Assortative mating

Assortative mating (also referred to as positive assortative mating or homogamy) is a mating pattern and a form of sexual selection. It means that individuals with similar phenotypes mate with one another more frequently than would be expected under a random mating pattern. Some examples of similar phenotypes are body size, skin coloration or pigmentation, and age. Assortative mating can increase genetic relatedness within the family and is the inverse of disassortative mating. Assortative mating (also referred to as positive assortative mating or homogamy) is a mating pattern and a form of sexual selection. It means that individuals with similar phenotypes mate with one another more frequently than would be expected under a random mating pattern. Some examples of similar phenotypes are body size, skin coloration or pigmentation, and age. Assortative mating can increase genetic relatedness within the family and is the inverse of disassortative mating. Disassortative mating (also known as negative assortative mating or heterogamy) means that individuals with dissimilar genotypes or phenotypes mate with one another more frequently than would be expected under random mating. Disassortative mating reduces the genetic similarities within the family. Positive assortative mating occurs more frequently than negative assortative mating. In both cases, the nonrandom mating pattern result in a typical 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). Several hypotheses have been proposed to explain the phenomenon of assortative mating. Assortative mating has evolved from a combination of different factors, which vary across different species. Assortative mating with respect to body size can arise as a consequence of intrasexual competition. In some species, size is correlated with fecundity in females. Therefore, males choose to mate with larger females, with the larger males defeating the smaller males in courting them. Examples of species that display this type of assortative mating include the jumping spider Phidippus clarus and the leaf beetle Diaprepes abbreviatus. In other cases, larger females are better equipped to resist male courtship attempts, and only the largest males are able to mate with them. Assortative mating can, at times, arise as a consequence of social competition. Traits in certain individuals may indicate competitive ability which allows them to occupy the best territories. Individuals with similar traits that occupy similar territories are more likely to mate with one another. In this scenario, assortative mating does not necessarily arise from choice, but rather by proximity. This was noted in western bluebirds although there is no definite evidence that this is the major factor resulting in color dependent assortative mating in this species. Different factors may apply simultaneously to result in assortative mating in any given species. Assortative mating in animals has been observed with respect to body size and color. Size-related assortative mating is prevalent across many species of vertebrates and invertebrates. It has been found in the simultaneous hermaphrodites such as the land snail Bradybaena pellucida. One reason for its occurrence can be reciprocal intromission (i.e. both individuals provide both male and female gametes during a single mating) that happens in this species. Therefore, individuals with similar body size pair up with one another to facilitate this exchange. Moreover, it is known that larger individuals in such hermaphroditic species produce more eggs, so mutual mate choice is another factor leading to assortative mating in this species. Evidence for size-related assortative mating has also been found in the mangrove snail, Littoraria ardouiniana and in the Japanese common toad, Bufo japonicus. The second common type of assortative mating occurs with respect to coloration. This type of assortative mating is more common in socially monogamous bird species such as the eastern bluebirds (Sialia sialis) and western bluebirds (Sialia mexicana). In both species more brightly colored males mated with more brightly colored females and less brightly colored individuals paired with one another. Eastern bluebirds also mate assortatively for territorial aggression due to fierce competition for a limited number of nesting sites with tree swallows. Two highly aggressive individuals are better equipped to protect their nest, encouraging assortative mating between such individuals. Assortative mating with respect to two common color morphs: striped and unstriped also exists in a polymorphic population of eastern red-backed salamanders (Plethodon cinereus).