Evolution of Homosexuality

Many scientists have puzzled over how homosexuality (especially exclusive homosexuality) evolved. How could a behavior that appears to reduce reproductive success survive the rigors of natural selection? Many researchers have suggested some hidden adaptive value: (1) exclusive homosexuals may help their relatives raise more offspring (kin selection, parental manipulation); (2) genes that are maladaptive in males might be especially adaptive in females, and vice versa; (3) genes for exclusive heterosexuality may be less adaptive than combinations of genes that permit some homosexuality (balanced polymorphism, heterosis, hybrid vigor) (Kirkpatrick, 2000; Sommer, 1990; Werner, 1998). Clear evidence for or against these different ideas is still lacking.

Most theorists have considered only adaptation, but evolutionary arguments must also account for how changes might have arisen throughout our phylogenetic history. Werner (1998) suggested an evolutionary sequence of ever greater male-male cooperation among primates that progressed gradually from systems that marked territories in more solitary animals, to systems that marked dominance and subordination in multimale groups, to systems that marked alliances in more complex social animals.

Only small changes needed to occur to move from one system to another. The scent deposits in urine or other bodily secretions that marked territorial boundaries began to mark some animals as subordinate "guests" in a dominant's territory. In addition to "paying homage" to dominant individuals by inhaling their markings, subordinates also had to hide or avoid penile erections while observing the erection displays of the dominant males (who had exclusive sexual rights to the group's females), and perhaps also tolerate the dominant's mounting behaviors. In many of these groups adolescent males practiced these dominance displays by alternating roles with each other. In more complex animal societies this adolescent behavior continued among adult males who could mark alliances by alternating subordinate and dominant roles. As these alliances became more complex, the same-sex behaviors came to resemble human homosexuality more and more.

In a complex animal society a male with genes that encouraged only submission might fail to reproduce for lack of trying, but a male that could act only as a dominant might also fail to reproduce. A little submissiveness helps avoid dangerous fights and facilitates the formation of alliances. In every generation some males may be too dominant and others too submissive to reproduce, but their genes will be passed on through those who have a little of both personalities.

In line with this theory, one of the most peaceful and cooperative of primates, the bonobo, probably also has the highest incidences of "homosexual" behavior, especially among females. As De Waal (1989) points out, sex is probably the major way that these animals reconcile conflicts and maintain peace.

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