In human genetics, assortative mating is a form of non-random mating in which pair bonds are established on the basis of phenotype i.e. observable characteristics, which is thought to underlie mate selection in a variety of animal species as well.
“I had a patient once who dreamed she kept her husband in the deep freeze except for mating. Lots of men feel that way.” – Robert Johnson
Two Coccinella Septempunctata, or Ladybirds mating
In human relations, the assortative mating process may lead a person to choose a mate according to religious, cultural, or ethnic preferences, professional interests, or physical traits.
Positive assortative mating, or homogamy, exists when people choose to mate with persons similar to themselves; this type of selection is very common.
Negative assortative mating is the opposite case, when people avoid mating with persons similar to themselves.
“All nature’s creatures join to express nature’s purpose. Somewhere in their mounting and mating, rutting and butting is the very secret of nature itself.” – Graham Swift
The sexy son hypothesis suggests that females obtain future fitness benefits from mating with polygynous males through the inheritance by their sons of traits contributing to mating success. It proposes that a female animal’s optimal choice among potential mates is a male whose genes will produce male offspring with the best chance of reproductive success.
What matters are her so-called sexy sons’ future breeding successes (like that of their promiscuous father) in creating large numbers of offspring carrying copies of the female’s genes.
Sexual selection by direct and/or indirect benefits as well as sexual conflict determines the evolution of animal mating systems. Female mating preferences are widely recognized as being responsible for the rapid and divergent evolution of male secondary sexual traits. In 1976, Richard Dawkins wrote in The Selfish Gene:
‘In a society where males compete with each other to be chosen as he-men by females, one of the best things a mother can do for her genes is to make a son who will turn out in his turn to be an attractive he-man. If she can ensure that her son is one of the fortunate few males who wins most of the copulations in the society when he grows up, she will have an enormous number of grandchildren. The result of this is that one of the most desirable qualities a male can have in the eyes of a female is, quite simply, sexual attractiveness itself.’
The Fisher’s runaway process for species development consists of the two phases which he outlines in his genetical theory of natural selection:
Female preferences initially evolve because the preferred traitis favored by natural selection and hence the offspring are more likely to carry the beneficial trait.
Whenever appreciable differences exist in a species, which are in fact correlated with selective advantage, there will be a tendency to select also those individuals of the opposite sex which most clearly discriminate the difference to be observed, and which most decidedly prefer the more advantageous type.
Once female preferences exist, males with the trait are even more fit (both a natural and a sexual selection advantage). There will then be an ever increasing selective force favouring stronger preferences and more extreme traits (Fisher’s runaway process).
The further development of the character trait will still proceed, by reason of the advantage gained in sexual selection, even after it has passed the point in development at which its advantage in Natural Selection has ceased.
Males and females have different constraints on reproduction:
Females tend to be limited by fertility,
Males limited by mating success;
This disparity leads to:
Males having a much larger variance in reproductive success;
Female “choosiness” (the bases for which are not always clear);
This choosiness leads to extreme male traits, often at odds withmale viability fitness.