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Assortative mating (also called assortative pairing) takes place when sexually reproducing organisms tend to mate with individuals that are like themselves in some respect (positive assortative mating) or dissimilar (negative assortative mating). In evolution, therefore these two types of assortative mating have the effect of reducing and expanding the range of variation, respectively, when the assorting is cued on heritable traits.

It is mirrored by selective fertilization in plants.

Assortative mating has been invoked to explain sympatric speciation. For some populations there are two different resources for which different genotypes are optimum. Intermediates between these two genotypes are less favorable. It is then favourable if the organisms can recognize mates that are optimized for the same resources as they are themselves. If mutations that make such recognition possible appear, these will be selected for.

For example, Munday et al. (2004) notes the speciation of a daughter species from the parent species of coral-dwelling goby fish. The species live in a small area of rare coral in the ocean around Bootless Bay in southern Papua New Guinea which the parent species shun. The daughter species has become reproductively isolated from the parent species even though the parent species surrounds the daughter species so there is no geographic isolation. According to Munday, the speciation in the early stages would depend on assortive mating in which the evolving goby fishes would prefer to mate with other fish that preferred to spawn in the same area of rare coral.

From a population/evolutionary genetics standpoint, genetic counseling is a strategy of negative assortative mating uniquely found in humans.

• Munday, Philip L., Lynne van Herwerden, and Christine L. Dudgeon. 2004. "Evidence for sympatric speciation by host shift in the sea." Current Biology 14 (16), pp. 1498-1504.

• Disassortative mating
• Sexual selection