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.
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.
For our purposes, the implication is significant: Anything that diminishes, even slightly, the reproductive performance of any gene should (in evolutionary terms) be vigorously selected against. And homosexuality certainly seems like one of those things. Gay men, for example, have children at about 20 percent of the rate of heterosexual men. I haven’t seen reliable data for lesbians, but it seems likely that a similar pattern exists. And it seems more than likely that someone who is bisexual would have a lower reproductive output than someone whose romantic time and effort were devoted exclusively to the opposite sex.
Across cultures, the proportion of the population who are homosexual is roughly the same. What maintains the genetic propensity for the trait?
Nor can we solve the mystery by arguing that homosexuality is a “learned” behavior. That ship has sailed, and the consensus among scientists is that same-sex preference is rooted in our biology. Some of the evidence comes from the widespread distribution of homosexuality among animals in the wild. Moreover, witness its high and persistent cross-cultural existence in Homo sapiens.