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Assortative mating (also referred to as positive assortative mating or homogamy) is a mating pattern and a form of
sexual selection Sexual selection is a mode of natural selection in which members of one biological sex choose mates of the other sex to mate with (intersexual selection), and compete with members of the same sex for access to members of the opposite sex ( ...
in which individuals with similar
phenotypes In genetics, the phenotype () is the set of observable characteristics or phenotypic trait, traits of an organism. The term covers the organism's morphology (biology), morphology or physical form and structure, its Developmental biology, dev ...
or
genotypes The genotype of an organism is its complete set of genetic material. Genotype can also be used to refer to the alleles or variants an individual carries in a particular gene or genetic location. The number of alleles an individual can have in a ...
mate with one another more frequently than would be expected under a random mating pattern. A majority of the phenotypes that are subject to assortative mating are body size, visual signals (e.g. color, pattern), and sexually selected traits such as crest size. The opposite of assortative is
disassortative mating Disassortative mating (also known as negative assortative mating or heterogamy) is a mating pattern in which individuals with dissimilar phenotypes mate with one another more frequently than would be expected under random mating. Disassortative mat ...
.


Causes

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 Jumping spiders are a group of spiders that constitute the family Salticidae. As of 2019, this family contained over 600 described genera and over 6,000 described species, making it the largest family of spiders at 13% of all species. Jumping spi ...
'' Phidippus clarus'' and the
leaf beetle The insects of the beetle family Chrysomelidae are commonly known as leaf beetles, and include over 37,000 (and probably at least 50,000) species in more than 2,500 genera, making up one of the largest and most commonly encountered of all 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.


In non-human 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
vertebrate Vertebrates () comprise all animal taxa within the subphylum Vertebrata () (chordates with backbones), including all mammals, birds, reptiles, amphibians, and fish. Vertebrates represent the overwhelming majority of the phylum Chordata, with c ...
s and
invertebrate Invertebrates are a paraphyletic group of animals that neither possess nor develop a vertebral column (commonly known as a ''backbone'' or ''spine''), derived from the notochord. This is a grouping including all animals apart from the chordate ...
s. 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 bluebird The eastern bluebird (''Sialia sialis'') is a small North American migratory thrush found in open woodlands, farmlands, and orchards. The bright-blue breeding plumage of the male, easily observed on a wire or open perch, makes this species a f ...
s (''Sialia sialis'') and
western bluebird The western bluebird (''Sialia mexicana'') is a small North American thrush. Taxonomy The western bluebird was formally described by the English naturalist William John Swainson in 1832 and given the binomial name ''Sialia mexicana''. Six sub ...
s (''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 The red-backed salamander (''Plethodon cinereus'') is a small, hardy woodland salamander species in the family Plethodontidae. It is also known as the redback salamander, eastern red-backed salamander,Integrated Taxonomic Information System nter ...
''). Assortative mating is also found in 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 can indicate better overall condition or such individuals might have better genes, or be better suited as parents.


In humans

Assortative mating in humans has been widely observed and studied, and can be broken down into two types of human assortative mating. These are * genetic assortative mating (assortative mating with mate choice based on genetic type and phenotypical expression) * and social assortative mating (assortative mating with mate choice based on social, cultural, and other societal factors) Genetic assortative mating is well studied and documented. In 1903 Pearson and colleagues reported strong correlations in height, span of arms, and the length of the left forearm between husband and wife in 1000 couples. 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. However, the same result does not apply to females selecting male faces. Genetically related individuals (3rd or 4th cousin level) exhibit higher fitness than unrelated individuals. Assortative mating based on genomic similarities plays a role in human marriages in the United States. Spouses are more genetically similar to each other than two randomly chosen individuals. 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 Population structure (also called genetic structure and population stratification) is the presence of a systematic difference in allele frequencies between subpopulations. In a randomly mating (or ''panmictic'') population, allele frequencies are ...
(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. In addition to genetic assortative mating, humans also demonstrate patterns of assortative mating based on
sociological Sociology is a social science that focuses on society, human social behavior, patterns of social relationships, social interaction, and aspects of culture associated with everyday life. It uses various methods of empirical investigation and ...
factors as well. Sociological assortative mating is typically broken down into three categories, mate choice based on
socio-economic status Socioeconomic status (SES) is an economic and sociological combined total measure of a person's work experience and of an individual's or family's economic access to resources and social position in relation to others. When analyzing a family's ...
, mate choice based on racial or ethnic background, and mate choice based on religious beliefs. Assortative mating based on socio-economic status is the broadest of these general categories. It includes the tendency of humans to prefer to mate within their socio-economic peers, that is, those with similar social standing, job prestige, educational attainment, or economic background as they themselves. This tendency has always been present in society: there was no historical area when most of the individuals preferred to sort, and had actually sorted, negatively into couples or matched randomly along these traits. Still, this tendency was weaker in some generations than in others. For instance, in the 20th century in the Western world, late Boomers had weaker aggregate preferences for educational homogamy than early Boomers had when being young adults; also, the members of the early Generation-X were typically much less "picky" about spousal education than the members of the late Generation-X were. This trend is evidenced by the search criteria of online dating site users. Another form of sociological assortative mating is assortative mating based on racial and ethnic background. Mentioned above in the context of the genetically similar preferring to mate with one another, this form of assortative mating can take many varied and complicated forms. While the tendency mentioned above does exist, and people do tend to marry those genetically similar to themselves, especially if within the same racial or ethnic group, this trend can change in various ways. It is common, for example, for the barriers to intermarriage with the general population experienced by a minority population to decrease as the numbers of the minority population increase. This assimilation reduces the prevalence of this form of assortative mating. However, growth of a minority population does not necessarily lead to decreased barriers to intermarriage. This can be seen in the sharp increase in the non-white Hispanic population of the United States in the 1990s and 2000s that correlated with a sharp decrease in the percentage of non-white Hispanics intermarrying with the general population. Religious assortative mating is the tendency of individuals to marry within their own religious group. This tendency is prevalent and observable, and changes according to three main factors. The first of these is the proportion of available spouses in the area who already follow the same religion as the person searching for a mate. Areas where religious beliefs are already similar for most people will always have high degrees of religious inbreeding. The second is the social distance between the intermarrying religious groups, or the physical proximity and social interactivity of these groups. Finally, the third factor is the personal views one holds towards marrying outside of a religion. Those who greatly value adherence to religious tradition may be more likely to be averse to marrying across religious lines. Although not necessarily religious, a good example of humans mating assortatively based on belief structure can be found in the tendency of humans to marry based on levels of charitable giving. 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. Assortative mating also seems to be very widespread among people with mental disorders.


Effects

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 which 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 disorders. 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; however, this risks the breakdown of coadapted gene complexes, leading to outbreeding depression. 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 The white-throated sparrow (''Zonotrichia albicollis'') is a passerine bird of the New World sparrow family Passerellidae. Etymology The genus name ''Zonotrichia'' is from Ancient Greek (, ) and (, ). The specific ''albicollis'' is from Lati ...
(''Zonotrichia albicollis''). This bird exhibits two color morphs – white striped and tan striped. In both sexes, 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. Like other animals, humans also display these genetic results of assortative mating. What makes humans unique, however, is the tendency towards seeking mates that are not only similar to them in genetics and in appearances, but those who are similar to them economically, socially, educationally, and culturally. These tendencies toward using sociological characteristics to select a mate has many effects on the lives and livelihoods of those who choose to marry one another, as well as their children and future generations. Within a generation, assortative mating is sometimes cited as a source of inequality, as those who mate assortatively would marry people of similar station to themselves, thus amplifying their current station. There is debate, however, about whether this growing preference for educational and occupational similarities in spouses is due to increased preferences for these traits, or the shift in workload that occurred as women entered the workforce. This concentration of wealth in families also perpetuates across generations as parents pass their wealth on to their children, with each successive generation inheriting the resources of both of its parents. The combined resources of the parents allow them to give their child a better life growing up, and the combined inheritances from both parents place them at an even greater advantage than they would be with their superior education and childhoods. This has an enormous impact on the development of the social economic structure of a society.


Economics

A related concept of 'assortative ''matching has been developed within
economics Economics () is the social science that studies the production, distribution, and consumption of goods and services. Economics focuses on the behaviour and interactions of economic agents and how economies work. Microeconomics analyzes ...
. This relates to efficiencies in production available if workers are evenly matched in their skills or productivity. A consideration of this assortative matching forms the basis of Kremer's 1993 O-ring theory of economic development.


See also

*
Directional selection In population genetics, directional selection, is a mode of negative natural selection in which an extreme phenotype is favored over other phenotypes, causing the allele frequency to shift over time in the direction of that phenotype. Under dir ...
* Disruptive selection *
Endogamy Endogamy is the practice of marrying within a specific social group, religious denomination, caste, or ethnic group, rejecting those from others as unsuitable for marriage or other close personal relationships. Endogamy is common in many cultu ...
*
Genetic sexual attraction Genetic sexual attraction is a concept in which a strong sexual attraction may develop between close blood relatives who first meet as adults. There is no evidence for genetic sexual attraction being an actual phenomenon, and the hypothesis is re ...
*
Koinophilia Koinophilia is an evolutionary hypothesis proposing that during sexual selection, animals preferentially seek mates with a minimum of unusual or mutant features, including functionality, appearance and behavior. Koinophilia intends to explain t ...
* Matching hypothesis *
Negative selection (natural selection) In natural selection, negative selection or purifying selection is the selective removal of alleles that are deleterious. This can result in stabilising selection through the purging of deleterious genetic polymorphisms that arise through random ...
* Reinforcement (speciation)


References

{{speciation Mating Sexual selection Fertility