Biologist Robert Vrijenhoek has been studying the Mexican Poeciliid fish for more than 30 years. Some species of Poeciliopsis reproduce sexually while others reproduce asexually. Vrijenhoek found that the genetic diversity produced by sexual reproduction allowed the sexual fish to survive a parasite more successfully than the asexual fish. Invoking the Red Queen hypothesis, Vrijenhoek suggests the sexual populations are able to keep up or adapt to new selective challenges, while the asexual populations, essentially clonal, are not. From Evolution: "Why Sex?" Also featured: Meredith Small.

Many say that our culture is obsessed with sex, and uses sexual references to sell everything from fragrances to cars. And wouldn't life be dull without it? Yet from the viewpoint of evolutionary theory, sex isn't the most efficient way of reproducing. In fact, scientists have been asking for decades, why does sexual reproduction even exist?

If the goal of life is spreading one's genes far and wide, asexually reproducing organisms seem to have the edge. They do not have to invest time and energy finding a mate. Asexual individuals can pass on their genes twice as fast as those reproducing sexually, because they pass on all their genes -- and only their genes. When reproduction is sexual, half of the genes handed on to the next generation are those of the other parent. The offspring may lose some of the genetic traits that made each parent successful.

There are many hypotheses but, until recently, little hard evidence on the advantages of sex. They stem from the notion that the genetic variation created in sexual reproduction is worth the cost. And at least in some environments, that variation must give a competitive edge over asexual organisms that can spread their genes efficiently, but vary little from one generation to the next.

One explanation is the increasingly popular Red Queen hypothesis, referring to the huffy chess piece in Lewis Carroll's Through the Looking Glass. In Looking Glass Land, the Queen tells Alice, "It takes all the running you can do, to keep in the same place." According to the Red Queen hypothesis, sexual reproduction persists because it enables many species to rapidly evolve new genetic defenses against parasites that attempt to live off them.

Scientists from Rutgers University in New Jersey have tested this idea by observing different groups of small fish called topminnow in Mexico. Some populations of the topminnow reproduce sexually, while others reproduce asexually, so they provide the perfect opportunity to test these ideas. The topminnow is under constant attack by a parasite, a worm that causes something called black-spot disease.

The researchers found that identical populations ("clones") of the asexually reproducing topminnows harbored many more black-spot worms than did those producing sexually, a finding that fit the Red Queen hypothesis: The sexual topminnows could devise new defenses faster by recombination than the asexually producing clones.

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