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Punctuated equilibrium (Punctuated equilibria, PE or Punk-eek) is a theory popularized by Stephen Jay Gould and Niles Eldredge. The theory postulates that evolutionary change is not constant but that speciation may occur rapidly and the species then change little for long periods of time. The theory explains the patterns shown in the fossil record. While many of the basic ideas in the theory are widely accepted, it has been criticised for putting emphasis on species selection.
The biogeographical aspect
Punctuated equilibrium is often misunderstood, even by scientists, in diverse ways. Fundamentally and originally, all punctuated equilibrium is an application of Mayr's theory of allopatric speciation -- which argued that speciation occurs in small peripheral populations cut off from the rest of the species -- to the fossil record. Gould and Eldredge pointed out that if Mayr's theory was true, then the pattern observed in the fossil record would tend to be a jerky succession of species, rather than an absolutely smooth progression. That's it, really. It does not apply to processes other than speciation, although for some reason the term gets dragged out when any kind of relatively rapid change is being discussed.
Misunderstandings of Punk Eek
Creationists commonly advance punctuated equilibrium as concession on the part of the paleontological community that evolution is not borne out by the fossil record, largely due to the way in which S. J. Gould presented it. More specifically, they say that PE tries to explain away an alleged lack of transitional fossils. Gould recognized his error in presentation and later lamented the misrepresentation of punctuated equilibrium on the part of the creationists. In 1981, he said,
Punctuated equilibrium solely attempts to explain the rate and pattern of evolutionary change as inferred from the fossil record. Attempts to attribute to this postulate more than it explains are without substantiation.
Controversy and Critique
Punctuated equilibrium as advanced by Stanley (1975, 1981, 1982), Gould & Eldredge (1972, 1977), Eldredge (1984, 1985), Eldredge & Stanley (1984) and Gould (1980, 1982) and most vociferously espoused by Gould throughout the latter portion of his career, has come under significant criticism by a number of evolutionary biologists. While almost all agree that evolutionary rate is not constant at any one level and can be either bradytelyic, horotelyic, or tachytelyic (Simpson 1944), the specific presentation of punctuated equilibrium by Gould et al as a process of evolution fundamentally at odds with the neodarwinian paradigm, is seriously flawed. Indeed the very argument that evolution progresses continuously in a punctuational manner, has not been borne out by explicit review of the fossil record by which Gould and his colleagues made their argument in the first place. Most damaging are the studies of Philip Gingerich on morphologic change in Cenozoic mammals demonstrating a more or less continual pattern of gradualism (1976, 1980, 1982), although further research into the pattern of morphologic change in mammals also fails to underwrite Gouldian punctuated equilibrium in the fossil record: see Hurzeler (1962), Chaline & Laurin (1986), Fahlbusch (1983), Harris & White (1979), MacFadden (1985), Krishtalka & Stucky (1985) and Maglio (1973). Carroll (1987; 1997) has emphasized that the defense of punctuated equilibrium using operational taxonomic units, per the method of Stanley (1981), is inherently flawed due to the limitations of the Linnean terminological infrastructure employed by taxonomic science, and emphasized that attention to anatomical detail and not taxonomic convention, precludes creating punctuated equilibria where none exist. Levington (2001) concluded that fossil evidence for punctuation is slight, while that of gradualism is much more impressive. He referred to Ernst Mayr (1904-2005) as the founder of the punctuated equilibrium hypothesis.