Mating does not always occur by random pairing among all members of a population. Nonrandom mating changes which genotypes are produced even when it does not initially change allele frequencies.

A particularly important case is inbreeding, mating between genetically related individuals.

Related individuals are more likely to carry copies of alleles inherited from shared ancestors. Their offspring therefore have an increased probability of receiving two copies of the same ancestral allele.

The immediate population-genetic effect is typically:

• more homozygotes than expected under random mating;
• fewer heterozygotes;
• little or no direct change in allele frequencies solely from the pairing pattern.

For example, a population may have allele frequencies $p=0.6$ and $q=0.4$. Hardy-Weinberg random mating predicts heterozygosity

$$2pq=2(0.6)(0.4)=0.48.$$

If close relatives mate more often, the observed heterozygote frequency can fall below 0.48 even though $p$ and $q$ remain near 0.6 and 0.4.

This distinction is important: genotype frequencies can change without immediate evolutionary change in allele frequencies.

Inbreeding can nevertheless affect evolution indirectly. Recessive alleles are exposed more often in homozygotes, allowing selection to act on phenotypes that were previously hidden in heterozygotes.

Other forms of nonrandom mating include preference for similar or dissimilar phenotypes. If mate choice is itself associated with differential reproductive success, it can become part of sexual selection.