Theory of sexual selection needs updating

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 to build and manage relationships as well as to procreate. "Female choice, I'm sure, has much more to do with managing male power than it does with trying to obtain good genes," she contends.

Other sexual traits, indicates Roughgarden, may represent a "market economy" dedicated to trading sexual opportunity for ether resources. In many species, some individuals act as helpers to dominant males and reap some rewards in the process. Male waterbucks, for example, establish a territory along a lakeshore and wait for a female to enter. Subordinate "satellite" waterbucks help to defend the territory, and, in turn, may mate with a few females and get a shot at inheriting the territory when the dominant male retires.

Homosexual behavior, too, is common, but unexplained, by Darwin. Over 300 vertebrates, including monkeys, flamingos, and mate sheep, practice such behavior. Homosexuality in some species appears to play a social role. For instance, bonobos (pygmy chimpanzees) will have sex with same-sex partners to calm tensions after a squabble; or to make sure that a large amount of food is shared. For some species, though, humans included, homosexual behavior may have no adaptive value at all. "Looking for any single conceptual framework to explain homosexual behavior is an unattainable goal," maintains Vasey.

While two types of sex cells exist--sperm and egg--it is more difficult to sort individuals into these binary classes. Several species have more than just male and female genders, where gender is defined as the body and sexual behavior of an individual. In some species, an individual's body may be difficult to classify as male or female. Most plants and some fish are hermaphrodites--capable of producing eggs and sperm. Various lizards are unisexual. There are no male whiptail lizards, and females will mount each other, prompting hormonal changes that result in cell division--a true "virgin birth."

A single individual also may switch from male to female, or vice versa, and back again--that is, may switch from producing sperm to producing eggs--due to a change in hormones triggered by external circumstances. In any coral reef, for example, about 25% of the fish have changed sex in their lifetime. Over 50 species of angelfish, wrasse, parrot fish, and groupers have gone from male to female or vice versa. Other invertebrates, such as shrimp and oysters, also may do the same.