). If the average generation is 7 years, then the given timespan corresponds to 10000 generations. Thus, on average, the limb grows at the minute, gradual rate of only 0.005 cm per generation (= 50 cm / 10000 generations).
The theory is often referred to as an explanation for purported "gaps in the fossil record", i.e. the so-called "missing links". However, this confuses two levels of evolution. It merely explains the small jumps that are observed in fossil lineages within or between closely related fossil species, not the transitions between major categories of organisms. Due to the rarity of preservation and the likelihood that speciation occurs in small populations during geologically short periods of time, transitions between species are uncommon in the fossil record.
Relation to Darwinism
The sudden appearance and lack of substantial gradual change of perhaps most species in the geologic record, from their initial appearance until their extinction, has long been noted, including by Charles Darwin, who appealed to the imperfection of the record as an explanation. Because Darwin stressed the gradual nature of evolution, to clearly contrast it with the then-popular catastophism, it is often incorrectly assumed that he insisted that the rate of change must be constant or nearly so. But in The Origin of Species he wrote, "the periods during which species have undergone modification, though long as measured in years, have probably been short in comparison with the periods during which they retain the same form." Thus, punctuationism in general is consistent with Darwin's conception of evolution; it also appears to be compatible with the independent proposals of the theory of evolution by natural selection of Patrick Matthew and Alfred Russel Wallace.
However, in the theory of punctuated equilibrium, "peripheral isolates" are considered to be of critical importance for speciation. But Darwin wrote, "I can by no means agree ... that immigration and isolation are necessary elements.... Although isolation is of great importance in the production of new species, on the whole I am inclined to believe that largeness of area is still more important, especially for the production of species which shall prove capable of enduring for a long period, and of spreading widely." (Darwin, pp. 106-107)
Darwin explained the reasons for this belief as follows:
- "Throughout a great and open area, not only will there be a greater chance of favourable variations, arising from the large number of individuals of the same species there supported, but the conditions of life are much more complex from the large number of already existing species; and if some of these species become modified and improved, others will have to be improved in a corresponding degree, or they will be exterminated. Each new form, also, as soon as it has been improved, will be able to spread over the open and continuous area, and will thus come into competition with many other forms ... the new forms produced on large areas, which have already been victorious over many competitors, will be those that will spread most widely, and will give rise to the greatest number of new varieties and species. They will thus play a more important role in the changing history of the organic world." (Darwin, pp. 107-108)
Thus, punctuated equilibrium contradicts some of Darwin's ideas regarding evolution.
See also:
References
- Eldredge, Niles, and Gould, Stephen Jay:"Punctuated equilibria: an alternative to phyletic gradualism" in: T.Schopf (Hrsg.), Models in Paleobiology, 82-115, Freeman, Cooper and Co., San Francisco, (1972); reprinted in: N.Eldredge, Time frames, Princeton Univ.Press, Princeton, N.J., 1985, 2
- Gould, Stephen Jay (1980). The Panda's Thumb, chapter 17. New York: W.W. Norton & Company, Inc.
- Dawkins, Richard (1986). The Blind Watchmaker, chapter 9. New York: W.W. Norton & Company, Inc.
- Darwin, Charles (1872). The Origin of Species (Sixth Edition).
External links
- Talk Origins Archive (http://www.talkorigins.org/faqs/punc-eq.html)
- A 1991 essay by Stephen Jay Gould reflecting on punctuated equilibrium. (http://www.stephenjaygould.org/library/gould_opus200.html)
- ISCID Encyclopedia of Science and Philosophy. (http://www.iscid.org/encyclopedia/Punctuated_Equilibrium)