Critics claim that evolutionary biology is, at best, guesswork. The reality is otherwise. Evolutionists have nailed down how an enormous number of previously unexplained phenomena—in anatomy, physiology, embryology, behavior—have evolved. There are still mysteries, however, and one of the most prominent is the origins of homosexuality. The mystery is simple enough. Its solution, however, has thus far eluded our best scientific minds. First the mystery. 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.