By Julianna Kettlewell
BBC News Online science staff
A popular explanation for why we have frequent sex has been challenged by a report published in Science magazine.
Sex has long been a puzzle to evolutionary biologists
According to the Red Queen Hypothesis, sex exists to help organisms protect themselves against parasites.
Parasites are constantly developing new ways to take advantage, so animals need to evolve defences quickly - and sex, say some, allows them to do this.
But scientists have constructed a model, which suggests this "arms race" alone is not enough to account for sex.
Evolutionary biologists are obsessed with sex and why we have it.
It is one of nature's great mysteries because there are not many obvious reasons why we should do it - but plenty why we should not.
Firstly, sex is a very inefficient way to make babies. Asexual organisms can produce twice the amount of young than their sexual counterparts.
"Clones have a tremendous advantage," explained Curt Lively, an evolutionary biologist from the University of Indiana, US.
"If you have a sexual population and you introduce a clone, that clone will have an advantage, because its intrinsic growth rate is higher. So the clones should take over."
Secondly, if being overrun by clones is not enough, sex is dangerous. You may catch a nasty disease while engaging in the messy act and, even if you don't, your offspring are likely to inherit shoddy genes from their father.
"It is a paradox why so many organisms have sex," said the paper's co-author Sarah Otto, from the University of British Colombia, Vancouver, Canada.
"If you are a parent who has survived to reproduce you probably have a good gene combination, so shuffling them about is not going to benefit you."
But sex does exist - in great abundance. Natural selection, for some reason, chose it. The clones have not taken over and the risk, big as it might be, is not big enough to make sex a bad idea.
Red Queen to the rescue?
The Red Queen Hypothesis takes its name from the character in Lewis Carroll's Through The Looking-Glass, who tells Alice she has to run as fast as she can to stay in the same place.
The idea is that organisms have to keep evolving - keep adopting new genetic combinations - to "outwit" pathogens.
"The theory states that parasites are selected to target the most common genotype, which is now this clone," Professor Lively told BBC News Online. "So if the parasites are successful, and very virulent, they can prevent that clone from taking over the sexual population.
"But for this theory to work, there have to be an awful lot of parasites about, and they have to have very dramatic effects."
And there is the rub. According a mathematical model developed by Sarah Otto and her colleague Scott Nuismer, there are not enough parasites about to explain why organisms have so much sex.
Too much sex
Having sex every now and again might be an advantage, Dr Otto believes. Doing it occasionally should fox the parasites. But doing it frequently probably just spoils winning genetic combinations.
According to her model, if evading parasites was the only objective, organisms should reproduce sexually sometimes, but asexually often.
"If you actually do the maths, the hosts that are common in the population at the current moment in time have been doing a pretty good job at evading their parasites.
Sex is ridden with costs - but it must be worth it
"A little sex makes enough of the combinations present, but having more sex breaks apart the combinations that are working to evade the parasites."
Since we - and many other organisms - have more than a little sex, we might have to look beyond the Red Queen for the whole answer.
"What the Red Queen can't explain is why creatures have more than a minimal amount of sex," said Dr Otto. "If organisms only had sex very rarely, then it could be the case that the Red Queen could explain that."