Disassortative mating

Disassortative mating (also known as negative assortative mating or heterogamy) is a mating pattern in which individuals with dissimilar phenotypes mate with one another more frequently than would be expected under random mating. Disassortative mating reduces the mean genetic similarities within the population and produces a greater number of heterozygotes. The pattern is character specific, but does not affect allele frequencies.^[1] This nonrandom mating pattern will result in deviation from the Hardy-Weinberg principle (which states that genotype frequencies in a population will remain constant from generation to generation in the absence of other evolutionary influences, such as "mate choice" in this case).

Disassortative mating is different from outbreeding, which refers to mating patterns in relation to genotypes rather than phenotypes.

Due to homotypic preference (bias toward the same type), assortative mating occurs more frequently then disassortative mating.^[2][3] This is due to the fact that homotypic preferences increase relatedness between mates and between parents and offspring that would promote cooperation and increases inclusive fitness. With disassortative mating, heterotypic preference (bias towards different types) in many cases has been shown to increase overall fitness.^[4] When this preference is favored, it allows a population to generate and/or maintain polymorphism (genetic variation within a population).

The fitness advantage aspect of disassortative mating seems straightforward, but the evolution of selective forces involved in disassortative mating are still largely unknown in natural populations.