The Universe of Discourse

Sat, 21 Jul 2007

Homosexuality is not hereditary
A just read a big pile of blog comments that all said that homosexuality couldn't be hereditary, because if it were, natural selection would have gotten rid of it by now.

But natural selection is more interesting than that. This article will ignore the obvious notion of homosexuals who breed anyway. Here is one way in which homosexuality could be entirely hereditary and still be favored by natural selection.

Suppose that human sexuality is extremely complicated, which should not be controversial. Suppose, just for concreteness, that there are 137 different genes that can affect whether an individual turns out heterosexual or homosexual. Say that each of these can either be either in state Q or state S, and that and that any individual will turn out homosexual if any 93 of the 137 genes are in state Q, heterosexual otherwise.

The over-simplistic argument from natural selection says that the Q states will be bred out of the population, and that S will be increasingly predominant over time.

Now let's consider an individual, X, whose family members tend to carry a lot of Q genes.

Suppose X's parents have a lot of Q genes, around 87 or 90. X's parents' siblings, who resemble them, will also have a lot of Q genes, and have a high probability of being homosexual. Having no children of their own, they may contribute to X's welfare, maybe by caring for X or by finding food for X.

In short, for every gay uncle X has, that is one additional set of cousins with whom X does not have to compete for scarce resources.

This could well turn out to be a survival advantage for X over someone from a family of people without a lot of Q genes, someone who is competing for food with a passel of cousins, none of whom ever really get enough to eat, someone whose aunt might even try to kill them in order to benefit her own children.

Perhaps X turns out to be homosexual and never breeds, but X probably has some siblings, in which case X might be an advantageous gay uncle or lesbian aunt to one of his or her own nieces or nephews, who, remember, are carrying a lot of the same genes, including the Q genes.

It might not actually work this way, of course, and in most ways it probably doesn't. The only point here is to show that natural selection does not necessarily rule out the idea of inherited homosexuality; people who think it must, have not exercised enough imagination.

(Now that I have finished writing this article, it occurs to me that the same argument applies to bees and ants; most individuals in a bee or ant colony are sterile. Who would be foolish enough to argue that this trait will soon be bred out of the colony?)

The moral of this story:

Time and time again, biologists baffled by some apparently futile or maladroit bit of bad design in nature have eventually come to see that they have underestimated the ingenuity, the sheer brilliance, the depth of insight to be disovered in one of Mother Nature's creations. Francis Crick has mischievously baptized this trend in the name of his colleague Leslie Orgel, speaking of what he calls "Orgels Second Rule: Evolution is cleverer than you are."
Daniel Dennett, Darwin's Dangerous Idea, p. 74.

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