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Hypermedia Online Publishing: the Transformation of the Scholarly Journal


2.3.3 Punctuated equilibrium and speciation

Evolution in real ecologies occurs on a time scale that is too long for an individual to observe. If one wishes to look at how organisms in an ecology adapt and change, it is necessary to look at the past. The standard model for historical evolutionary change is called phyletic gradualism. This argues that species gradually and continually evolve into new species over time.

An alternative model was first proposed by Niles Eldredge and Stephen Jay Gould in 1972 [Eldredge and Gould, 1972]. Their key breakthrough was to realise that "a standard biological account, Mayr's peripatric theory of speciation in small populations peripherally isolated from a parental stock, would yield stasis and punctuation when properly scaled into the vastness of geological time" [Gould and Eldredge, 1993, p. 223]. This theory, which they named punctuated equilibrium suggests that rather than occurring gradually, evolution proceeds in fits and starts. They propose that the development of new species occurs when parts of the breeding population become cut off from the rest of the population in different environments. They adapt to the new environmental challenges by evolving into new species that no longer need to change [Benton, 1993, p. 33]. If the surrounding environment then also changes, a few of the newly developed species are already preadapted and will quickly out-compete their ancestral relatives. In the fossil record, this then shows up as a sudden change from one organism to another. This addition to standard evolutionary theory has now been "accepted ... as a valuable addition to evolutionary theory" [Gould and Eldredge, 1993, p. 223].



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© Andrew Treloar, 2001. * http://andrew.treloar.net/ * andrew.treloar@its.monash.edu.au