Assortative mating
Examples of species that display this type of assortative mating include the jumping spider Phidippus clarus and the leaf beetle Diaprepes abbreviatus.
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.
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.
[8] Evidence for size-related assortative mating has also been found in the mangrove snail, Littoraria ardouiniana and in the Japanese common toad, Bufo japonicus.
Eastern bluebirds also mate assortatively for territorial aggression due to fierce competition for a limited number of nesting sites with tree swallows.
[11] 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).
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.
[16] Assortative mating based on genomic similarities plays a role in human marriages in the United States.
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).
[18] At the same time, individuals display disassortative mating for genes in the major histocompatibility complex region on chromosome 6.
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.
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.
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.
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.
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.
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.
Also, positive assortative mating for behavioral traits allows for more efficient communication between the individuals and they can cooperate better to raise their offspring.
[citation needed] 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.
[citation needed] Positive assortative mating is a key element leading to reproductive isolation within a species, which in turn may result speciation in sympatry over time.
Speciation from assortative mating has occurred in the Middle East blind mole rat, cicadas, and the European corn borer.
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.
