The sine qua non for any trait to have evolved is for it to correlate positively with reproductive success, or, more precisely, with success in projecting genes relevant to that trait into the future. So, if homosexuality is in any sense a product of evolution—and it clearly is, for reasons to be explained—then genetic factors associated with same-sex preference must enjoy some sort of reproductive advantage. The problem should be obvious: If homosexuals reproduce less than heterosexuals—and they do—then why has natural selection not operated against it?
The paradox of homosexuality is especially pronounced for individuals whose homosexual preference is exclusive; that is, who have no inclination toward heterosexuality. But the mystery persists even for those who are bisexual, since it is mathematically provable that even a tiny difference in reproductive outcome can drive substantial evolutionary change.
J.B.S. Haldane, one of the giants of evolutionary theory, imagined two alternative genes, one initially found in 99.9 percent of a population and the other in just 0.1 percent. He then calculated that if the rare gene had merely a 1-percent advantage (it produced 101 descendants each generation to the abundant gene’s 100), in just 4,000 generations—a mere instant in evolutionary terms—the situation would be reversed, with the formerly rare gene occurring in 99.9 percent of the population’s genetic pool. Such is the power of compound interest, acting via natural selection.