MICHELE NORRIS, host:

After decades of trying, scientists have finally demonstrated that sex is useful.

NPR's Jon Hamilton tells us about a new study. It shows how hooking up helps a species remain one evolutionary step ahead of dangerous parasites.

JON HAMILTON: Sex doesn't make much sense from a biological perspective. For one thing, this approach to reproduction requires males who consume resources even though they may not contribute much beyond their DNA.

Levi Morran of Indiana University says, imagine you're a mom who wants a lot of grandchildren.

Dr. LEVI MORRAN (Postdoctoral Fellow, Biology Department, Indiana University): If you were able then as this mother to produce daughters that didn't need a male to reproduce, you could actually produce more grandchildren than another mother who wasted her time and resources producing males that can't directly bear offspring.

HAMILTON: So why, then, are there so many species that have sex?

Morran says one common explanation dates back to the 1970s, when biologists came up with something called the Red Queen hypothesis. It's named after the Red Queen in "Through the Looking-Glass."

At one point, Alice and the Queen run as fast as they can but never get anywhere. Researchers think something similar is going on with species: They're evolving as fast as they can just to stay even with their competitors and enemies.

And Morran says that's where sex comes in.

Dr. MORRAN: One thing that sex would allow individuals to do that self-fertilization would not is rapidly adapt to environmental conditions.

HAMILTON: In other words, it would let them evolve faster. That makes sense because sex lets a species mix and match from two different sets of genes. Without sex, it's a lot like cloning. Each generation looks like the previous one.

Most scientists accept the Red Queen hypothesis. But Morran wanted to demonstrate it in the lab, and he also wanted to show why it's so important to adapt quickly.

So he did an experiment with worms, which can reproduce sexually or asexually. Morran says he exposed the worms to a bacterial parasite that can do bad things to them.

Dr. MORRAN: It actually - as gross as it sounds, it actually digests the host from the inside out. So it's a pretty nasty pathogen.

HAMILTON: Morran says this parasite is constantly evolving in ways that help it infect the worm.

Dr. MORRAN: And then you have the host evolving on the other side of the equation trying to evade the parasite.

HAMILTON: Morran and his team used genetic engineering to create a group of worms that could only reproduce through self-fertilization.

Dr. MORRAN: When we allowed a parasite to co-evolve with them, they rapidly went extinct.

HAMILTON: Then, he says, they tried the experiment on another group of worms that could only reproduce by having sex.

Dr. MORRAN: Even though their bacterial parasites became more infective, those sexual populations were able to adapt and become better at evading their parasites.

HAMILTON: Finally, they looked at unaltered worms, which can choose to reproduce sexually or asexually. And what they found was that the worms responded to an evolving parasite by adding lots of males to their population and having a lot more sex.

Morran says the results are good news for both sex and males.

Dr. MORRAN: You know, they are certainly expensive, but I think this argues that they are worth it.

HAMILTON: Other scientists say the implications of Morran's study are profound.

Michael Brockhurst from the University of Liverpool notes that nearly every species that have sex, including humans, is in an evolutionary race with parasites. He says he welcomes the results of the new study.

Dr. MICHAEL BROCKHURST (Institute of Integrative Biology, University of Liverpool): To help explain one of this kind of longstanding mysteries in evolutionary biology that we've been banging our head against for 30 years, and now potentially we have a kind of clinching experimental evidence.

HAMILTON: The new research appears in the journal Science.

Jon Hamilton, NPR News.

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