Homosexuality and evolution

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The point of evolution is the survival of the species. The species survives by making babies. From an evolutionary standpoint, sex between two men or two women is a waste of energy. But if homosexuality offered no evolutionary advantage, why didn't it die out as an evolutionary dead end. The human tail was lost when man moved from the trees to the savannah, because it no longer conferred an evolutionary advantage. But homosexuality is still with us and has been observed in 1500 species.

1. Does the persistence of homosexuality indicate that this behavior has an evolutionary advantage?

2. If it does not, then why has homosexuality survived evolution's pruning?

3. If it does, then what is the evolutionary advantage conferred by homosexuality?

No. Selection can operate at different levels: gene, individual, group, and there is a remote possibility that it might operate at the level of ecosystems. Most evolutionary biologists think that group selection can only operate under a special set of circumstances. "Survival of the species" is not just group selection, but a pretty large group. The larger the group, the more special the circumstances have to be for group selection to work.


Yes.


If you believe that reproduction is the only function of sex. But then hidden ovulation and sex when a woman is not fertile is equally a waste of energy. Whatever argument you make, based on lack of opportunity to make babies, applies equally to hidden ovulation, to sex after menopause, and to homosexuality. So it's a good guess that sex might have other functions.


None of our nearest relatives has a tail. Chimps, bonobos, gorillas, orangutans, and gibbons all lack a tail. The most parsimonious inference is that the tail was lost before our last common ancestor with gibbons.


Probably. Look up bonobos.


In bonobos, it has social function. In Laysan albatross, two females without males may be able to raise a chick together, where a single female couldn't. They still need a male donour. I don't know how they decide who lays an egg or whether they just ignore a second egg.

I stumbled upon an interesting Ted talk about that. But I cant find the one I saw, but here is another Ted talk that covers the same ground as the other one I saw.

Apparently the latest research shows that the odds of a guy being gay go up sharply the more older brothers he has because of epigenetics having to do the mother's womb. Its nature's way evening out the sex ratio in the tribe.

It's always been explained to me that evolution is really the survival of everything that's not so costly it prevents reproduction, i.e. it's not that efficient. There isn't necessarily a direct evolutionary benefit to a characteristic just because it's present/observable.

Thanks for the comment Dan. Well, if something can't reproduce, it usually won't survive. But two men can't reproduce, and still, nature keeps turning out homosexuals.

I think of evolution as adaption to new environments. So the question becomes "How might homosexuality allow a society to adapt to new environments?" I guess it could be useful in the case of overpopulation. It might also be useful in allowing groups of men to cooperate in taking down big game. It could act to solidify a group's cohesion for protection too. Such is the case with the Sacred Band of Thebes, which was composed of pairs of homosexual lovers, who really hang together in battle, as each battled for the safety of his lover. And I suppose it could do something similar for female couples, had any historical information been researched on women warriors and hunters?

Well, I mean, I'm definitely not an expert (I'm thinking about studying a high-school level biology curriculum) but I SUPER SERIOUSLY think evolution is the result of random mutations that allow adaptation to environments--it isn't that adaptation itself. A characteristic can develop that doesn't adapt to an environment and that's still evolution.

Think about a peacock's tail: it's really not that adaptive. It takes resources/nutrients to grow, and then once it's grown it can get stuck in underbrush or predators can grab onto it, allowing it to get killed and eaten. Still, it remains, because even though it's costly, it's not costly enough to guarantee the individuals that express it will die before they reproduce.

I'm saying it's the same with homosexuality: evolutionarily speaking, there's not necessarily any "purpose," or net evolutionary benefit, to homosexuality just because it's present in a population. It's just there and doesn't make people die that fast, so it stays.

But if I had to say there was a benefit to it, I'd say it's related to the theory that grandparents are evolutionarily beneficial because they help raise kids after they're no longer reproducing offspring of their own. Like, gay people can contribute to society, so.

Again, though, I don't think this is the "purpose" of homosexuality in evolution.

That was another hypothesis. That gays help raise the children of heterosexual couples.

Not much evidence has come forth to prove it though.


Fish can actually change their sexual gender. If a community of little fish in an isolate stream have too many of members of one gender, and not enough of the other- some individual fish will just BANG!- morph into the opposite sex to even out the sex ratio.

Humans do not have the ability to spontaneously sex-change. But we do seem to have subtle mechanisms like that. For example its been long observed that during war time more boy babies are born than girls (as if to replenish the men killed in war). Like during WWII. I doubt that its because that women's bodies somehow "know" that men are dying thousands of miles away on Omaha Beach, or on Guadalcanal. But their bodies may pick up cues that there is a shortage of adult men right there in the community via pheromes. If all the draft age men in a community vanish into the armed services their absence could be sensed by a womens chemical receptors as she goes about shopping etc. And that could trigger her body favoring making male babies to even out what her body thinks is a lopsided sex ratio in the tribe.

Similarly there is that thing in that video I posted above about evidence that boys are more likely to be gay if they have many older brothers.

And the "helping raise kids" theory may have some weight as well.

I'm sure that evolution does not assign purposes to the various paths it takes. As you say, it just uses mutations to adapt to and survive in the environment. The mutations simply have a staying power that the normal genetic makeup can't emulate. So what doesn't work dies off. So why are there homosexuals? It could be because they provide some advantage or it could be that they are just there; not providing any benefit, but not being detrimental to survival.

You may not be an expert, but you obviously have a great love of learning. I do too. I like to think about what constitutes our consciousness.

I dunno, I think I just listen to a lot of NPR, ha ha.

I do think the homosexuality thing has to do with consciousness, though, in that it takes a lot to conceptualize "masculinity" or "femininity" or whatever that it takes to be attracted to someone. Hence why I don't think it's just a mutation but many mutations, probably related to abstract reasoning. Like art or something, I don't know.

On my radar is The Penguin History of the World, which is a single-volume history of the world, which the first chapter glosses over evolution and its position is that the history of the evolution of humans is the history of the (or an) evolution of conscious thought and the ability to choose. Like, it talks about how our ability to consciously choose our behavior allows the rate of change in culture/society to outstrip the rate of change of random genetic mutation by many probably thousands or millions of times.

But I don't believe homosexuality is a choice, which I guess strains my resistance to evolutionary psychology. I dunno.

Ah free will choice. I do not see how that is a possibility for a human being. Since we are born knowing nothing, We must learn everything, including the criteria which guide our choices. Is it a free choice if we only ever heard about 8 of a possible 15 alternatives? If we find ourselves in a situation we have never experienced before, we might freeze and do nothing. Is that a free will choice? What's more, we have little to no control over what experiences that life will throw our way. The deck is stacked against free choice, because of all the cultural instruction which was foisted upon us, regardless of whether we wanted it or not. We are not even conscious of how it guides our choices. Everything that we have experienced sets up the single path that our choices must follow. In a word, I'm deterministic. I think that everything happens for a cause. Free will choices are Saints and villains don't just pop into existence. People learn to be saints or villains, as a result of the experiences that life throws their way.

Hmmm...it appears that I have just hijacked my own thread

Homosexuality likely exists for the same reason autism and sickle cell anemia exist. The factors that give rise to homosexuality also provide an evolutionary benefit.

I'll use the example of sickle cell anemia simply because it illustrates the point well and not to imply that homosexuality is a disease. I do think its fair to say homosexuality confers an evolutionary disadvantage in regards to passing on one's genes. In heterozygous individuals the sickle cell trait confers a resistance to malaria along with no appreciable decrease in cardiovascular health. In homozygous individuals it is associated with significant cardiovascular problems and early mortality.

Likewise the traits for homosexuality likely increase the chances of having biological children when not manifesting as full homosexuality. What those traits are, or why they're advantageous I have no idea and would be hesitant to speculate.

I kind of jumped the gun and made another post that I've deleted since before looking things about this up, like the idea there's a genetic link to homosexuality in humans. That actually really surprises me.

I'm curious where you get the idea about increasing the chances of having children, though? Is that speculation?

Kind of in defense of things I was saying in the post I deleted (which are more or less in line with what I've posted before in this thread), here's a quote from the New Scientist article I looked at:



I think sexuality has to do with consciousness, consciousness is biological, and biology is messy.

The peacock's tail is the textbook example of sexual evolution of a handicap. Only males with otherwise good genes are healthy enough to grow that big tail and physically fit enough to escape predation. The females who pick the males with the most expensive tails get the good genes that can produce those tails for their offspring.. The sons will have the good genes and handicap, but the daughters will have the good genes without the handicap. That's what selects for the female preference. Sexual selection is one way to evolve traits that are a drawback for natural selection.


If it reduces reproductive success, it goes, unless there is a compensating benefit.


That would work.


They would have to contribute not just to society, but to those people most likely to share the genes that predispose towards homosexuality: their own relatives. Gay people being really good aunts and uncles would do.

It also matters that arranged marriages were the norm for much of human history. There was pressure to have enough sex with an opposite sex spouse to produce children. Reproductive success might have been not much different than in a marriage of heterosexuals, and that would have removed any selection pressure against homosexuality. Today's homophobes are doing the same thing. The more their prejudice pushes homosexuals to remain in the closet, the more likely it is that they pass on whatever genetic predisposition they may have. If the homophobes took a long view, they should welcome gay people coming out and encourage them to adopt. I guess they are afraid the cultural tradition of homophobia would die out before homosexuality does.


Statistical variation is another possibility. The central limit theorem states that when you have many causes with small effects, the overall distribution approaches the bell curve of the normal distribution. Apply that to preference for men or women, and have one bell curve for men's preferences, another for women's preferences. If the curves are wide enough, their tails will include men preferring men and women preferring women. Nothing to get excited about, just part of the normal variation.


Not all fish, only some species. Here is one:

That is the rigorously investigated theory of evolution. Traits that increase the chances of passing on traits are passed on. Therefore since homosexuality decreases the chance of having biological children, than the traits that give rise to homosexuality must increase the chances of having biological children when not manifesting phenotypically as homosexuality.

Speculation would be if I were to make any remarks of what those traits are when I have no evidence or theory to support it.

I'm interested in this specific claim:



It sounds to me like you're saying the genes for homosexuality increase fertility if they're not manifesting as full homosexuality.

My understanding of evolution is that genes as molecules mutate randomly mostly due to errors in transcription, which causes differences in the expressed traits those genes code for, and if these differences in expression (differently shaped beaks, for example) cause individuals expressing them to die at a faster rate than they can reproduce, they die out due to natural selection.

By analogy, I'm saying the genes for a beak that isn't particularly good at opening nuts on a bird that primarily survives by eating nuts, for example, won't be selected out if there's not a strong evolutionary pressure on the population--if their food is plentiful, or they're not being hunted very often, etc.

Humans are apex predators and incredibly adaptive to their environments, or rather they adapt their environments to themselves very well. The estimates I've heard for homosexuality in the population is something like 2-10%. Even if we assume that no one who's homosexual has children, which I don't think has ever been the case, that slows down reproduction 2-10%, all else being equal. I don't see why there'd have to be some kind of increase in reproduction or survival due to the genes that are linked to homosexuality.

I think homosexuality is costly to reproduction, but the idea I can't get out of my head is that it's not costly enough for natural selection to eliminate it. There isn't another species competing for our ecological niche, for example.

For clarity's sake, I'm saying:

Logically possibly. I don't think it has to.



Lack of evolutionary pressure.



Being like grandparents raising kids past their reproductive years. Also being like women not displaying fertility externally so humans have to have sex all the time. Human relationships are really important to how society functions. I'm not sure we can be sure this outweighs the reduction in reproduction it causes in evolutionary terms, though.