The recent article in the Quarterly Review of Biology has some good points. It does not argue that homosexuality is adaptive, which would be silly. It does not argue for Lamarckian epigenetics, which would also be silly. I should also point out that they’ve given up on sexually antagonistic selection, since the GWAS surveys pretty much rule it out.
They note that variations in hormonal levels before birth don’t seem to cause much trouble, and conclude that humans adjust to those levels – homeostasis, basically. They suggest that this is implemented by epigenetic changes, and that sometimes those epigenetic changes are not properly reset in the next generation. Leaky epigenetics.
Of course, this hardly ever results in funny-looking genitalia – you’d think it would, but then natural selection has made that kind of error rare. Of course, it should have made epigenetic leakage that cause low-fitness behaviors like homosexuality equally rare. Which is the problem with this hypothesis: it rests on the assumption that natural selection in humans has been on a mysterious, all-expenses-paid vacation when it comes to mating. They suggest that maybe sexual behavior have changed since the human-chimp split and therefore there just hasn’t been enough time to thoroughly canalize sexual development in humans.
I guarantee that speech is newer than that. What fraction of people are unable to talk? Gee, being mute is far less frequent than homosexuality, even though speech sure seems more complicated. Why is speechlessness so rare? Speech is damn useful, and natural selection has made failure rare. But it’s not more useful than reproduction its own self.
Rice suggested sexually antagonistic genes, earlier. What fraction of known, common, fitness-reducing syndromes are known to be caused by that? Gee, none of them. What fraction are known to caused by leaky epigenetics? None.
What fraction are known to be caused by infectious organisms? Practically all of them. So you know it can’t be that, right? Life is really a vast murder mystery – it’s always the one you least suspect.
The Emmdees say that when you hear hoofbeats, think horses, not zebras. When explaining homosexuality, people think of pterodactyls and unicorns.