1 Punctuated Equilibria

The paper Punctuated Equilibria: An Alternative to Phyletic Gradualism [Eldredge and Gould, 1972] described a new perspective on evolution. Phyletic gradualism, a hypothesis proposing that evolution proceeds at a constant rate and new species evolve gradually from ancestral species, was the common world view of evolution and the authors contended that it was insufficient to explain the fossil record. Since 1930 the model of allopatric speciation was being considered, but not seriously as the cause of evolution. In allopatric speciation, new species evolve in geographical isolation from ancestral species. The authors suggested that this model more clearly explains why the fossil record is incomplete.

The theory of punctuated equilibria says that individuals and species are resistant to change, they have an innate property that prevents easy change. The previous belief that gene flow was sufficient to keep a group of subpopulations similar enough to remain a species is contrasted with the fact that the species and individuals were probably not kept coherent by gene flow but because they had a strong resistance to change. However, peripheral isolates, populations that are at the edge of a species range, with small population sizes and considerably different environments provide enough ``distance" to allow for the hard and uncommon event of speciation. Thus, one sees long periods of stasis, or equilibria, where species do not change at all, and then small bursts of success by peripheral isolates in different environments.