Assortative mating is a mating pattern in which individuals with similar genotypes and/ or phenotypes mate with one another more frequently than would be expected under a random mating pattern. Examples of similar phenotypes include, but are not limited to, body size, skin coloration/ pigmentation, and age. Assortative mating, also referred to as positive assortative mating or homogamy, may increase genetic relatedness within the family. Assortative mating can be contrasted with disassortative mating (also known as negative assortative mating or heterogamy) in which individuals with dissimilar genotypes and/ 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, due to the nonrandom mating pattern, there is a deviation from the Hardy-Weinberg Principle.
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 is prevalent among many socially monogamous species of birds. Monogamous species are often involved in bi-parental care of their offspring. Since males are equally invested in the offspring as the mother, both genders are expected to display mate choice, a phenomenon termed as mutual mate choice. Mutual mate choice occurs when both males and females are searching for a mate that will maximize their fitness. In birds, female and male ornamentation may indicate better overall condition and/or such individuals may have better genes, and/or may be better suited as parents. Positive assortative mating typically occurs when the benefits due to increased fitness of the resulting offspring outweigh the costs of individuals being excessively choosy while selecting their mates. There must be a significant fitness advantage in choosing a partner.
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 the larger of females. Examples of species that display this type of assortative mating include the jumping spider Phidippus clarus and the leaf beetles 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.
Examples of assortative mating in animals
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 dependent 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).
Assortative mating in humans
Assortative mating in humans has been widely observed and studied. Strong correlations in height, span of arms, and the length of the left forearm exist between husband and wife in 1000 couples (Pearson and colleagues, 1903). Assortative mating with regards to appearance does not end there. Males prefer female faces that resemble their own when provided images of three women, with one image modified to resemble their own (Kocsor F., 2011). However, the same result does not apply to females selecting male faces.
Outside of physical appearance, assortative mating in humans occurs over a wide array of traits. These include socio-economic, educational, religious, political attitudes, racial and ethnic, cultural, personality and psychological. In fact, evidence has been found for assortative mating in regards to altruism. Couples show similarities in terms of their contributions to public betterment and charities, and this can be attributed to mate choice based on generosity rather than phenotypic convergence (A. Tognetti, 2014). Greenwood et al (2015) show that couples sort according to education levels and that this tendency has increased over time.
Assortative mating based on genomic similarities plays a role in human marriages in the United States. Spouses are more genetically similar than two randomly chosen individuals (Guo G. , 2014). The probability of marriage increases by roughly 15% for every 1-SD increase in genetic similarity. However, some researchers argue that this assortative mating is caused purely by population stratification (the fact that people are more likely to marry within ethnic subgroups such as Swedish-Americans).
At the same time, individuals display disassortative mating for genes in the major histocompatibility complex region on chromosome 6. Individuals feel more attracted to odors of individuals who are genetically different in this region. This promotes MHC heterozygosity in the children, making them less vulnerable to pathogens. Apart from humans, disassortative mating with regards to the MHC coding region has been widely studied in mice, and has also been reported to occur in fish (Raphaelle Chaix, 2008).
Effects of assortative mating
Assortative mating has reproductive consequences. Positive assortative mating increases genetic relatedness within a family, whereas negative assortative mating accomplishes the opposite effect. Either strategy may be employed by the individuals of a species depending upon which strategy maximizes fitness and enables the individuals to maximally pass on their genes to the next generation. For instance, in the case of eastern bluebirds, assortative mating for territorial aggression increases the probability of the parents obtaining and securing a nest site for their offspring. This in turn, increases the likelihood of survival of the offspring and consequently fitness of the individuals.  In birds whose coloration represents well being and fecundity of the bird, positive assortative mating for color increases the chances of genes being passed on and of the offspring being in good condition. Also, positive assortative mating for behavioral traits allows for more efficient communication between the individuals and they can cooperate better to raise their offspring.
On the other hand, mating between individuals of genotypes that are too similar allows for the accumulation of harmful recessive alleles, which can decrease fitness. Such mating between genetically similar individuals is termed inbreeding which can result in the emergence of autosomal recessive diseases. Moreover, assortative mating for aggression in birds can lead to inadequate parental care. An alternate strategy can be disassortative mating, in which one individual is aggressive and guards the nest site while the other individual is more nurturing and fosters the young. This division of labor increases the chances of survival of the offspring. A classic example of this is in the case of the white-throated sparrow (Zonotrichia albicollis). This bird exhibits two color morphs - white striped and tan striped. In both genders, the white striped birds are more aggressive and territorial whereas tan striped birds are more engaged in providing parental care to their offspring. Therefore, disassortative mating in these birds allows for an efficient division of labor in terms of raising and protecting their offspring.
Positive assortative mating is a key element leading to reproductive isolation within a species, which in turn may result speciation in sympatry over time. Sympatric speciation is defined as the evolution of a new species without geographical isolation. Speciation from assortative mating has occurred in the Middle East blind mole rat, cicadas, and the European corn borer.
- Directional selection
- Disruptive selection
- Matching hypothesis
- Negative selection (natural selection)