Punctuated equilibrium

Punctuated equilibrium (pl. punctuated equilibria) is a part of evolutionary theory that states that evolution, particularly speciation, occurs 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.

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 catastrophism, it is often incorrectly assumed that he insisted that the rate of change must be constant or nearly so. However, Darwin saw evolution as a slow process that continued without jumping around and ahead. He figured that if you study the fossils of organisms, you will see long intervals in the geological layers. Throughout these layers, you can see at which times a certain species may have become extinct and then replaced by other life forms. But in The Origin of Species Darwin 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.

Related Topics:
Patrick Matthew - Alfred Russel Wallace

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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)

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Darwin explained the reasons for this belief as follows:

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:"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)

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Thus, punctuated equilibrium contradicts some of Darwin's ideas regarding evolution.

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