Maintenance of genetic variability under mutation and selection pressures in a finite population.

Formulas are developed for the distribution of allele frequencies and the mean and variance of heterozygosity under mutation and selection pressures. In large populations, even slight selection drastically changes the shape of the distribution of allele frequencies and reduces heterozygosity. On the other hand, the number of rare alleles in a sample is much less affected by selection. Under genic selection, heterozygosity may decrease with increasing population size. As a test statistic, the variance of heterozygosity can be used to detect the presence of selection, though it is not efficient when selection is very slight.