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How does that leave room for a genetic basis for homosexuality?
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Because selection happens at the gene, not the individual.
For example, some studies have shown that sisters of homosexual men may be hyperfertile. This suggests that the "gene for homosexuality" has different effects depending on gender.
Examples similar to this idea are rampant. A more extreme example is saying "Why hasn't the gene that predisposes worker behavior in ants been selected out?"
In all actuality, we're most likely talking about a bunch of genes which makes blanket statements like "If queerness is genetic it would have been selected out by now." ridiculous since different combinations of alleles may have wildly different influences in fitness leading those pesky gay genes to not only avoid being selected out but actually selected for.
For example, lets take that most feminine of neuropeptides - oxytocin.
Let's say certain combinations of alleles make men with high oxytocin levels and this causes them to be gay. WHy hasn't that damn oxytocin gene been selected out?
Well it's responsible for bonding between mother and child, between woman and mate, lactation, labor, and recently implicated, along with its macho partner vasopressin, in general social behaviors. Pretty important, right?
Here's a thread.