Biology and Sexual Orientation - Sexual Orientation and Evolution

Sexual Orientation and Evolution

Sexual practices that significantly reduce the frequency of heterosexual intercourse also significantly decrease the chances of successful reproduction, and for this reason, they would appear to be maladaptive in an evolutionary context following a simple Darwinian model of natural selection—on the assumption that homosexuality would reduce this frequency. Several theories have been advanced to explain this contradiction, and new experimental evidence has demonstrated their feasibility.

Some scholars have suggested that homosexuality is adaptive in a non-obvious way. By way of analogy, the allele (a particular version of a gene) which causes sickle-cell anemia when two copies are present may also confer resistance to malaria with a lesser form of anemia when one copy is present (this is called heterozygous advantage).

The so-called "gay uncle" hypothesis posits that people who themselves do not have children may nonetheless increase the prevalence of their family's genes in future generations by providing resources (food, supervision, defense, shelter, etc.) to the offspring of their closest relatives. This hypothesis is an extension of the theory of kin selection. Kin selection was originally developed to explain apparent altruistic acts which seemed to be maladaptive. The initial concept was suggested by J.B.S. Haldane in 1932 and later elaborated by many others including John Maynard Smith, W. D. Hamilton and Mary Jane West-Eberhard. This concept was also used to explain the patterns of certain social insects where most of the members are non-reproductive.

Brendan Zietsch of the Queensland Institute of Medical Research proposes the alternative theory that men exhibiting female traits become more attractive to females and are thus more likely to mate, provided the genes involved do not drive them to complete rejection of heterosexuality.

In a 2008 study, its authors stated that "There is considerable evidence that human sexual orientation is genetically influenced, so it is not known how homosexuality, which tends to lower reproductive success, is maintained in the population at a relatively high frequency." They hypothesized that "while genes predisposing to homosexuality reduce homosexuals' reproductive success, they may confer some advantage in heterosexuals who carry them." and their results suggested that "genes predisposing to homosexuality may confer a mating advantage in heterosexuals, which could help explain the evolution and maintenance of homosexuality in the population.". However, in the same study, the authors noted that "nongenetic alternative explanations cannot be ruled out" as a reason for the heterosexual in the homosexual-heterosexual twin pair having more partners, specifically citing "social pressure on the other twin to act in a more heterosexual way" (and thus seek out a greater number of sexual partners) as an example of one alternative explanation. Also, the authors of the study acknowledge that a large number of sexual partners may not lead to greater reproductive success, specifically noting there is an "absence of evidence relating the number of sexual partners and actual reproductive success,either in the present or in our evolutionary past."

Important new evidence on a plausible mechanism for the evolution of "gay genes" has emerged from the work of Camperio-Ciani. They found in two large, independent studies that the female relatives of homosexual men tended to have significantly more offspring than those of the heterosexual men. Female relatives of the homosexual men on their mother's side tended to have more offspring than those on the father's side. This indicates that females carrying a putative "androphilia genes" complex are more fecund than women lacking this complex of genes, and thereby can compensate for any decreased fertility of the males carrying the genes. This is a well known phenomenon in evolution known as "sexual antagonism," and has been widely documented for many traits that are advantageous in one sex but not in the other. This provides solid experimental evidence of how "gay genes" could not only survive but thrive over the course of evolution.