Non-Random Mating

The Hardy-Weinberg principle assumes random mating: every individual is equally likely to mate with any individual of the opposite sex, whatever its genotype. In real populations this is rarely true.Non-random mating happens when individuals choose mates, or are more likely to mate with some individuals than others. Common forms are:Sexual selection: individuals (often females) choose mates with particular features, such as brighter plumage or larger antlers, or males compete for access to mates. Individuals with the favoured phenotype produce more offspring, so the alleles responsible increase in frequency. The long tail of the male peacock is a classic example.Assortative mating: individuals mate with others that have a similar phenotype, for example a similar body size. This increases the proportion of homozygous genotypes.Inbreeding: mating between close relatives, which is more likely in small or isolated groups. It also increases homozygosity, so harmful recessive alleles are more often expressed.Assortative mating and inbreeding change genotype frequencies (there are fewer heterozygotes than the Hardy-Weinberg equation predicts), even if allele frequencies stay the same. Sexual selection changes allele frequencies, because some individuals leave more offspring than others.Either way, the observed genotype frequencies no longer match p² + 2pq + q² = 1, so the population is not in Hardy-Weinberg equilibrium.