Assortative mating is a nonrandom 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. For example, it is common for individuals of similar body size to mate with one another. Less commonly, in negative assortative mating (also known as disassortative mating), individuals with diverse traits mate more frequently than would be expected in random mating. Both cases cause the frequency of certain genotypes to differ greatly from the frequencies predicted by the Hardy-Weinberg Principle, which states that allele and genotype frequencies should remain constant under a random mating system. Assortative mating often increases the proportion of homozygous individuals. By contrast, disassortative mating tends to result in a greater number of heterozygotes.
Assortative mating occurs on a variety of traits and may strengthen the mating bond to increase fertility. The mating strategy may work to increase genetic relatedness, which can facilitate communication and altruism. Conversely, disassortative mating reduces genetic relatedness between family members. Ultimately, the effect of assortative mating is to increase inclusive fitness, meaning the sum fitness of the individual and all of the individual’s offspring. However, mating between individuals who are too genetically similar is considered inbreeding and reduces fitness by putting an individual at a greater risk of harmful recessive traits. Although genetic diversification is generally seen as an adaptive strategy in unpredictable environments, decreasing genetic diversity can provide stability in predictable environments.
Following biological species concept, speciation occurs when a population split into two non-interbreeding populations. Assortative mating is one of the possible isolating mechanisms for splitting a population. The evolution of assortative mating to avoid the cost of hybridization, known as reinforcement, is important for speciation. 
Assortative mating may be required for sympatric speciation, meaning the evolution of a new species without geographic isolation. Instead, isolated mating via assortative mating may trigger speciation. This has been observed in the Middle East blind mole rat, cicadas, and the European corn borer.
Assortative mating is believed to be the cause of the speciation of a daughter species from the parent species of coral-dwelling goby fish. The species live in a small area of rare coral in the ocean around Bootless Bay in southern Papua New Guinea which the parent species shun. The daughter species has become reproductively isolated from the parent species even though the parent species surrounds the daughter species so there is no geographic isolation. The speciation in the early stages would depend on assortative mating in which the evolving goby fishes would prefer to mate with other fish that preferred to spawn in the same area of rare coral.
In humans, assortative mating occurs along many dimensions, including religious beliefs, physical traits, age, socioeconomic status, intelligence, and political ideology, among others.
An experiment published in the Archives of Sexual Behavior had test subjects choose their preferred image out of three with one image modified to resemble the test subject, another a non-resembling attractive face, and a face more attractive than the resembling face, as determined by an outside group. The study found that male subjects preferred the female faces that resembled their own, while females did not prefer the male faces that looked like their own. Furthermore, both males and females rated the most attractive faces highly, as attractiveness may signal genetic quality.
In a 2011 study from the University of Vienna, researchers found that marriages between men and women with equivalent levels of education were less likely to suffer from reproductive failure, or childlessness. Though the study found no difference in the mean number of children in marriage between couples with similar and different levels of education, individuals with similar levels of education also had a lower than average age at first marriage.
Controversially, some have suggested assortative mating may play a role in the number of children diagnosed with autism. Autism researcher Simon Baron-Cohen is studying the prevalence of autism in children born to Massachusetts Institute of Technology graduates. The research aims to see if there is a correlation between assortative mating in high functioning MIT graduates, who may fall on the autism spectrum, and rates of autism in their children. Baron-Cohen has found a correlation between rates of autism in regions of the Netherlands where a high concentration of people work in IT and engineering.
It is believed that the American Robin practices assortative mating on plumage color, a melanin based trait. However, in female American Robins, dark plumage indicates reproductive performance, while dark plumage in males indicates health and decreased parental investment. Nevertheless, positive assortment based on plumage color may explain the American Robin’s unique mating pattern of remaining together throughout the breeding season but never reuniting in subsequent breeding seasons. Because assortative mating based on plumage color adequately predicts reproductive success, there are higher levels of fidelity and less need for extra pair relations.
- Directional selection
- Disruptive selection
- Matching hypothesis
- Negative selection (natural selection)
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