Can Uniform Selection Retard Random Genetic Divergence Between Isolated Conspecific Populations?
Evolution 38 (3): pp. 495-504 (1984)

Uniform selection is generally supposed to cause convergence between populations while drift has the effect of causing divergence. However, selection toward the same phenotype in identical, finite populations can result in greater eventual genetic divergence than pure drift. The question of whether uniform selection with drift results in more or less variation in final genotype composition than does pure drift is a quantitative problem that depends on the relationship between the model of selection, the intensity of selection, population size, and the frequencies of alleles at the beginning of the selective process. A model is presented in which two populations begin at the same frequency at a diallelic locus and are subjected to directional selection. For a given initial frequency of the favored allele (p), selection intensity (s), and population size (N), the model was used to calculate the probability that the two populations would become fixed for alternative alleles, under selection with drift and under pure drift. Selection with drift results in a greater probability of alternate fixation than pure drift when the initial frequency of the favored allele is less than a critical value, p*, determined by the effective population size and the selection intensity. The critical initial frequency p* is not very restrictive. A model of the expected divergence under multilocus selection and drift was also presented. The proportion of the initial allele frequency space in which selection with drift causes a greater number of alternate fixations than pure drift was calculated, for various values of Ns and various numbers of loci. When selection is weak (Ns = .1 or less) selection results in greater divergence than pure drift in about half the initial frequency space, regardless of the number of loci subjected to selection. It has been asserted that uniform selection, rather than gene flow, is responsible for slowing the rate of random genetic divergence between isolated conspecific populations (Ehrlich and Raven, 1969; Levin, 1979). The results of this study suggest that weak uniform selection acting on small isolated populations would be likely to increase the rate of genetic divergence between populations.
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