[theapportioner]

What I would consider legitimate debate on evolution -- not the legitimate crap that creationists ballyhoo:

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Sex and the single monkey

Feb 20th 2003 | DENVER
From The Economist print edition

Darwin's theory of sexual selection is under attack. The first of four reports from this year's meeting of the American Association for the Advancement of Science

HOW the peacock got his tail is one of evolutionary biology's best-known fables. It was first told by none other than the master of evolutionary theory himself: Charles Darwin. It is, Darwin said, all down to the fickle, female peahen. Because she prefers to mate with males with the flashiest tails, large-tailed males have more offspring.

What is less well known is that this theory of sexual selection was created as something of a fudge. Darwin proposed it in response to criticism of his broader theory of evolution by natural selection. That explained how an animal (such as a giraffe) became adapted to its environment through natural variations between individuals. Different neck-lengths in giraffes may confer an advantage on
those with the longest necks.

The problem with this theory, said Darwin's critics, is that it does not account for animals such as the peacock, the stag beetle and the mandrill (illustrated above), whose males have elaborate and bizarre traits. Far from being adaptations to their environment, these showy traits seem likely to be a disadvantage.

Thus, to explain variations between the sexes, Darwin put forward the theory of sexual selection. Females, he said, are the choosy sex; males compete to win female attention. Evolutionary biologists later came to believe that this difference arose from the different size of the gametes—sperm and eggs. Because female gametes are larger than male gametes, females invest more as parents in producing them and so are pickier in their choice of a mate. Males, on the other hand, produce cheap sperm and are promiscuous.

Everybody's doing it
The problem, says Patricia Gowaty, of the University of Georgia, is that the real world does not work like this. At this year's meeting of the American Association for the Advancement of Science (AAAS), in Denver, she explained that her experiments with a number of species, in particular fruit-flies and mice, show that males can be just as picky about their sexual partners as females. Joan Roughgarden, of Stanford University, points out that female monkeys often solicit males and are rebuffed. Why should this be, she asks, if sperm are so cheap?

What worries the minority of scientists who are openly challenging sexual-selection theory is that it is unable to account for much of the diversity of sexual behaviour that exists. If, for example, as sexual-selection theory assumes, mating is primarily about sperm transfer, why do some animals mate between a hundred and a thousand times more often than is needed for conception alone? For animals with strong social structures, the answer could be that mating is a public symbol with social consequences. It is used to create and maintain relationships and alliances, and also to obtain sexual gratification.

Dr Roughgarden, however, believes that the basic mistake is to think that a difference in the size of sperm and eggs translates into differences in behaviour and life history. She says that there are many ways in which animals refute the predictions of sexual selection. For one thing, some fish can make both eggs and sperm during their lifetime, so it is difficult to say categorically that an individual is male or female. Even in such fish as the blue-gilled wrasse, whose biological sex is determined at hatching, gender roles (ie, the behaviours typically associated with one sex or the other) are not.

For, besides fish that behave like “normal” males and females, there are “feminised” males—fish that look like females but have male gametes. These reproduce by helping dominant males to mate with females. In doing so, they gain more reproduction opportunities themselves.

The society of blue-gilled wrasse is by no means the most complicated in sexual terms. Animal societies with up to three male genders and two female genders have been described. Even when only two genders exist, there are cases where male choice of females is the norm. For example, sea-horse males incubate the young in a special pouch and thus provide parental investment that is worth competing for if you are female.

Most troubling, perhaps, to the theory of sexual selection, is the high incidence of homosexuality. Homosexual behaviour in animals, though well demonstrated in the literature written by scientists who actually observe what animals get up to, has tended to be glossed over by theoreticians. But Paul Vasey, a researcher at the University of Lethbridge in Alberta, Canada, has spent many years studying both theory and practice in Japanese macaques.

Female macaques often form homosexual consortships. These are temporary but exclusive relationships that involve frequent sexual activity. Females in a consortship will mount each other tens or hundreds of times. In one group that Dr Vasey observed, females mounted each other as often as once every two minutes. Yet his observations suggest these consortships serve no adaptive function. He has spent many years testing hypotheses that might explain the behaviour, such as alliance forming, the relief of social tension and the communication of dominance. There is, he says, not a shred of evidence for any of them. Female mounting behaviour may have evolutionary roots, but he reckons the reason for it now is sexual gratification. That gratification is involved is known because when a female mounts another female she thrusts her pelvis against the mountee and masturbates her clitoris using her tail.

This activity, of course, excludes males. In one study, Dr Vasey found that when male monkeys courted a female involved in a homosexual consortship, 95% of such females rebuffed him and chose to remain with their girlfriend. This suggests, he says, that it is not simply males who are competing for sexual partners, as Darwin's theory predicts, but both males and females. And homosexual behaviour is documented in at least 15 other species, including Canada geese, gorillas, chimpanzees and humans.

Such examples may not be enough to topple sexual selection, and it is likely that this part of Darwin's theory does indeed hold good for many species. But as Dr Roughgarden warns supporters of that theory, although any one of these problems with it might be overlooked, the “sheer number of difficulties is hard to deny. If these are not enough to falsify sexual-selection theory, then what would be?” Sex, it seems, has come a long way since Darwin.