rubbercok3000

What, if any, is the evolutionary purpose of homosexuality? Genetically is it just a reccesive trait? Population control? Or perhaps weeding out of inferior genes?

Lobstrosity

I personally don't think it has a "purpose." I think it's merely an inescapable side-effect of the way humans have evolved to differentiate the sexes. Men and women are incredibly similar genetically. Hell, men even have nipples, even though they serve absolutely no purpose and never have. Is it really so surprising that such similarity occasionally can result in cognitive confusion over whom one should find sexually appealing (with the "should" being taken from an evolutionary "reproduction = good" standpoint)?

The reason why men and women take on different physical and psychological traits is due almost entirely to hormones, not genes. I personally suspect that if you get that hormone mixture slightly incorrect during your formative years (e.g. from your mother in the womb or as a child while your brain is developing), brain structures responsible for determining attraction will form incorrectly. Sometimes one's attraction "switch" is turned on for all humans. Sometimes it's just on for your own sex.

I think nature found an incredibly economical way to encode both men and women: rather than have massive amounts of genetic material specific to just one sex, you have a tiny bit of gender-specific genetic material coupled with hormone-controlled development. My belief is that this economy simply makes it easier for the process to not work out quite right every time (and I mean "right" from the stand point of reproduction, not from the stand point of morality). I seriously doubt that homosexuality can be largely genetic (though perhaps genes make up a small component of if) because evolution would have weeded those genes out long ago--homosexuals in general will not reproduce with anywhere near the frequency of heterosexuals, especially long ago when you didn't have the church trying to force them to be "normal." Therefore, I maintain that homosexuality is not specifically an evolved trait but rather a small side-effect of other evolved traits. The evolutionary costs are that the human reproduction rate takes a hit due to 10% (or whatever the true percentage is) of the population's being homosexual. The benefit is genetic simplicity. I suspect the benefits simply outweighed the cost (i.e. the existence of a small percentage of homosexual individuals is not enough to drive evolution to take more drastic measures to ensure it stops).

Paul2

I read somewhere that it developed as a method to curb overpopulation. to bad i can't remember where that was.

Vorkosigan

Why, it evolved in Africa, like all other human traits.

pz

Yes, that is too bad -- because I think your source would be a very silly thing to read.

I think Lobstrosity's post is almost right. One thing I might quibble with is that it has never been demonstrated that homosexuality has any reproductive cost beyond that of any of the other elaborate and inefficient sexual strategies our species uses. It's also not the case that evolution will invariably weed out features that impair reproductive rates.

MrDarwin

This question comes up periodically, and has been discussed at great length. Try searching the evolution/creationism archives for past discussions.

Lobstrosity

What I mean by reproductive cost is that a homosexual organisms almost certainly not reproduce at all in its lifetime (assuming the organism does not its cognitive abilities to push aside its instinctual sexual drives). I didn't mean to imply that this was the only source of reproductive cost suffered by a population of organisms.

One of the points I was trying to make is that if gene X means "will not reproduce," you will not find gene X in a population after a few generations, save for mutations alone. Therefore, if there was a simple gene (or even a more complex series of genes) that produced homosexuality, one would expect homosexuality to be solely the result of mutations, and that wouldn't account for anywhere near the incidence of homosexuality in humans (and probably many other species as well). Because homosexuality persists at a non-negligible level even though in general homosexuals themselves do not produce offspring, it can reasonably be assumed that it is the product of the human sexual-differentiation strategy itself and not merely that specific individual's genes.

I would only suggest that evolution weeds out those individuals who reproduce poorly due solely to their own genes. I am clearly not suggesting that evolution would weed out poor reproduction due to something as complex as homosexuality.

pz

I know what you meant, and that's my point: there is no evidence that homosexuals will "almost certainly not reproduce at all". It's an assumption. It's at least mostly false. I know many homosexuals who do not have kids, but I also know many more heterosexuals who do not, either. I also know homosexuals who have kids and families, or who want to have kids.

Here's another attribute of our species that has a reproductive cost: chastity. The fact that we do not encourage our daughters to sleep around promiscuously and engage in sex the instant they reach a fertile age almost certainly has a far greater impact on our rate of population growth than the presence of men and women who enjoy sexual practices that do not lead to reproduction (and that includes heterosexual men and women, too, who often deposit gametes in orifices where they have no hope of encountering a complementary gamete, or even go to elaborate measures to suppress gamete production!). Strangely, though, while many people seem to be baffled and anguished by the fact that evolution hasn't exterminated homosexuality (I'm not including you in that category), these other practices don't seem to elicit the same response.
That's the assumption not in evidence.I agree, but for different reasons.This line of reasoning has some flaws. Natural selection weeds out those individuals, but evolution is more than natural selection -- and clearly, evolution does not work quite so sharply. There are traits that are due solely to genes, but have persisted in the population for quite some time: cystic fibrosis and Huntington's, for instance.
Yes, it wouldn't.

Lobstrosity

Then perhaps you neglected to read my disclaimer "assuming the organism does not its cognitive abilities to push aside its instinctual sexual drives." Yes, human homosexuals will have an understanding of how reproduction works and will do so out of the desire for children or out of denial of their true desires. I was trying to more generally address homosexual organisms, not specifically humans. Perhaps that's pointless, however, because maybe human homosexuality has no parallel in other species (I honestly don't know and was simply assuming it would exist elsewhere--I've heard of monkeys and dolphins exhibiting homosexual behavior, but I do not know if this is actually a parallel of human homosexuality). In the animal world, short of homosexual females being raped by males, it's hard to envision a homosexual organism ever copulating. They are not mating because they know it will produce babies, they are mating because an instinctual drive is urging them on. This is why I made my argument against the sole genetic origins of homosexuality. If you do not find this sort of homosexuality in other species, however, then yes, I concede my whole genetic analysis does not apply. This is because as you indirectly pointed out, humans are strange. Much of our actions are based on understanding and exploit technology. As such it becomes difficult to use human behaviors as a general rule of thumb for what one would expect to see in nearly all other mammalian species. I know that a fair number of human homosexuals reproduce or adopt.

Perhaps I should have said "prunes," then. I wasn't trying to imply anything about the "sharpness" of the effect, merely that the effect was in place to greatly diminish the presence of such phenotypes in a population. Clearly detrimental genes can stick around if they are recessive and/or still allow the organism to successfully reproduce.

MortalWombat

It is my hypothesis (not formal, mind you, just a thought kicking around inside my skull) that male homosexuality, at least, is a "tolerated by-product" of male horniness in general.

That is, male horniness might have been the overriding trait that was selected for in order to propagate the species, and the small percentage of males that end up being horny for other males for whatever reason (genetic, developmental, environmental, who knows?) is not enough to overcome the positive selective advantage of horniness in general.

Female homosexuality, on the other hand, I have no hypothesis about. Other than it makes many horny males even hornier.