The Impact of Historical Contingency on Gene Phylogeny

Abstract

The integration of Darwin’s theories on natural selection (Darwin, 1872) into genetics in the first half of this century resulted in the interpretation of most genetic variation as having either some positive or negative selective consequence (Gould and Lewontin, 1979). In the last few decades, as the molecular evolution of protein and DNA sequences was considered, this Darwinian viewpoint gave way to neutral mutation-random drift hypothesis, which supposes that the majority of mutations have little or no effect on organismal fitness (Kimura, 1968, 1983; Avise, 1994). DNA sequences are randomly fixed in a population with selection only occasionally restraining the outcome. Taken at its extreme this theory suggests that each new mutational event is unaffected by previous mutations, since most of these mutations were also neutral. However, as this theory has matured in light of the data on complex gene phylogenies, a gentler view has developed. This includes “nearly neutral” mutations, which are thought of as being intermediate between neutral and selected mutations (Ohta, 1992).