12 December 2012, 10.21am AEST<quoted text>
If it was an actual link...
New ideas about the evolution of homosexuality
When I give talks about the relevance of evolution to modern life, I can count on one regular question interrupting an orderly transition from lecture theatre to bar. Sometimes it comes with a “bet-you-didn’t-think-of-that- one” sneer. Far more often it is asked earnestly and with palpable empathy. The Question?
How do you explain homosexuality?
The very real fact that a large proportion of people across the world are sexually attracted to members of the same biological sex provides a giant obstacle to a Darwinian view of life.
And I am always happy to field this question because it allows me to explore, with the audience, some of the layered complexity of evolution. But a simple, definitive answer remains, for now, beyond reach. That may start to change, however, with a paper published today in The Quarterly Review of Biology.
Born this way
Evolutionary biology can be a bit of a blunt instrument. Especially when it seeks to explain big, categorical differences – like sex differences. Focusing at a broad scale means a lot of nuance and individuality gets ignored or trampled. Which is why so many authors – quite unfairly – write off all biology as determinism.
Fortunately, biological ideas about homosexuality tend to be more welcome than biological ideas about, say, gender. That may be because so many gay people strongly feel they were “born this way”. And because ideas about homosexuality being a choice or a curable condition proliferate in all but the most enlightened places.
But being “born this way” isn’t necessarily the same thing as the traits involved being genetically determined. Genetic claims require genetic evidence. In support of a genetic basis, sexual orientation has a moderate to high heritability. Heritability being the statistic that describes how much of the variation in a trait is due to genetic differences among individuals.
But despite the statistical vapour-trail indicating a substantial genetic basis, the search for major genes involved in homosexuality has been far less fruitful. And then there remains the problem so beloved of seminar questioners. How could any such genes have persisted through millenia of selection if they lead to sexual preferences that do not produce offspring?
The idea that “gay people don’t have children” is simplistic and, historically, wrong. Being gay does not necessarily mean not having a family, and throughout history many – perhaps most – homosexuals spent time in heterosexual unions, having children. And yet even if a small proportion did not, this could have exerted strong evolutionary selection against any genes involved.
But perhaps those genes provided some other kind of evolutionary advantage that outweighed the direct cost of having fewer kids. Here, theories lie thick upon the ground. First, there is the idea that homosexual relatives provide exceptional help to their heterosexual relatives who are raising families. Any genes that raise the chances of homosexuality, then, are passed on through relatives. And the extra help means more nieces and nephews carrying those genes.
The second group of ideas hinges on the idea is that genes that make reproductively successful females can impose costs when they find themselves expressed in males. And the opposite can happen for genes that enhance male fitness. Some support for this idea exists as well, including evidence that families in which females tend to be highly fertile also have a higher proportion of gay men than one might expect by chance.