The male of the species proves an aid to evolution   Â

Female perplexity about the purpose of men may have been solved by an experiment that has shown how males are a vital ingredient of evolution.

Biologists have long puzzled over the function of males and why sex is so common in the animal world, given that it appears to be disadvantageous for females to practise it. If females went in for asexual reproduction, they would produce twice as many offspring who would all share the mother's genes.

In simple mathematical terms, sex makes no sense and the production of males – who frequently contribute little to rearing offspring – seems to be a waste of valuable resources.

The latest study, involving the ubiquitous fruit fly, offers a possible explanation for the role of the male in ridding harmful mutations from the gene pool and increasing the number of beneficial traits.

William Rice and Adam Chippindale, biologists from the University of California, Santa Barbara, found that the rate of "progressive evolution", where beneficial traits accumulated, was faster in populations of flies that reproduced sexually compared with those forced into asexual reproduction.

The disadvantages of sex applied to males as well as females because being a father carried huge costs, Dr Rice said. "My son only has half my genes; the other half are from his mother. Only half of my genome is getting into the population," he said.

"However, if I were an asexual female, my offspring would carry all of my genome. I would put twice as many genes into the next generation. With asexual reproduction you get two times as many offspring and two times as many genes into the population."

The classic explanation for sex is that it mixes genes between males and females and creates genetic diversity. There has, though, been little hard evidence to support such a generalised theory.

Now, Dr Rice has shown in 34 separate experiments with fruit flies just how the benefits of sex can result in a rise in beneficial mutations. They split up the fruit flies into two groups, one with mixed genes and one without.

They introduced a mutation for red eyes into each population, which is known to be beneficial compared with the gene for white eyes, and monitored its frequency over a period of 18 months involving many hundreds of generations.

In the groups that were allowed to reproduce sexually, the red-eye gene increased to a point where it was almost "fixed" permanently at 100 per cent, completely replacing the white eye alternative.

The asexual groups on the other hand were never able to rid themselves of the "evolutionary baggage" of the white eye mutation.

"We knew that there were advantages to sexual diversity, but it was not clear why," Dr Rice said. "Now we have evidence that the rate of progressive evolution is faster in sexual populations."