It's official: sex is good for you. Well, at least it's good for yeast.

From evolution's standpoint, sexual reproduction seems to have many disadvantages over the asexual variety.

It wastes time and energy, it mixes up perfectly good genes, and females, who do nearly all of the reproduction work, get to pass on only half their genetic material. So scientists have wondered, why bother with sex?

But on the plus side, August Weismann, a 19th century theorist, proposed that sexual reproduction helps speed up natural selection by allowing good genes to spread more quickly through a population, and bad genes to disappear faster. That makes sex good for the species, and hence worth the effort.

Although generally accepted, Weismann's theory has been difficult to prove in the lab. But a team of British scientists has at last shown just what is so good about sex. In yeast, at least.

Yeast comprises tiny organisms that reproduce both ways -- sexually and asexually.

So Matthew Goddard and a team of scientists from Imperial College London made two different versions of otherwise identical yeast, one that could reproduce both ways and one that could only perform the asexual kind of reproduction.

They found that under normal conditions, both sorts of yeast fared just as well. But under extreme conditions, the sexy yeast did better.

"Our results indicate that sexual reproduction can provide a selective advantage during adaptation to new environments, and these data are consistent with Weismann's ideas," they wrote in the journal Nature. But there is still more to study.

"A challenge now is to understand the nature of the mutations that underlie adaptation, and to extend these techniques to larger plants and animals," they wrote.