Assortative mating refers to the tendency of (some) animals to mate with individuals that share similar traits. For example, having the same fur colour, or size. The effect is particularly strong in fishes, less so in birds. It also seems to apply, in some cases, to human populations.
The evolutionary advantages for assortative mating are unclear, and have been discussed for many decades. From an evolutionary standpoint, the trait would seem to restrict genetic diversity when compared to purely random mating.
There are several mainstream theories which attempt an explanation, but none is generally accepted.
See : Assortative Mating in Animals , The American Naturalist. 181 (6): E125–E138
Our results raise many further questions. These include the need to identify the proximate mechanisms that generate assortment, the underlying evolutionary forces that lead to weak positive assortment, its population genetic consequences, and the potential effects of such nonrandom mating on evolutionary and genetic inferences."
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