SCIENCE / What is sex for?: The Great Unsolved Mysteries: 4 Male and Female

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For reproduction is the obvious answer - and the wrong one. An asexual human race would be twice as prolific and Nature did not invent sex just so we could have fun in bed. Recent research, Matt Ridley reports, indicates maleness is necessary for health.

MOST OF US lose little sleep over sex. It seems pretty obvious why we do it. It is fun, and the reason it is fun is because evolution designed it that way, by breeding only from those who found it fun and not from those who did not: sex is essential for reproduction.

Yet sex is one of the great mysteries of life: 'the queen of evolutionary problems', as one scientist calls it. The problem is not reproduction, but why reproduction need be sexual. There are plenty of asexual ways of reproducing: from dandelions and greenfly to elm trees and diatoms (tiny single-cell algae), all sorts of creatures reproduce by dividing themselves. Many, such as the greenfly, have sex only once every five or six generations; others get by without ever having sex at all. They simply form a bud or split their bodies in half, or lay eggs or set seed without waiting for sperm or pollen. No need to waste energy on males; let everybody be a fully productive member of society and bear babies.

'What use is sex?' John Maynard Smith, one of the greatest evolutionary theorists of our time, once asked. 'Since sex continues, it must have some short-term advantages.' The mystery is what these advantages are. An asexual human race would be twice as prolific as a sexual one, because it would take one, not two, to make a baby. Why do so many species, including ourselves, find it necessary to mix together the genes of two individuals when making a third?

The problem was raised in the 19th century by a German evolutionist named August Weismann, then 'solved' in the 1930s by two great geneticists, Ronald Fisher and Hermann Muller. Sex, they pointed out, was good for the species because it enabled individuals to share genetic inventions and so adapt smoothly to new circumstances (this was dubbed the 'Vicar of Bray' theory after the 16th-century cleric who alternated between Catholic and Protestant rites as the monarch changed).

However, the problem was declared unsolved again in the 1960s when Professor George Williams of the State University of New York and his colleague Professor Maynard Smith noticed a large flaw in the argument. They established that things that are good for the species never survive at the expense of things that are good for the individual. Asexual reproduction should therefore have prevailed among humans since it would allow an individual to swamp its rivals long before the rivals' ability to share genetic inventions among themselves could come to the rescue. Sex provides a long-term advantage but asexuality seemingly provides an overriding short-term one. 'There is a kind of crisis in evolutionary biology,' said George Williams. 'Nothing remotely approaching an advantage that could balance the cost of meiosis (sex) has been suggested. The impossibility of sex being an immediate reproductive adaptation would seem to be as firmly established a conclusion as can be found in current evolutionary thought.'

The short-term advantage of asexuality is more than just a theory. Computer simulations and field observations alike have proved it correct: in one example, sexual minnows soon died out in a pond because of competition from asexual ones (so long as the sexual minnows were closely related to each other - which is an important clue).

In the 1970s, theories to explain the evolution of sex proliferated like libidinous rabbits. There were 'lottery' theories (one exceptional offspring is worth a thousand mediocrities), 'tangled bank' theories (variety is the spice of life), 'genetic-repair' theories (without sex, genetic defects accumulate) and many more. A promising idea, championed by Alexey Kondrashov of Moscow University, was that sex serves to purge damaging genetic mutations from offspring.

None of these ideas proved satisfactory. Lottery theories and tangled banks suggested that sex would be commonest where in fact it is rarest: among fast-spreading, short-lived species in disturbed habitats or harsh conditions. Genetic-repair theories suggest that sex should be common at high altitudes where ultraviolet rays damage DNA. It is not. And the mutation-purging theory, while not disproven, seems to predict too slow and gradual an advantage for sex.

One theory, however, has proved to fit the facts better every time a new test is done. The Red Queen theory - named after the character in Alice Through the Looking Glass who lives in a world where however fast you run you never get anywhere - essentially argues that sex is vital in the fight against disease.

The argument goes as follows. A virus or a bacterium invades your body by plugging into a protein molecule on the surface of one of your cells: the fit must be exact, like a key in a lock. Different individual parasites may have different keys, and different individual people may have different locks. Sex is about changing the locks.

Imagine a time, not so long ago, when infectious disease was the main cause of death among human beings, when malaria, smallpox, plague and cholera were our enemies, not cancer and heart disease (essentially diseases of an old age our ancestors rarely reached). In every generation, some people were immune because they had the wrong locks for the prevailing strain of parasite.

But not for long. Whenever one particular kind of lock was common, parasites grew better at picking it because the strain of parasite with the appropriate key thrived at the expense of its rivals. So the advantage kept shifting to new locks - and new keys. Sex gave you access to a huge library of other genetic locks in other people to supply to your offspring. It was a race that was never won but never lost.

There is now all sorts of strong evidence that this idea is right. The molecular locks have been found, chiefly among a group of human genes that are famously diverse between individuals, called the major histocompatibility complex. Some versions of such genes are excellent predictors of whether people are susceptible to malaria. Natural experiments on Mexican minnows have shown how sexual strains lose out in competition with asexual strains except in the presence of fungal disease - so long as there is genetic diversity available to the sexual ones. And computer simulations by William Hamilton of Oxford University, the Red Queen's principal champion, have shed light on the extraordinary dance of genetic change that a parasite induces in a sexual but not an asexual host.

Few scientists are yet prepared to accept that the mystery of sex has been definitively solved by the Red Queen. But scientists are like that: they are no more likely to agree on closing a subject (and so foregoing future grants to investigate it) than crime writers are to kill off a favourite detective. The Red Queen is, for the moment, the solution to the mystery of sex. We are sexual beings because sex is a necessary weapon in the battle against parasites. Bill Hamilton believes he is at last in sight of a satisfactory explanation for sex, and 'Mother Nature's bizarre invention of maleness'. Males are needed to fight disease, he explains. 'I can now tell people that we males are necessary for health.'

In recent years, though, Red Queen-like theories have been popping up elsewhere. The idea of evolutionary treadmills is proving a useful one. Consider the peacock, adorned with the most absurdly picturesque plumage which he employs to seduce peahens. Why? What can it possibly matter to a peahen that her mate has a gorgeous tail? Yet the peacock is not alone. Throughout the animal kingdom elaborate colours, ornaments, songs and displays are used by one sex, usually the male, to persuade members of the other to mate with them. The more ornamented the male, the more selective and discriminating the females are.

And here lies the clue to why and how such ornaments evolve. If females are discriminating, then a female that does not discriminate but picks a plain-looking male will have plain sons, who will fail to attract other females. Once established, a sexual preference for ornaments is a treadmill that no female dare step off, so the female will get more and more

discriminating while the male gets more and more ornamented, in a sort of arms race between the sexes.

This notion has been around since the 1930s, when it occurred to the geneticist Ronald Fisher, but in recent years it has been subjected to all sorts of experimental tests. And disease has again reared its head. Perhaps competition between ornamented males to seduce choosy females results in success for the males with the most disease-resistant genes, a commodity the females eagerly seek. The ornaments themselves may be designed to reveal parasite infestation: a cockerel's comb is discoloured by many diseases. Again, the main champion of this theory is Bill Hamilton of Oxford.

Sexual selection may explain the increase in human brainpower over the last three million years. Geoffrey Miller, now at the University of Sussex, has argued that what he calls 'protean expressiveness' has been an important criterion of mate choice in human beings. His view is an example of a larger group of theories now in vogue among evolutionists to explain the sudden explosion of human intelligence. They reject the old view that large brains were useful to early human beings for tool-making, fire-using, scavenging or hunting, because such activities require only a sufficiency of intellect, not an ever-increasing quantity, and those arguments also applied to almost every other savannah primate.

What pressure could have caused humankind to need an accelerating brain size? What on earth is the use of being able to understand calculus, or learn the part of Hamlet? The answer must lie with the Red Queen: however big the brain gets, a bigger one is always better, because it is needed to understand, predict, outwit, deceive, seduce, manipulate and empathise with other people of similar brain size. This idea started with Nick Humphrey of Cambridge University. It suggests that the main thing our brains are designed to do is understand each other, a task that gets harder as brains get bigger.

So intelligence, like beauty, is relative, not absolute. It matters only to the extent that it is greater or lesser. There is no such thing as beautiful or intelligent; only more or less so.

'The Red Queen: Sex and the Evolution of Human Nature' by Matt Ridley has just been published by Viking Penguin, price pounds 17.99

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