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Punctuated equilibrium is a theory of evolution which postulates that changes such as speciation can occur very quickly, with long periods of little change (equilibria) in between. This theory explains 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.
The theory and history of the concept
The theory of punctuated equilibrium was proposed by Niles Eldredge and Stephen Jay Gould
in the 1970s. It relies heavily on Ernst
Mayr's concept of peripatric speciation, and it is
usually contrasted with phyletic gradualism, though critics, notably Richard Dawkins, have argued that phyletic gradualism is merely a straw man.
The actual differences between the various evolution theorists are not as large as the opponents of punctuated equilibrium have
suggested (Gould himself said that punk eek, as it is affectionately called, did not in fact refute Darwin's gradualism theories,
just added to them the ideas of catastrophism and stasis) and the current debate is often more a debate on the relative degree of
punctualism and gradualism than one between two fully different extremes.
Misconception of the theory
The theory of punctuated equilibrium is often misstated as being 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.
However, transitional fossils at higher taxonomic levels
are abundant.
Simulations of punctuated evolution
Recently, computer simulations have provided some insight into how punctuated evolution may work: The equilibrium periods show
a gradual accumulation of neutral mutations, and the jump occurs when some
beneficial combination of them reaches a certain threshold percentage.
See also: Adaptive radiation
Literature
- N.Eldredge, S.J.Gould:"Punctuated equilibria: an alternative to phyletic gradualism in: T.Schopf (Hrsg.), Models in
Paleobiology, 82-115, Freeman, Cooper and Co., San Francisco, (1972); wieder abgedruckt in: N.Eldredge, Time frames, Princeton
Univ.Press, Princeton, N.J., 1985, 2
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