It looks like the practice of same-sex lovin’ may have evolved more than once, and for more than one reason. Researchers look to the animal kingdom for hints about the origins of homosexuality, and a recent review of that research suggests that those origins probably vary depending on species, environment and chance. The article doesn’t have much to say about human sex or homosexual preference (the authors point out that animals probably don’t have sexual identities the way people do), but it does seem to imply that there could be more than one evolutionary/biological basis for homosexuality in Homo sapiens.
First, there are the adaptive hypotheses, which provide an explanation for same-sex behaviour that would boost the biological fitness of one or more of the individuals involved. For example, several species, including bottlenose dolphins, seem to use same-sex behaviours to promote social bonding. Others may have evolved them as a form of intrasexual conflict. Indirect insemination, as in the flour beetle, provides a third possible adaptive advantage. Then there is the practice hypothesis, that individuals are honing their skills for mating, which seems to hold good for male fruit flies at least. Several other adaptive explanations have been invoked to explain same-sex behaviour in humans, including kin selection – helping to further the genes you share with close family members – and “over-dominance” – the idea that certain genes somehow increase fitness in individuals who possess a single copy of them but are associated with same-sex behaviour in people with two copies. Then there is “sexually antagonistic selection” – the idea that alleles promoting same-sex behaviour in men are favoured by selection because they increase the reproductive chances of their daughters.
There are also various non-adaptive explanations. Mistaken identity could indeed be one cause. Van Gossum’s damselflies exemplify another idea, known as the prisoner effect, in which depriving individuals of interaction with the opposite sex prompts them to mate with members of their own sex. Then there is the evolutionary by-product hypothesis – selection for some other independent trait, such as high sexual responsiveness, might make individuals more likely to participate in same-sex sexual behaviour. It has also been suggested that same-sex behaviours appear when organisms are imperfectly adapted to their environment.
New Scientist: Homosexual Selection: The Power of Same-Sex Liasons
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