In this scenario, the male penis serves as a barrier against cross-copulations between members of two different species, an act that would either be a sterile waste of reproductive energy, or would result in the birth of genetically weak offspring. Thus, the males of a species must all have uniquely shaped keys to match the genital locks of their corresponding females. Charles Darwin himself believed that the evolution of reproductive organs was driven by such practical concerns as the maintenance of species purity. But now a researcher from Sweden has published results strongly suggesting that here, at least, the great man was wrong. In a detailed study of insect genitalia and its association with insect sexual practices, Goran Arnqvist of the University of Umea offers evidence supporting another theory for the divine absurdity of phallic diversity.
Sexual selection stands with natural selection as a mighty engine of evolutionary change.
But while natural selection favors practical traits that help an animal survive in its habitat, like a fur coat to weather winter or a long tongue to pick out ants from an anthill, sexual selection favors traits that, for whatever reason, lend a male an advantage in the harsh competition for access to a female's eggs. As Arnqvist demonstrates in his new report, the hand of sexual selection does not stop at molding the properties necessary for attraction and courtship. Because females of many species mate with multiple partners, he says, it is not enough that a male have the wherewithal to solicit a copulation. He must also possess features that keep his sperm in the race once mating has occurred. The thing that Darwin did not realize, but that has become apparent to us in recent years, is that sexual selection can operate in different way, not just on the level of mating, but on the level of fertilization as well.