There are a bunch of plausible theories that ethologists have come up with for evolution of homosexuality. This is one of them - and it does show up in other experiments, e.g. with rats: if you make their population denser, a higher proportion exhibits same-sex behavior.
But it's not the only one. Another is that species that employ K strategy (low reproduction rates, high investment into offspring to ensure its survival) and that are social, gradually develop some kind of social safety net mechanisms for offspring, in cases where e.g. the parent dies or is sick and unable to provide proper care. One way to do that is to have some proportion of the population that does not breed on their own, thus ensuring that they are always available to rear others' offspring. Since evolution generally takes the path of least resistance, it's easier to achieve this by redirecting sexual drive than by switching it off entirely. And this can even make sense from a "selfish gene" perspective of one individual, because if you help enough of your close relatives (who share many genes with you) keep their kids alive, it may actually be more effective than betting on a few children of your own surviving to carry more shared genes.
(If this theory is right, that has interesting implications for all those laws restricting adoption by same-sex parents...)
The K strategy theory looks reasonable, but I don't see how evolution could develop a population-control mechanism like the parent comment postulated. If you start with an overly-dense population that's half straight and half gay, the straight ones are still at an evolutionary advantage relative to the gay ones. It doesn't matter that they'll all die when they reach the reproduction rates of a 100% straight population, that's still where the evolutionary forces will take them.
In case of population density, it might be epigenetic.
But also, don't forget that evolution does not "care" about organisms, only about genes; and a gene can be carried across generations without manifesting itself. Suppose it is beneficial to carry the "gay gene", because it means that more of your descendants don't die, even if some of them won't procreate - producing a net gain in the propagation of your genes on the whole, ironically. That would trigger selection, once such a gene appears.
>Suppose it is beneficial to carry the "gay gene", because it means that more of your descendants don't die, even if some of them won't procreate
That doesn't work because even though less of your gay descendants will starve, the straight organisms in the population will also reap the benefits and they'll still be at an evolutionary advantage.
I didn't mean to imply that all descendants would be gay, only that all would carry the gene and pass it to their descendants in turn. Most would still breed.
And in your original example, the straight organisms still die in the end. So over the lifetime of multiple colonies, the ones that survive would carry more "cooperative" genes, no? I mean, isn't this basically how biological altruism develops in general?
>I didn't mean to imply that all descendants would be gay, only that all would carry the gene and pass it to their descendants in turn. Most would still breed.
It doesn't matter. It's a gene that, in the long term, slightly increases the chance of every other individual reproducing while immediately greatly decreasing its own chance. That just doesn't work.
>over the lifetime of multiple colonies, the ones that survive would carry more "cooperative" genes
Evolution already works extremely slowly when its forces are applied to individual organisms. Any theory that requires evolution to act on entire populations is extremely unlikely. Also, you'd need a way for the gay gene to spread among a colony population in the first place in order for that colony to be selected in the way you postulate.
Another problem is that if ever a cooperative (in this particular sense of abstaining from reproduction) population meets an uncooperative one, eventually the uncooperative genes are going to take over again. It would need to speciate before that happens.
>I mean, isn't this basically how biological altruism develops in general?
I'm not really familiar with the concept, but it's easy to predict some things: Biological altruism will generally mean an organism helping another at a small or no cost and/or the benefited organism is a close relative that likely shares the gene. The gay gene fits neither criteria. The examples on Wikipedia do: https://en.wikipedia.org/wiki/Altruism_%28biology%29#Example...
Lastly, nature has other ways to solve overpopulation. Organisms can evolve to require less food, change their diets to something else, become aggressive towards other individuals of the same species (killing the competition), or best of all: combining those last two things by resorting to cannibalism (this was the actual result of an experiment that applied evolutionary pressure over populations of insects).
> It's a gene that, in the long term, slightly increases the chance of every other individual reproducing while immediately greatly decreasing its own chance. That just doesn't work.
Don't forget that all those other individuals in vicinity carry some of the same genes - the closer, the more genetically related they are, in general. You'd need to compute how much that small but broad effect can add up to the total percentage of genes propagated indirectly.
I get that it doesn't sound very "common sense", but I don't think it can be rejected outright without doing the numbers and modeling first. Which is exactly what people in the field do, and if they say that it's plausible (i.e. that they can come up with models that are consistent with observations, and that demonstrate stable propagation), then it can't be dismissed out of hand. Look up "group selection" for the kind of models that they use.
Also, that Wikipedia article gives some far more extreme examples of biological altruism, like self-sacrificing ants - noting that this is still evolutionary viable because of how much shared genetic material is there in the rest of the colony (indeed, one could say that the very social structure of ant colonies with non-reproducing workers is an extreme example of biological altruism taken to 11 by evolution). And then we have our own social mores, which have included literal self-sacrifice for the sake of one's "tribe" as a virtue for pretty much as long as we can trace human culture back - and it's not on its way out, either. Clearly, we aren't ants, but in this case, is it really a qualitative difference, or simply the result of the same process applied to different starting conditions and different evolutionary pressures, producing different points on the same scale?
Side note: when I was digging into modern evolutionary theory, one surprising (to me) thing that came up is that there have been substantial revisions in the estimated speed of evolution as a process since Darwin's days. Sexual selection in particular is a mechanism that turned out to work much faster than originally anticipated, and can in fact push other traits that wouldn't be viable otherwise over the viability threshold. One theory is that it developed in the first place because faster selection - i.e. adaptability - is itself a selected-for trait, so it's kinda getting very meta. And there's a fringe but intriguing argument that our human culture (and its building blocks, like language) is, essentially, a further development along these lines to reduce the lag even further - memes can change faster than genes, so population groups that rely less on genetically hardcoded behavior and more on transmitted social mores, are more adaptable. But of course the genes are still there, so those transmitted behaviors that can be implemented on top of hardwired stuff are going to be stronger, even if the original effect was very subtle - e.g. modern concepts of fairness and justice (presumably evolved from hardwired behaviors like parochial altruism).
In the case of ants, the beneficiary of the altruism/self-sacrifice is the queen, which always carries the gene. That's why I said:
>Biological altruism will generally mean an organism helping another at a small or no cost and/or the benefited organism is a close relative that likely shares the gene.
Ants and bees are extreme examples of the later.
In your theory, the only beneficiaries of fewer individuals reproducing are the straight (and bi) individuals who are still reproducing. It's a really bad theory to explain homosexuality and I don't know why anyone would insist on it when it's so easy to come up with better ones. Just think of something that would benefit the individuals carrying the genes themselves. Maybe an attraction to muscles (or some other manly characteristic in that species) that leads to stronger offspring when the individual mates with muscular females, but can go too far sometimes leading to homosexuality.
>one surprising (to me) thing that came up is that there have been substantial revisions in the estimated speed of evolution as a process since Darwin's days. Sexual selection in particular is a mechanism that turned out to work much faster than originally anticipated, and can in fact push other traits that wouldn't be viable otherwise over the viability threshold. One theory is that it developed in the first place because faster selection - i.e. adaptability - is itself a selected-for trait, so it's kinda getting very meta.
That's actually really interesting! Got any good links on that? The speed of evolution is something that sometimes bothers me when looking at highly complex adaptations.
Most of my reading about this was in Russian, and I don't remember the exact sources that were cited there. This particular stuff was mentioned kinda in passing in a broader conversation on evolution of human behavior, as an explanation of how it could get so complicated so fast. But searching around, it looks like there are some very specific papers on it - e.g. this looks interesting:
"This paper presents theoretical arguments and simulation results in support of our view that sexual selection creates new fi tness peaks (and thus new niches), helps species escape from
old local optima to find new, better peaks, and promotes speciation to increase the number of lineages searching for peaks."
But it's not the only one. Another is that species that employ K strategy (low reproduction rates, high investment into offspring to ensure its survival) and that are social, gradually develop some kind of social safety net mechanisms for offspring, in cases where e.g. the parent dies or is sick and unable to provide proper care. One way to do that is to have some proportion of the population that does not breed on their own, thus ensuring that they are always available to rear others' offspring. Since evolution generally takes the path of least resistance, it's easier to achieve this by redirecting sexual drive than by switching it off entirely. And this can even make sense from a "selfish gene" perspective of one individual, because if you help enough of your close relatives (who share many genes with you) keep their kids alive, it may actually be more effective than betting on a few children of your own surviving to carry more shared genes.
(If this theory is right, that has interesting implications for all those laws restricting adoption by same-sex parents...)