Evolution and the prevalence of homosexuality

Factual Questions
1

Disclaimer: I don’t think there is anything wrong with homosexuality.

Disclaimer 2: Sometimes I speak as if “evolution” does something or wants something. I know this is not the case, but it’s easier to communicate using this language.

The question: Given that natural selection is mostly very effective at selecting against traits that reduce fitness, how come homosexuality still exists in the population at such high levels (~5%)?

As a rule of thumb in evolutionary biology, the number of generations a mutation persists in the population is reciprocal on its effect on fitness - eg. if the effect is 1% it would take a 100 generations for the mutation to disappear. This is a very short period of time considering the timespan evolution has to act over.

You would clearly expect homosexual males to father less offspring than heterosexual males. (I focus on male homosexuality, since I find that most puzzling in this question.) Perhaps in pre-historic times most homosexual males would be in partnerships with women, but even in that case I would expect fewer offspring, given their limited sexual interest in their partners.

Suggested solutions and my thoughts on them:

  1. “Gay uncle hypothesis”

The gay uncle, having no offspring of his own, can give extra help to the offspring of his siblings.

My thoughts: While that’s true, the gay uncle could help his own genes more by having offspring himself.
2. “A gene for liking men”

What’s inherited is not male homosexuality, but sexual attraction to men. Thus female offspring would also inherit this, make them more attracted to men, and thereby increase their number of offspring.

My thoughts: I think that evolution can find alternative ways to achieve this, such as encoding it on the X chromosome. Also, I don’t think the increased fecundity of the female children would be enough to counterweight the reduced fecundity of the homosexual male children.
3. “The sneaky bisexual”

The homosexuality of the members of the tribe is well known, and the tribe leaders trust it so much that they let their wives be alone with the homosexual males. But then they were actually bisexual, and they proceed to fuck the wives.

My thoughts: No.
4. “Homosexuality creates greater in-group cohesion, and this is important.”

My thoughts: This would probably require that homosexual conduct would be accepted and not found repulsive by other members of the group. It would also require that group selection is an important factor.

I think (4) is most likely of these, but it seems more likely to me that the answer is something other than these.

2

The short answer is that, among any given population, genetic traits that do not interfere with overall species propagation will not be selected against.

There are many, many human traits that are just … there … without any underlying reason. Since those traits didn’t hurt overall species reproduction, such traits stay around.

3

Additionally:

It has always been the case that a significant percentage of homosexual men end up fathering children. I’m sure it’s no different for homosexual women (some percentage of them bearing babies). Sometimes in these kinds of discussions, there is an assumption that homosexual human pass on absolutely no genetic material directly to offspring – and that has never been the case.

4

Evolution occurs through populations, not individuals. Even in humans, we see traits that may be disadvantageous to individuals remain in certain populations because they increase the fitness of a population as a whole. The classic example is sickle cell trait: here, an inidividual who receives defective copies of the gene from both parents will develop sickle cell anemia early and (in pre-modern times) likely die before he or she gets a chance to reproduce. However, an individual who receives one defective and one normal copy of the gene from the parents (and is thus heterozygous) will not only remain asymptomtic from sickle cell disease, but will experience stronger resistance to the parasite that causes malaria. Because this mutated gene confers advantages to the survival of the population as a whole, it was selected for in populations living in areas where malaria was prevalent, even if it is harmful to certain individuals.

5

I don’t think it’s been shown that it even has to be genetic. It could all be due to hormones in the womb. If homosexuality is really that bad for individual reproduction it could simply be a defect in the animal body building process that isn’t amenable to evolutionary change because it’s too fundamental and baked into the way we’re built.

Alternatively, IIRC one popular theory is that if genes are responsible they may be useful in other contexts or increase reproductive success in straight animals. As long as this is true, some small homosexual population will exist as a consequence because they will be selected for in the straight population.

6

Much of human sexuality (regardless of partner genders) ignores reproduction. The majority of heterosexual sex happens when the female if not fertile unlike many other species. We’ve also got highly evolved social mechanisms and see disadvantges to not being touched or being isolated for long times. That seems to provide some support to 4. Human sexuality in general seems to support social/group success.

7

First, and most glaringly, you’re making a HUGE underlying assumption: that homosexuality is genetic. It’s not. Or at least not entirely. Studies of identical twins have repeatedly shown that while there appears to be a genetic component to sexuality, that is FAR from the only influence. Two people with identical genes can have different sexual orientations. Evolution can only act on heritable changes.

Second, you’re assuming that homosexuality reduces fitness. This may be true, particular in modern times, but I’d want to see hard evidence that this has been true for enough time to impact human evolution. Even today, there are plenty of homosexual people who have children for one reason or another. Back in the days when social pressures were higher to keep these urges secret, we would expect that even more homosexual people would have married and reproduced. Selective pressure needs to be applied for a long, long time in a species like humans, with high population numbers and long generation times, before it can have any effect.

Thirdly, you’re assuming that natural selection means that deleterious traits simply cannot exist in the population. That’s not true either. Natural selection takes time. LOTS of it, sometimes. During that time, the deleterious trait will be present in the population; perhaps even quite prevalent. Even if we were to accept that homosexuality is genetic and deleterious, it could take a very very long time for it to be reduced in the population. There are also quite a few mechanisms by which deleterious alleles can persist indefinitely even in the face of strong negative selection pressure - recessive alleles hiding out in heterozygotes is the most obvious example.

Finally, I’d just say that as a biologist, I think it is generally a big mistake to attempt to understand very complex human behavior and society using just this sort of very very simplistic Darwinian point of view. There are just too many other factors acting, and there’s a HUGE risk of slipping into an evolutionary “just-so” story way of explaining things, without bothering with any sort of evidence or research.

8

The only quibble I’d have with this one is that you see homosexual behavior in a lot of other species, including birds, where this description doesn’t necessarily match their behavior. That speaks to a much more fundamental cause, in my mind. If there’s a genetic component, it has to either be older than mammals or have multiple origins. If it’s a gestational anomaly, then it’s still working on some pretty much boilerplate element of physiology.

In regards to humans at least, I think we also have another interesting phenomena. While we like to assume that homosexuals are not reproducing, being homosexual has been punishable by death in a lot of places over history (and currently today). The effect? Homosexuals make sure to have a hetero marriage as cover, complete with children. Thus, the effort to kill off homosexuals actually has the unintended consequence of making sure they reproduce.

9

To add to Smeghead’s excellent post, even if there is a genetic component it is almost certainly multifactorial. That is, many different genes contribute to sexual orientation, some of which may be selectively beneficial in other combinations. It may be difficult or impossible to eliminate such genes even if there is significant selection in one particular combination.

An exclusive preference for homosexuality undoubtedly reduces reproductive success. Even if some homosexuals do have children they are certainly going to reproduce at a lower rate on average than heterosexuals. However, there are many other behavioral characteristics that probably reduce reproductive success, and we rarely ask if there is a genetic component maintaining them in the population. These may include simply being asexual, celibacy due to religious reasons, or a degree of shyness or social awkwardness that precludes ever getting a date. The percentage of the population going childless for these reasons may well exceed that of homosexuals.

10

Evolution works with what it’s got. (For what it matters, I know a pair of identical twins, one is gay and one is heterosexual and has 4 children…)

My theory - there’s a series of “recognizers” our brains are preprogrammed to recognize things - healthy, young, old, cute (as in babies and puppies are “cute” attractive), repulsive, etc. This is how evolution has shaped our brains and the way they work. In order to construct he complex mechanism of our brains, things have to connect “just right”. If during gestation the program (for lack of a better word) for what turns someone on accidentally develops or gets connected to the program designed for the other sex, then that’s a price to be paid for having a flexible system that allows a strong sex drive in the first place.

Using this hypothesis, then homosexuality or other diversions of sex drive are simply a side effect of the path evolution has chosen. To correct this would require backing up to a completely different method and creating a different method of connecting urges to sex drive.

So Mother Nature will tolerate a certain amount of “error” in order to get the result that works for reproduction, let’s say 90% of the time. You’re assuming there’s a gene that says straight/gay when likely it’s more like there’s a gene that says something similar to “make a toe, another toe, etc. up to 5 toes” and if it accidentally makes 4 or 6, well, that’s the result of the mechanism involved having occasional misconnects. The gene(s) may say something like “I’m detecting male development, find the ‘I like females’ program among many and hook it into this part of the brain”.

Also, based on human nature, female sexual urges probably didn’t figure much into their ultimate reproductive outcome, knowing what we do of typical male behaviour over the course of history. So there wouldn’t be any selection there anyway…

11

Another hypothesis: Mutations for homosexuality might be very common. Genetic regulation of behavior is always going to be incredibly complicated, so it’s not implausible that something might get mixed up somewhere along the line. So selective pressures might be constantly selecting against “gay genes”, but new ones just keep cropping up in their place.

Of course, the most likely hypothesis of all is that it’s all of these possibilities combined, to various degrees. Biology is almost never just a matter of a single cause and a single effect.

12

Why are you attracted to the opposite sex? Not just that you desire to procreate with some of them, but every part of attraction. I find it absolutely mindboggling that I’m specifically attracted to a (fairly large) subset of opposite-gendered humans, and I don’t really find it all that surprising that this obviously complex trait isn’t a perfect adaptation to promoting procreation.

I find the way I’m attracted to some opposite-gendered humans and not others, despite some of the former having obvious flaws “fitness”-wise and some of the latter being much “better”, much more befudling. And it appears to be a field where just-so-stories can be passed off as science.

13

I read that after a woman has given birth to a boy any subsequent boys have a higher probability of being homosexual – something about hormone suppression. So women who have just two boys are likely to have both heterosexual but a woman who has nine boys my end up with eight heterosexual and one homosexual. Even if we assume that the homosexual boys don’t reproduce, it would still be likely that eight hetero boys have a better chance of passing on the mother’s DNA than two hetero boys. There’s an evolutionary advantage to having more babies even if you increase the risk that some of the babies won’t reproduce.

14

We have little idea of what’s normal for humans. We can look at gorillas and we see a single alpha male with a harem and small balls. We can look at chimpanzees and see lots of rape. We can look at Bonobos chimpanzees and see an anything goes sexual free for all. Are humans a combination of these things, or something different altogether?

15

It might be added that there is not one single kind of homosexual preference either (despite the stereotypes of male homosexuals being effeminate and lesbians being butch). It’s not as if some wires got crossed and a male is attracted to other males in the just same way a female would be. Male homosexuals may behave in a feminine manner; in a hyper-masculine manner; or (probably the most common) in a way indistinguishable from the average heterosexual male. By the same token, they may be attracted to other males showing a range of behaviors from hypermasculine to feminine. The same goes for female homosexuals. This argues that there’s not one single cause for homosexuality, and certainly not one gene.

16

The OP missed another possibility:

Women related to homosexual males are more fecund (have more children):

17

There have also been studies showing it’s not uncommon for women to select one type of man to sire her children and another type of man to provide for her and her offspring.

A lot of what we think is normal for humans is a by-product of the emergence of farming and all the societal changes that came with it: the accumulation of wealth instead of sharing, ownership not only of women but men and everything else. A lot of that came in what, the last 30k years or so? That’s not long in evolutionary terms.

18

Modern San Fransisco, Paul Lynn-Ellen Degeneres homosexuality is different from the concept of plain sexual attraction to ones own gender. In the past people with those attractions just fell into life and societys norms and had families and children while just seeing their own gender on the side.

Its only in modern times where they can live openly and all.

19

Yes and no. To some extent, one’s sexual preference is a complex issue that (as some posters mentioned earlier) that is affected by cultural, as well as genetic, factors.

20

That study seems flawed for several reasons. 200 is a very small sample, and it’s a sample that’s limited to a Roman Catholic culture. That culture tends to have a more children due to religious practices. I’d be careful of putting too much faith in that finding.