EFFECTS OF RANDOM FLUCTUATION OF SELECTION INTENSITY ON GENETIC VARIABILITY IN A FINITE POPULATION

Abstract

The effects of random fluctuation of selection intensity on the expected number of heterozygous codons, average heterozygosity, and the relationship between gene frequency and heterozygosity are studied, assuming that the fluctuation of selection intensity is a random noise. In small populations these effects are relatively small, and the genetic variability in a population is determined mainly by the mutation rate and population size. In large populations, however, even a small amount of random fluctuation reduces the genetic variability to a considerable extent. A formula for the expected number of segregating codons per locus is also worked out. Roughly speaking, the effect of random fluctuation of selection intensity on this parameter is similar to that on the expected number of heterozygous codons per locus. In addition to the above studies, mathematical models of random fluctuation of selection intensity in diffusion approximations are discussed. It is shown that data on molecular evolution and polymorphism are more consistent with the predictions from the hypothesis that temporal fluctuation of selection intensity is a “random noise” than those from the alternative hypothesis that it is a “stabilizing factor for gene frequency.”