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.
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 relies heavily on Ernst Mayr's concept of peripatric speciation, and has been summarized by Gould as follows:
An unstated supposition is that, through competition, the descendant species eliminates the ancestral species. 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.
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.
The lack of substantial gradual change of perhaps most species in the geologic record, from their initial appearance until their extinction, is well known among paleobiologists, and has long been noted. Besides appealing to the imperfection of the fossil record, Charles Darwin considered punctuation as an explanation for this observation, writing in The Origin of Species, "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." 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 be constant or nearly so. Punctuation appears also to be compatible with the independent proposals of the theory of evolution by natural selection of Patrick Matthew and Alfred Russel Wallace.
See also: Adaptive radiation