For evolutionary biologists interested in homosexuality there is one big puzzle: That it exists at all, although it should not exist according to the basic assumptions of this discipline. Only such genetic dispositions that increase the reproductive success and the genetic fitness of their carriers can succeed in the evolutionary struggle for existence. But the need to have sex with members of ones own sex is not a good recipe for childbearing.In the past I attributed the homosexuality/evolution puzzle to gayness being a byproduct of heterosexual sex. Evolution gave us heterosexual sex for survival and it accidentally gave homosexual sex too. There are lots of byproducts of evolution that don't contribute to fitness, like perhaps music or language, though you can ask Steven Pinker and Noam Chomsky about that.
Now I see my mistake:
"Homosexuality is practically a form of sterilization", says psychologist Qazi Rahman from Queens College in London.Byproducts need to be survival neutral. They can't be antithetical to survival.
These figures represent a major challenge for any genetic theory, because a genetic trait that reduces the reproductive success would be mercilessly eliminated by natural selection, explains psychologist Edward M. Miller of the University of New Orleans.
But does homosexuality have a genetic component at all?
A study by US-researchers J. Michael Bailey and Richard Pillard reached a clear conclusion: With identical twin brothers of homosexuals, the probability that they are gay too amounts to 52 percent, with fraternal twins it is 22, with adopted siblings 11 percent.
So can we estimate the proportion of genetic influence on homosexuality? Only to a degree. The estimates range from 31 to 74 percent heritability in men and 27-76 percent heritability in women...
Nevertheless, the geneticist Dean Hamer of the National Cancer Institute in 1993 made a splash with the message that he had identified a “gay gene” on the X-chromosome which is inherited from the mother. Of 40 pairs of homosexual brother, 33 had five common markers in a section of DNA called Xq28.Like almost everything, there is at least a genetic component to homosexuality.
So what explains homosexuality's survival? One popular theory is that homosexual's may not reproduce, but they're especially good at promoting the survival of their relatives. But the problem with this is:
says psychologist Edward M. Miller: If the genetic merit of being gay would be the fitness increase in relatives, homosexuals would be better off by being completely asexual, and not to assume the risks and costs of the homosexual lifestyle.But there's an alternative explanation:
A man who carries a small dose of gay genes in his genome would, according to the theory, improve his success in the heterosexual mating game. That “certain something” that heightens sex appeal probably consist exactly of those essentials which make homosexuals different from heterosexuals in the first place. According to his theory, the alleged "gay genes" equip men who carry the heterozygous disposition with an above-average degree of feminine traits such as sensitivity, gentleness and friendliness. Gay genes therefore form a natural antidote against "hypermasculine" genes that turn men into rough machos.In other words, a tad of gay increases attractiveness to women, and creates fewer infections and miscarriages, thereby increasing fitness. Enough to counteract the counter-fitness of a lot of gay that makes a full blown homosexual.
This fits with the popular trope of the woman who wishes her gay male friend were only straight.
Homosexuality is one of many pieces of evidence that seemingly flies in the face of evolution. But every time we discover one of these evidences, we find some complicated way of preserving evolutionary theory and making the evidence fit. So I have to ask, is there any evidence at all that would overturn our current views on evolution?
-A Tad of Gay holds Sway is an excellent read.