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Why We Have Sex

Survival, Not Seduction, May Be the Key, Study Shows
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WebMD Health News

March 30, 2005 -- Take away the sweet talk and lust, and sex comes down to survival of the fittest, a new British study shows.

How did the researchers figure that out? They didn't ask this year's so-called sexiest man or woman alive. No surveys were done on modern mating habits; no calls were made to Dr. Ruth.

Instead, the scientists went straight to a source that can't conceal its sexual behavior: yeast.

The Birds, Bees, ... and Yeast?

Believe it or not, yeast can reproduce sexually. Of course, it's nothing like human sex. But, just like in humans, genes get jumbled around when yeast reproduces sexually.

More than a century ago, scientists suggested that sex had survival advantages -- and not just for yeast. Sex, said those Victorians, was how genes got mixed for optimum results.

Since then, the sexual landscape has undergone major changes, from the "sexual revolution" to AIDS. But the sex-and-survival theory still lacked strong proof, researchers write in a new report published by the journal Nature.

Plus, sex has its downsides. It takes time and energy, and the genetic upheaval doesn't always turn out for the best, say the researchers.

So, why bother? Is sex really worth the effort, in terms of survival? That's where the yeast weighs in.

Putting Sex Under a Microscope

The scientists cultivated two types of yeast. The yeasts were identical except for one thing: One type of yeast couldn't have sex. Instead, it reproduced asexually, basically splitting off little clones of itself. That meant no new gene combinations could be made by that type of yeast.

Grown in a lab, the two types of yeast got zero privacy so that reproduction and survival could be monitored. The two types were tested twice. First, they got comfy accommodations. In a harmless environment, both types fared equally well.

But that changed when the researchers made the conditions harsh. In that stressful setting, the sexual yeast had a survival advantage. It reproduced faster, while the asexual yeast lost ground.

It will take more work to figure out how the sexual yeast adapted to the harsh conditions.

What About Sex in People?

As for more complex creatures (like humans), much mystery still remains about sex.

"The challenge now is to understand the nature of the mutations that underlie adaptation and to extend these techniques to large plants and animals," write the researchers, who included Matthew Goddard of New Zealand's University of Auckland.

"We are still far from a definitive answer to the question of why sexual reproduction is so common," says Rolf Hoekstra of the genetics lab at Wageningen University in the Netherlands. Hoekstra commented in Nature on the yeast study.

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