Assortative Mating: Definition, Types, and Implications
Assortative mating, a non-random mating pattern, significantly influences the genetic and social structures of populations, including animals and humans. It occurs when individuals choose mates based on similar phenotypes or genotypes, leading to increased genetic similarities within families and potentially impacting the distribution of traits and allele frequencies over time. This article explores the definition of assortative mating, its various types, observed instances in both animal and human populations, and its reproductive and societal consequences.
Defining Assortative Mating
In population and evolutionary genetics, random mating is generally assumed. However, exceptions to this assumption include inbreeding, assortative mating, and selective mating. Assortative mating is "character specific but involves no gene frequency change," according to Lewontin et al. (1968). This means that while individuals with similar traits are more likely to mate, all genotypes contribute equally to the next generation. However, this definition does not account for selective mating models where individuals of the same phenotype have an increased probability of mating.
Types of Assortative Mating
Assortative mating can be broadly classified into positive and negative assortative mating.
Positive Assortative Mating (Homogamy)
Positive assortative mating, also known as homogamy, occurs when individuals with similar phenotypes mate more frequently than expected under random mating. This type of selection is very common and can be observed across various traits, including body size, coloration, and socio-economic status.
Negative Assortative Mating (Disassortative Mating)
Negative assortative mating, or disassortative mating, occurs when individuals with dissimilar phenotypes mate more frequently than expected under random mating. This type of mating promotes diversity by pairing individuals with differing traits, potentially leading to a wider range of allele frequencies and increased heterozygosity.
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Assortative Mating in Animals
Assortative mating in animals has been observed with respect to body size and color. It is also found in many socially monogamous species of birds.
Body Size
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. 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.
Examples:
- Land snail Bradybaena pellucida: Individuals with similar body sizes pair up to facilitate reciprocal intromission.
- Curculionid beetle Diaprepes abbreviatus: Size-assortative mating, male choice, and female choice.
- Mangrove snail, Littoraria ardouiniana: Size-dependent male mate preference and its association with size-assortative mating.
- Japanese common toad Bufo japonicus: Female polyandry and size-assortative mating in isolated local populations.
Coloration
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. 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.
Examples:
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- Eastern bluebirds (Sialia sialis) and western bluebirds (Sialia mexicana): More brightly colored males mate with more brightly colored females, and less brightly colored individuals pair with one another.
- White-throated sparrow (Zonotrichia albicollis): Exhibits two color morphs - white striped and tan striped.
Other Traits
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.
Examples:
- Eastern bluebirds (Sialia sialis): Mate assortatively for territorial aggression due to fierce competition for a limited number of nesting sites with tree swallows.
- Polymorphic salamander: Assortative mating has been observed.
Assortative Mating in Humans
Assortative mating in humans has been widely observed and studied, and can be broken down into two types of human assortative mating: genetic and sociological.
Genetic Assortative Mating
Genetic assortative mating is well studied and documented. Spouses are more genetically similar to each other than two randomly chosen individuals. 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.
Examples:
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- Height, span of arms, and length of the left forearm: Strong correlations have been reported between husband and wife.
- Facial resemblance: Males prefer female faces that resemble their own.
- Genomic similarity: The probability of marriage increases with genetic similarity.
- Major histocompatibility complex (MHC) region: Individuals display disassortative mating for genes in this region.
Sociological Assortative Mating
In addition to genetic assortative mating, humans also demonstrate patterns of assortative mating based on sociological factors as well.
- Socio-economic status: Humans tend 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. 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.
- Racial and ethnic background: People tend to marry those genetically similar to themselves, especially if within the same racial or ethnic group. 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.
- Religious assortative mating: Individuals tend to marry within their own religious group. This tendency is prevalent and observable, and changes according to three main factors: the proportion of available spouses in the area who already follow the same religion, the social distance between the intermarrying religious groups, and the personal views one holds towards marrying outside of a religion.
- Belief structure: Humans may marry based on levels of charitable giving.
Reproductive Consequences 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.
- Increased fitness: Assortative mating for territorial aggression increases the probability of the parents obtaining and securing a nest site for their offspring.
- Accumulation of harmful recessive alleles: 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.
- Inadequate parental care: Assortative mating for aggression in birds can lead to inadequate parental care.
- Breakdown of coadapted gene complexes: 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.
- Reproductive isolation and speciation: Positive assortative mating is a key element leading to reproductive isolation within a species, which in turn may result speciation in sympatry over time.
Societal Consequences of Assortative Mating
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.
- Inequality: 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. 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.
- Economic efficiencies: A related concept of 'assortative matching' has been developed within economics. This relates to efficiencies in production available if workers are evenly matched in their skills or productivity.
Assortative Mating and Linkage Disequilibrium
Assortative mating has been suggested to result in an increase in heritability and additive genetic variance through an increase in linkage disequilibrium. Theoretical analyses of assortative mating have concluded that it can generate linkage disequilibrium, and consequently influence estimates of heritability and additive genetic variance. However, it is not clear how much linkage disequilibrium can be generated by assortative mating. The amount and type of linkage disequilibrium generated by assortative mating depends upon the assumptions of the model of assortative mating.
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