<Li> Mutation will have a very subtle effect on allele frequencies . Mutation rates are of the order 10 to 10, and the change in allele frequency will be, at most, the same order . Recurrent mutation will maintain alleles in the population, even if there is strong selection against them . </Li> <Li> Migration genetically links two or more populations together . In general, allele frequencies will become more homogeneous among the populations . Some models for migration inherently include nonrandom mating (Wahlund effect, for example). For those models, the Hardy--Weinberg proportions will normally not be valid . </Li> <Li> Small population size can cause a random change in allele frequencies . This is due to a sampling effect, and is called genetic drift . Sampling effects are most important when the allele is present in a small number of copies . </Li> <P> Where the A gene is sex linked, the heterogametic sex (e.g., mammalian males; avian females) have only one copy of the gene (and are termed hemizygous), while the homogametic sex (e.g., human females) have two copies . The genotype frequencies at equilibrium are p and q for the heterogametic sex but p, 2pq and q for the homogametic sex . </P>

If there is a random change in the genetics of a small population it is termed