Evolutionary fitness of homosexuality

31/05/2012 § Leave a comment

Possibly out of the usual topic areas, but here goes. A preacher in North Carolina in the U.S. has been stirring up the usual anti-gay hatred. I grew up just one state north, so it is all depressingly familiar. I am fairly inured to intolerance because (a) there is so much of it and (b) picking fights with it only makes it stronger.

My first thought really is, haters gonna hate:

One of the arguments is that homosexuality is unnatural: because gays and lesbians won’t procreate, how could the gene be passed down? I built a toy model as an explanation. It didn’t quite work but let me explain the logic.

From an evolutionary perspective, genes want to reproduce themselves. Note that this isn’t about the longevity of an organism. It is about producing offspring who survive to produce offspring. Biologists know that this can be achieved with different strategies. One strategy is to protect and foster a few offspring. Another is to have lots of offspring and hope some survive.

Human genes have gone for the nurture-a-few strategy (in comparison to, say, 13-year cicadas). What if the genes could ensure not two adult guardians but three? That might give the offspring a larger chance of surviving. Maybe a set of genes could sort itself so that some offspring specialised in breeding and others specialised in providing material support for the next generation.

And that is why hereditary homosexuality can be ‘fit’ from an evolutionary perspective: it can provide extra aunts and uncles to protect the littlies. The genetic trick is to get the right proportion of breeders and non-breeders across the generations. That’s what I was working on modelling. I found that the ideal proportion of non-breeding adults was directly proportional to its impact on survival, a fairly trivial result.

So be glad for your gay relatives — they signal your own genetic fitness.

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