Homo sapiens have menopause - as some other mammals (e.g. some Cetacea) - a phenomenon where older females live for decades after the end of their own reproduction.
Perhaps homosexuality, that looks to hold at 10% levels in any population, may be just the same sort of adaptation?
Barnard's wonderful text "Animal Behavior: Mechanism, Development, Function and Evolution" offers some enlightening summary to the discussion of homosexuality in a biological context!
Brief Summary to ch 12, page 606:
What we know so far:
- homosexual behavior is recorded all throughout animal kingdom
- there are evidence for genetic basis to homosexuality in various species including humans
Three interrelated schools of thoughts:
Homosexuality is maladaptive - external stress creates homosexuality
reduce resource stress on family unit, youngest child tend to be gay (E.M. Miller 2000)
*** mitochondrial DNA carries male killing genes, with male being a dead-end (Hurst 1991)
Homosexuality as an adaptive early phase of development - homosexual acts are just target practice
- homosexuality is just males practicing sex on each other, most homosexual acts are performed by bisexuals with just as high fertility as straight males (Baker & Bellis 1995)
Homosexuality advantages in other aspects of reproduction - homosexuality may imply better parenting
- homosexual genes induces feminine attributes in males such as sensitivity, empathy and kindness hereby increasing likelihood of long term pair bonding and paternal care (E.M. Miller 2000)
*** original text:
A completely different suggestion is that homosexuality may reflect the interests of selfish genetic elements (see 2.4.3.2) inherited maternally on mitochondrial DNA for which males are effectively a dead end. Some insects have been shown to carry 'male-killing' genes on their mitochondrial DNA, so perhaps homosexuality is a similar, if more benign, rout to sidelining males (Hurst 1991)
There is a study that found that female maternal relatives of homosexuals have higher fecundity than those of heterosexuals [1]. This suggests that an allele predisposing to homosexuality may propagate because it increases reproductive success in some situations, not necessarily in the homosexual individual.
However, many factors can influence a persons sexuality, such as birth order (in males each additional older brother increases the odds of homosexuality by approximately 33%) [2], so genetics only tells part of the story, and I found no comparable explanation for female homosexuality.
Here's an interesting article if you want further reading [3].
Genetic view of adaptation:
Do note that this is looking at the view from the POV of a gene. Please read the comments below this answer, for a small discussion on the kin selection hypothesis. Do note that I do not study evolutionary psychology, or work on the level of the population.
This is a bit of a misconception, as nature really doesn't work like that. Nature works by allowing people to pass on genes, and then selecting those genes which are the best. Now the issue with homosexuality when viewing from an evolutionary angle is that, homosexuality by definition doesn't allow for genes to be passed on. I would also quote Richard Dawkins in this issue
“That’s a very tempting error,” Dawkins replied, “not with respect to homosexuality, but with respect to the idea that nature does population control. That’s not the way nature works.”
“It’s so tempting to think that things will be best for the species, so nature will do whatever it takes to keep the species going. But that’s just not the way evolution works.”
“Natural selection,” he continued, “works at the level of the individual gene. So in order for natural selection to favor population control, [it] would have to favor a gene or genes within an individual that limited that individual’s reproduction.”
“Homosexuality would do that, but of course, it wouldn’t actually be naturally selected because the individual [who] doesn’t have children doesn’t pass on the genes for not having children.”
“In order for natural selection to favor population control,” Dawkins said, “it would have to be the case that an individual who has too many children ends up rearing fewer” successfully.
He noted that this is exactly what ornithologist David Lack found when he studied avian clutch size — those birds that laid too many eggs were not able to care for all their hatchlings, whereas as those who laid too few were not selected for the obvious reason — they didn’t produce enough progeny.
hetero sibling of a hetero,hetero sibling of homo,homo sibling of homoandhomo sibling of hetero. $\endgroup$