Homosexuality and Evolution

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Kin selection might make a gene for homosexuality less deleterious than it might seem at first glance, which might mean that there wouldn’t be as much selection against it. But what I’m arguing against are ideas like, “Homosexuality keeps the species from being overcrowded.” But as we can see, that makes no sense…how can a gene for homosexuality get passed on to future generations when the people who carry it don’t have children? I’m arguing against the idea that homosexuality fulfills some function in human society.

Lets look at dwarfism. Dwarfism is caused by a mutation in a gene that builds growth hormone. Dwarfism isn’t maintained in the population because it is beneficial, or a balanced polymorphism, it is maintained by mutation pressure…any change in growth hormone and you don’t grow as tall as other people. But does homosexuality work this way? No, we can’t trace homosexuality in families, we can’t find who was a carrier, who expresses the trait, etc.
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Is there a gene for left-handedness? No, there is not. Some people are born left-handed, some are born right handed, and we don’t really know why. I don’t think homosexuality can be genetically caused because identical twins often do not share the same sexual orientation. It seems most gays are born that way, but that doesn’t mean that the cause is genetic, any more than the cause of left-handedness is genetic. If homosexuality was genetic, we would expect that children of gay parents would be more likely to be gay, but this doesn’t seem to happen…in fact, most gay-rights groups stress that children of gays have the same distribution of sexual orientations that children of straights have.

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I agree with this. It seems to me that many characteristics that are helpful to humans…large brains, behavioral flexibility, retention of juvenile characteristics into adulthood, extreme sociality…are also those that could allow homosexuality to develop.
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We know that some people are tall and some are short, and there are genes that control how tall you grow…tall parents have tall children at greater than chance rates. But that doesn’t mean that there are genes “for” tallness.
Let’s think about breast cancer for a minute. Some people have a gene that means they are much more likely to develop breast cancer. But why? This gene is not a “breast cancer gene”, the normal form of this gene is actually an enzyme that protects against cancer.

We may find that people who have certain alleles are more likely to be homosexual than people who have the alternate form of the allele. But that wouldn’t mean that this is a gene “for” homosexuality. I’m just saying that we need to be careful when we name alleles…just because a particular gene might increase or decrease incidence of this or that behavior doesn’t mean that that is what the gene is for. A gene is for making a protein, but these enzymes can have many different effects on the body.

When I say that I don’t think there is such a thing as a “gay gene”, I mean that there is no gene that all homosexuals share and that all heterosexuals lack. Of course I don’t have proof, what I’m saying is that it would be very surprising if that were the case. I’m saying there is no gene for homosexuality, shyness, bravery, stinginess, novelty-seeking, etc, etc, but rather genes that cause various levels of neurotransmitters, various levels of brain chemicals, etc, etc.

Is there a distinct module in the brain that recognizes an appropriate sexual partner? We have vision centers, speech centers. Maybe there is a sexuality center too. But I don’t think so. Ultimately, we know so little about the brain that it is almost pointless to wonder about this now, but somehow we can’t help it.

When I say that I don’t think there is a gay gene or genes, I’m saying that I don’t think we’ll ever be able to point to a variant allele in someone’s genome and say, “If this had been the alternate form, this person would have been straight,” even when if sequence everyone’s genome routinely. And of course, even though I’m saying homosexuality doesn’t seem genetic, that doesn’t mean that I think people “choose” homosexuality either. I can’t think of a single gay person I know who didn’t feel that they were just born that way, they didn’t know why, anymore than I know why I’m straight, I just am and I can’t explain it.

Before suggesting books for me, try reading a little better yourself. Where did I deny that there can be neutral mutations? The fact that there are such mutations does not mean that good mutations don’t survive, or that bad ones don’t vanish.

Thanks for all your answers, now I have more to think about then I did when i started. :slight_smile:

I know that in many cultures dwarfs were used for odd cultural roles, in some places considered simply entertaining, in others they were treated specially by the clergy. That might have something to do with their not being bred out.

I personally believe that there is a design behind mutation and evolution, that it is simply chaos shaped by statistical curves. Perhaps in another situation, in another era, dwarfs might have a reason or an opportunity to form a new culture composed soully of those sharing their mutation, a chance to speciate. Then, when they built up a sizable population, a kind of critical mass in the genetic pool, their genes could express in new ways unprecedented ways, because in the past the dwarf gene was mixed in with and overwhelmed by the human gene pool and never had a chance. This might even lead to a next step in human evolution, what was formerly a rare variant becoming the norm.

I think this is why evolution seems to jump around so wildly, instead of progressing at a normal rate. There are patterns to life that are meant to interact with other patterns to grow and make it more complex.

Damn I’m high.

But seriously, I think that many of these oddities that aren’t easily explained by evolutionary science as we know it are there because we do not fully understand how evolution works yet, that they make perfect sense in the long run.

Lemur does bring up an interesting point, though, in that perhaps a particular trait might need to be beneficial to the individual as well as the species. Perhaps on the individual level, homosexuality started out simply as a way to enjoy a pleasurable experience when members of the opposite sex weren’t available (I’m aroused, you’re aroused, but my regular mate/partner isn’t). As for the species benefit, it would limit the number of offspring in a tribe of people, as others have pointed out, keeping the number of offspring that the group needed to care for to a manageable number. Or maybe limiting tribe sizes wasn’t an issue for primitive man; does anyone know of any human practices in primitive civilizations where the purpose was to keep population sizes at a manageable level?

Badtz Maru: No. Dwarves tend to very reduced fertillity compared to tall people. It can be easily demostrated that dwarfism lowers your reproductive fitness, it is not neutral.

And have you ever heard of the concept of recessive genes? Most people who have achondroplasia (the gene that causes dwarfism) have parents who have normal height. Even if all dwarves were killed at birth it would not be bred out of the species. The mutation rate of the gene keeps introducing achondroplasia into the gene pool, while selective pressure removes it. But selection can only operate on individuals who express the gene, not people who are carriers. Let’s say that only 1% of the population has a copy of the gene for achondroplasia…1% of the population are carriers. Assuming random mating, 1/10,000 of the population would express the gene, would be dwarves. Let’s say they are all killed. Yay, we’ve protected the gene pool!

But no. We’ve only removed one copy of the gene. For every person who expresses the gene, there are 99 carriers and they are still alive and reproducing. The frequency of the gene is slightly decreased, but only by a little bit.

And the more rare the gene is, the less subject to selection it is. If the gene exists in only 1/10,000th of the population, then (again assuming randome mutation), then only one person in 100,000,000 will express the trait. That means only two or three people in all of America. So if the mutation rate is high enough, it can keep introducing the trait even if it is lethal. All it takes is for more mutations to occur than people expressing the gene.

The more common a bad recessive gene is, the more it is affected by selection, which will soon make it uncommon. But the more uncommon it is, the more it is protected against selection.

So every person has many many defective genes. It’s just that the odds of having a child with someone who shares the exact same bad mutations is very slim, unless they are related to you, that’s why inbreeding often causes birth defects.

So do you see how bad mutations can be maintained at startlingly high frequencies in a population, even with tremendous selection against them?

Caldazar: NO, I am not saying that perhaps a particular trait might need to be beneficial to the individual as well as the species. I am saying that a particular trait is never beneficial to the species, it is always beneficial to the individual.

Why wouldn’t we expect a gene that limits population size to managable numbers? Well, let’s say you have a bunch of sheep. If they get too crowded they stop having so many babies. But there’s one sheep, a mutant. The gene that normally would shut off her reproduction during crowded conditions is damaged, and doesn’t function properly. So she keeps on having babies. Let’s say she has twice as many babies as the average sheep who has a functional copy of the gene. Well, in the next generation, her two babies will also share the gene, and they will have twice as many babies. In the next generation their four babies will have
eight babies. Pretty soon the non-regulated sheep start to outnumber the regulated sheep. Sure, all the sheep are near starvation and are now crowded, filthy and diseased, but natural selection doesn’t care.

Genes that turn off your reproduction by definition won’t be passed on into future generations as fast as genes that do not turn off your reproduction. It doesn’t matter if it will be better for the species or not, natural selection doesn’t plan ahead, it is merely the preferential representation of variation in future generations.