Tuesday, 7 July 2015

How 'gay genes' might work

This business of 'gay genes' seems to me to be almost universally understood. First of all, they must be inheritable. Homosexuality can't just be a frequent mutation - it's too widespread. Secondly, they have to confer benefit to those who have the genes, and that's just about everyone, because homosexuality occurs in all populations. So how on Earth could that work? It works if you consider homosexuality to be part of an 'extended phenotype' (see the book 'The Extended Phenotype' by Richard Dawkins): genes for homosexuality don't produce homosexuality in the bodies they are in, but in others. That allows them to be inherited.

Imagine a group of animals that can end up living in dense populations. What would be a bad thing is for the breeding pairs to continue to produce large numbers of breeding offspring. Instead, when resources get limited, a better situation is if some children don't breed but end up helping their parents to raise their siblings. Does this happen in Nature? Yes, it does. A very clear example is some species of birds, where non-breeding young birds help their parents get food for their siblings. This division of labour into breeders and non-breeders results in a better chance of survival.

This must mean that the parent animals have genes that allow for the production of non-breeding offspring in certain situations. Not all offspring of course, but a certain proportion. This might happen because hormones react to population density. The point is that genes for non-breeders can exist throughout the population, and can be of real benefit.

There is slight evidence of a similar situation in humans. Later children in a family seem to be more likely to be homosexual. This makes sense, as the family has already produced breeding offspring, and what might be of more use is additional resource gatherers. This seems to work because the hormonal environment in the human uterus changes with each subsequent birth, and that might increase the probability of homosexuality.

This may be wrong, but it does show that genes for homosexuality don't need to act in the bodies of homosexuals - they could simply be genes that change the hormonal environment in the uterus with time. Also, homosexuality can be of real benefit in a population at low levels, as it provides additional support systems for families - even as simple as more hands to gather food and fight mammoths!

There is not going to be a simple 'gay gene'. The situation is far too subtle.

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