Assortative Mating

speciation

Assortative mating is the phenomenon where a sexually reproducing organism chooses to mate with individuals that are similar (positive assortative mating) or dissimilar (negative assortative mating) to itself in some specific manner. In evolution, these two types of assortative mating have the effect, respectively, of increasing or reducing the range of variation (trait variance), when the assorting is cued on heritable traits. Positive assortative mating, therefore, results in disruptive natural selection, and negative assortative mating results in stabilized natural selection.

Assortative mating has been invoked to explain sympatric speciation (the process through which new species evolve from a single ancestral species while inhabiting the same geographic region). For some populations there are two different resources for which different phenotypes (genetic traits) are optimum. Intermediates between these two phenotypes are less favorable. It is then favorable 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.

In humans, genetic counseling can lead to a strategy of negative assortative mating. Genetic counseling is the process by which patients at risk of an inherited disorder, are advised of the consequences and nature of the disorder, the probability of developing or transmitting it, and the options open to them in management and family planning. Assortative mating among computer scientists with Asperger syndrome could be contributing to the rise in juvenile autism spectrum disorders in Silicon Valley. Homogamy is marriage based on assortative mating.

Positive assortative mating is believed to be the cause of 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 near 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. The speciation in the early stages would depend on assortative 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.

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