Theory of sexual selection needs updating.

• Position: Darwinism

Darwin may have been wrong about sex--or at least too narrow minded. Charles Darwin's theories of natural selection are well established and generally accepted: "Survival of the fittest" leads to the evolution of a particular species over time, and species evolve from other species. However, a third theory has piggybacked upon the success of these other two: Darwin's theory of sexual selection, which explains the evolution of physical and behavioral traits that increase the odds that an animal will reproduce. These same traits do not necessarily help the animal survive, as do naturally selected ones. The male praying mantis, for example, will sacrifice himself for love--the female begins to eat him even as they copulate. He does not live long after finding his mate, but does pass on his genes.

Darwin postulated that females are "coy," mating rarely and choosing their partners carefully, presumably betting their odds on the males with the best genes to contribute to their offspring. For their part, males are "ardent" and promiscuous, and fight amongst themselves for female partners. Later theories added that males are so eager because they have less to lose by making babies; unlike eggs, sperm are plentiful and small. Moreover, females usually do most of the work to raise the offspring.

Sexual selection theory helped Darwin explain many traits, especially in males, that otherwise seemed maladaptive. The unwieldy tail on the mate peacock, for instance, makes him more vulnerable to predators but more attractive to females. Yet, many behaviors do not fit sexual selection theory. "I see females competing for males all the time. I see males ignoring females that are desperate to copulate with theme," notes Paul Vasey of the University of Lethbridge, Alberta, Canada, who studies Japanese macaques (rhesus monkeys). A great deal of empirical evidence exists that refutes Darwinian sexual selection. It is difficult to tell just how many exceptions there are to the rule because observations may have been skewed by Darwinian biases.

"The exceptions are so numerous they cry out for explanation," says biologist Joan Roughgarden of Stanford (Calif.) University, who has outlined a stunning array of behaviors that fail to fit the mold in Evolution's Rainbow: Diversity, Gender and Sexuality in Nature and People. Roughgarden thinks that a more comprehensive theory of sexuality should take into account social, as well as sexual, selection. Mating can function...