In case you were wondering why we bother with sex, University of Oregon scientists have produced compelling scientific evidence to explain the mystery.

The answer may seem obvious. "Behold, it was very good," the Bible's book of Genesis points out after man and woman begin multiplying fruitfully. But evolutionary biologists have been scratching their heads about sexual reproduction for decades because it's far more efficient for living things to reproduce solo.

The ability to self-fertilize offers huge theoretical advantages. For one thing, you wouldn't need to search and compete for a mate, so you could forget about speed dating, personal ads and trying to look or sound impressive.

And as a "selfing" organism, all of your offspring would be capable of making babies. Among competitors committed to sexual mating, only half of their offspring are capable of bearing young. All else being equal, selfers can multiply at twice the rate of "outcrossers" that need mates.

Biologists have come up with theories to explain the evolution and persistence of sexual mating. For instance, it should be a good way to purge unhealthy genetic mutations.

"If a new, harmful mutation arises in an organisms that is reproducing by self-fertilization, there is a good chance that mutation will become stuck in that group," UO biology professor Patrick Phillips says. "When you have sex with other individuals, you have a chance to get the normal type gene back."

Hooking up with others to reproduce also gives individuals frequent opportunities to add new genes to their repertoire, which should help them adapt more readily to a changing environment.

But scientists had no experimental evidence to support these theories -- until now. Phillips and colleagues Levi Morran and Michelle Parmenter enlisted the help of a millimeter-long nematode called C. elegans. The worm species is composed of males and hermaphrodites, which can either self-fertilize or mate with males.

The UO biologists bred one population that could reproduce only by selfing and another that could reproduce only by sexual intercourse with another worm. Then they compared their ability to hold up after generations of exposure to hostile conditions.

In one experiment, the worms had to cope with a chemical that triggered genetic mutations at four times the normal rate.

"The selfers did horribly," Philips says. In 50 generations, a pileup of mutations eroded their fitness. The sexually mating worms showed no loss of fitness.

In a second experiment, researchers challenged the worms with a strain of bacteria that kills 80percent of infected individuals. After 40 generations, the sexually mating worms adapted swiftly to the pathogen, showing a 150 percent increase in their ability to survive. The selfers failed to adapt at all.

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"Why there is sex is actually not easy to understand, and although we do have a number of hypotheses, progress has been hampered by lack of experimental evidence," says Marcel Salathé, a post-doctoral researcher at Stanford University. "This paper is a missing piece in this puzzle because it provides such evidence."

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