Not neccessarily. Humans are social creatures, our genes' survival is not solely dependent on our individual ability to pass them on, since we share genes with our siblings and parents as well as offspring. For example, families with some non-reproducing offspring may fair better due to having a higher adults/children ratio.

If such a gene existed (which perhaps it doesn't), it might benefit families who had members carrying it.

This is the same reason some species have evolved alarm-calling when a predator is nearby. It benefits the collective (who share the gene), but at an obvious detriment to the individual's ability to pass on their genes in this case.

In addition, one suggestion is that a possible “gay gene” would be something that causes extremely strong attraction to men in both men and women with that gene. Women with this gene would then be more likely to father children, which could counter the evolutionary pressure resulting from the male offspring with that gene not having any offspring.

(I’m massively oversimplfing of course, my bio knowledge is fairly limited).

Uh, I know we’re all about redefining gender roles these days, but I’m not yet ready for women to father children...

In my EvoBio class a decade ago, my professor told us the theory that homosexuality could be adaptive based on birth order because it would reduce fraternal rivalry: the family would have an additional adult to care for the young without being a reproductive competitor.

This doesn't make much sense, though. Every additional human is a very expensive investment of time, resources, and effort on the family, and offers no material return on investment for at least a decade in terms of productivity. True, while the gay son might not produce offspring to compete with his brother's kids, it also means he brings no additional mate and offspring to help out the family.

A common analogy here is that in lots of insects, you see whole groups of non-reproductive members, and yet the idea of a "non-reproductive worker insect" hasn't "evolved into extinction."

(I'm not commenting on whether being gay has a genetic basis or whether it's an example of kin selection, because honestly, I have no idea.)

The point is that non-reproductive insects don't have specific set of genes that make them non-reproductive - it is not an inheritable trait. If it was, then it would have evolved into extinction.

Of course they do (have genes that make them non-reproductive) and of course it hasn't (evolved out of extinction).

There is nothing about a possible genetic basis (whether complex and indirect or simple and direct) for an individuals non-reproductiveness that implies it has to result in extiguishing itself.

Humans are social enough that genetics which benefit the group at the expense of the individual are perfectly selectable.

Heck it even works for utterly antisocial species.

All that's required for a trait to persist is for it to be good for any members by any means. It doesn't have to be good for the individual carrier, and it doesn't have to be good in a way that we happen to understand.

A more formal description:

> If a gene copy confers a benefit B on another vehicle at cost C to its own vehicle, its costly action is strategically beneficial if p_B > C, where p is the probability that a copy of the gene is present in the vehicle that benefits. Actions with substantial costs therefore require significant values of p. Two kinds of factors ensure high values of p: relatedness (kinship) and recognition (green beards).

The Selfish Gene is probably the most famous way of looking at that concept. https://en.wikipedia.org/wiki/Gene-centered_view_of_evolutio...

non-reproductive workers benefit the reproductive insects. For instance, worker bees benefit the queen. The queens that lay the eggs in the most beneficial ratios are most likely to survive and pass on.

For this analogy to be equivalent, it should lead to having gay children benefit the adults enough to allow for more successful breeding.

I do am not commenting on one's gayness has a genetic basis, because I also have not idea.

The trouble with that argument is that it also works with,

"If there was a genetic cause for shortsightedness, why is it still around?"

"If there was such a thing as a genetic disease, why haven't they evolved out?"

"If not having X-ray vision was genetic, wouldn't the absence of X-ray vision have been selected out?"

You really can't expect evolution to accomplish its "goals," per se. It is sort of a gentle flow down a lazy river, towards adaptation.

Not really. Say I'm shortsighted, and still a potential mating partner due to a variety of other traits (being a billionaire, or looking suspiciously similar to Ryan Gosling). In that case, I would be able to transmit my genes to my eventual offspring.

The case with gay genes is different. Regardless of any other trait, I'm genetically prone to not have any offspring. Thus, eventually my genes would be less and less common.

Incorrect.

All that's required for a trait to persist is that it benefits the pool.

If the pool benefits from have 1% of members being blind, then so they shall be.

We don't have to understand the benefit for it to exist either.

You're presenting group selection, natural selection at the level of the group, as an established theory. But that's not the case.

From Wikipedia, quoting evolutionary biologist Jerry Coyne: "Group selection isn't widely accepted by evolutionists for several reasons."

https://en.m.wikipedia.org/wiki/Group_selection

>All that's required for a trait to persist is that it benefits the pool.

Or that it's less bad than whatever beneficial trait it's coupled to.

Humans consume a lot of water in order to shed heat. Our backs are pretty f-d up. Our brains waste a lot of energy and require a very long childhood. But on the flip side we're basically terminator compared to the African fauna our primitive ancestors hunted.

I'm assuming you're right since I don't know much about this subject, but how does that mechanically work? How does a trait, even if it is beneficial to the community, get passed down if the members that have that trait don't have offspring?

It could be recessive, so it only gets expressed if an individual has two copies of the recessive gene.

"If there was a genetic cause for shortsightedness, why is it still around?" - because myopia doesn't significantly affect your chances to produce offsprings.

It seems reasonable that being gay does significantly reduce your ability to have children; therefore it is reasonable to expect that if it was an inheritable trait it would have been selected against.

Everyone keeps saying that, but it's simply not true.

We have understood how this is not true for decades or even hundreds of years, and it's not even some impossible concept to grasp.

All that's required for a trait to persist, is for it to benefit the pool, not the individual.

The only way the seemingly obvious but incorrect idea you descibed applies is, a trait which is good for the group at the expense of the individual, if it's 100% effective like sterility unlike say alarm-calling which merely carries a risk for the individual, is that such a trait can never grow to where all members exhibit it.

But it can absolutely be strongly selected for maintaining whatever the optimal percentage is. IE, if it benefits the pool for 10% of members to be sterile, then the percentage of sterile members will not decrease through the mechanism you and so many imagine, but will stay at 10% for as long as the benefit exists.

And everybody keeps saying THAT. The problem is, that's a secondary effect. A secondary effect is going to have to be pretty strong to counter an obviously anti-competitive primary effect. You can construct scenarios, but I haven't seen any that seem particularly compelling. It seems to me that the default position of "it's not genetic" is much better supported by the data.

> All that's required for a trait to persist, is for it to benefit the pool, not the individual.

This is a well debunked belief. Genes are selfish, and they only reproduce to the benefit of the gene.

Group selection does not occur.

An example is evolutionary stable strategies, where cheaters persist in the population, even though the pool would be better off if all individuals in a pool were selfishly altruistic towards pool members.

A much better example of counter-intuitive evolution, one that's literally textbook, is sickle-cell anaemia. It's a genetic disease (bad) that offers protection against malaria (good), so it's more common in areas where malaria is a fatal problem for significant numbers of people. This illustrates the principle that genes do more than one thing, and can be simultaneously adaptive and maladaptive at the same time. As a hypothetical example, maybe the same genes that make a man fractionally more likely to be gay could protect his sister from ovarian cancer or stop him going bald.

A lot of the musings I've seen in this direction have been along the lines of

(1) some people are bisexual and thus would occasionally reproduce and

(2) if sexuality promotes "togetherness" and "group cohesion", then having "our gay uncle who 'has connections' and is always willing to help out the family group" is still beneficial for the family unit and thus a gay gene would not be selected against within that family lineage.

Probably not a majority of the population, but there's no reason for such a "gay gene" (if it were to exist) to be driven to zero.

Not if it is recessive. Example with a recessive "gay gene": Both parent have one normal allele and one "gay" allele. The repartition of the offspring will be:

- 25% both normal alleles (normal phenotype) - 50% one normal allele (normal phenotype) - 25% both gay alleles (gay phenotype)

But homosexuality is not a gene. I believe that the [fetal androgen exposure theory](https://en.wikipedia.org/wiki/Prenatal_hormones_and_sexual_o...) is the one currently preferred.

There are several possibilities.

1. If having gay relatives improved the chances that you will have children and they would survive to themselves reproduce, then evolution could favor maintaining a gay gene.

Remember that for nearly all of our species' existence, and that of the pre-humans we evolved from, we've lived most of our lives in arrangements where our close relatives lived near us and most people in the region were also related to us.

Your children have half your genes. Your siblings' children have a quarter of your genes. So in effect two of your siblings children are equivalent to one of your own as far as getting your genetic material into the next generation goes.

If being gay meant that your sibling does not have their own kids to take care of and so they devote effort that would have gone to raising their own kids to helping their straight siblings' with their kids or to doing things that help the village that those with kids don't have the time for, that might greatly increase the survival rate of those kids enough to make up for not having kids of their own.

2. Being gay doesn't mean you can't have kids. Throughout most of our history, many many people have had kids with people they have not been attracted to. That goes for both gay people and straight people.

3. What if it were a bisexual gene rather than a gay gene? Having people in your tribe that can form with both men and women the kind of close bonds the people form for the sexual partners has some obvious advantages.

There is also the 'beneficial sibling' theory where a homosexual sibling improves the reproductive success of their siblings/close family via being able to devote more energy to their siblings offspring instead of their own or are able to gather/utilize more resources that leads to a familial advantage.

Not necessarily.

* the same gene can have different phenotypes in different individuals. One phenotype may be net negative but the benefit from the other makes it net positive.

* in social animal gene can be carried by relatives. Extreme example is ants and bees, most of them are not fertile but they help to spread their genes by helping those who are.

More than just that, as far as I can see, there is no reason this only applies to social species.

If it benefits the pool that x% of members have a trait, even full sterility, then the genetics will result in x% having that trait, as long as the benefit continues to exist.

I'm not sure what an example mechanism might be for some mountain cats or spiders or whatever that the species benefits from 4% of their members being sterile, but I see no reason that there couldn't be one, and it doesn't require the unfortunate exhibitors of the trait to preserve and occasionally produce the next exhibitor of the trait.

Let's assume the benefit is improved female fertility. If it's an autosomal trait, half of the carriers will be male, and lead to the vast majority of them refusing to have offspring. That would require the sisters to have a pretty spectacular bump in the number of offspring to offset their gay brothers. Indeed, since the trait is clearly polygenic, all the sisters do not inherit the trait equally, if at all. The sisters who do inherit it would have to be spectacularly fecund to compensate.

It might persevere through kin selection. For siblings who share the gene, such an individual would have all the benefits of a male/female (security, hunting, care-giving), but none of the mating competition. One could argue if such a gene existed, it would be quite an altruistic one.

Not necessarily. If the gene is recessive, it will be passed on. The question then is: what happens when that gene expresses itself?

The naive reaction is: individuals with the "gay gene" won't reproduce and it will eventually die out. A more nuanced perspective is that while that individual won't reproduce, their presence in the community helps the clan's survival, so a community with the gene is overall stronger, so the gene will stay around.

I would think that being "gay" doesn't 100% prevent someone from fathering / mothering children.

This. In the past, there was a lot of prominent personalities who were suspected of being gay, but left progeny.

Social pressure to have children, especially if there is a title / wealth / prestige to inherit, would certainly play a role.

Not only social pressure. It’s a fact that some homosexual people do want to procreate.

Depending on other conditions (environmental and genetic), a particular gene may not be expressed in the same manner (or at all). You can find simple examples in recessive alleles. The phenotype of such genes will only be expressed when both alleles are of a compatible type.

Another striking example: Sickle cell disease will only be deadly if both alleles carry the unfortunate mutation. But the unhealthy allele is unlikely to be eliminated by evolutionary pressure because carrying one allele provides significant protection against malaria.

The interactions between genes and their environment is usually more complicated that these simple examples, but I hope it illustrates that evolutionary pressure may not suffice to erase some apparently unsustainable alleles from the population.

The striking example that you gave is more striking than that. Having one allele actually brings protection from malaria. So the recessive is selected for..as long as not too many people have it.

That is why sickle-cell anemia mostly shows up in people whose ancestors came from places with a long history of malaria.

The other posts cover possible reasons why being gay may be actually good for the utility function of a gene.

There is another hypothesis for why this hypothetical gene might not evolve into extinction. Being gay may just be a side effect of it, with some other benefits being the reason this gene stays in our gene pool.

The argument around this is that it's of social benefit rather than individual. Particularly when child/infant mortality is high and women have lots of children, it could be helpful to have a pool of adults supporting the child-rearing of others, especially their immediate siblings.

Well yes and no. It might actually give a benefit to other siblings. It could manifest itself only in certain circumstances in the womb or in the environment if something helps the phenotype manifest itself...

For example there is a correlation with sibling order and homosexuality. The more boys your mother had before you the more chance you have of being homosexual. We're talking about a non-negligible increase of over 30%: https://en.wikipedia.org/wiki/Fraternal_birth_order_and_male...

Interestingly, I've heard about many people who went from "straight", married with children to leaving their families because they've found out they're gay. I even knew one whose first sexual experiences were with women, and he decided he was gay later. In his "straight phase" he could have had fathered children if he weren't careful. Curiously, I've never heard about opposite cases (spontaneous [1] gay -> straight transition). Make of it what you want.

[1] Spontaneous = ignoring attempts to "cure" gay orientation.

What if the "gay gene" is tied to the "sexually attracted to our parents gene" ?

And evolving out the former would cause the later to go which is detrimental to the overall health of the species.

Not necessarily, as long as it's recessive.

Not an expert either here though.

Yes, it would; being recessive just makes the process take longer.

I've pointed out on other HN threads that homosexuality has much lower concordance in identical twins (around 40%) than almost any other trait, making it an especially unlikely candidate for direct genetic causes.

If recessive, you'd expect the wild type and 'gay' alleles to be in Hardy Weinberg Equilibrium. However, this obviously isn't the case.

If homosexuality provided a species or community benefit, it could be selected for in that way.

Perhaps akin to aging phenomena that benefit the species through mortality.

Also not if for most of humanity the mates with this hypothetical gene were repressed into heterosexual relationships.

Common mutations are inescapable.

I view this question - and the deliberation over it - as coming from present-day society's latent homophobia.

Doesn't the question apply to straight people, and anyone else, who don't reproduce?

If you take a simplistic view of genetics where you draw Punnet squares and check if the offspring has GG alleles, maybe. Even if there are social benefits to there being some gay people around, that would still make those alleles pretty unlikely to be passed on. That doesn't imply there isn't a genetic component. Real genetics is a lot more complicated than that.

The fact that something is genetic doesn't imply there is no social component. Genetic factors could lead to a predisposition for a certain behavior that would still be altered by environment. There could be many genes that each has some small effect but none of which is an on/off switch. Those would lead to more complicated selection pressures.

We are pretty sure there is a genetic component. We are pretty sure that it isn't just "if you got the gay allele then you's gay".

Also, some gay (or mostly gay) people have children. This is because a) most people are at least a little bit bisexual, b) people may want to have children even if the sex act isn't appealing, and c) there are social pressures to conform.