Sexual reproduction may not be the best evolutionary strategy. So why do we do it?

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Most of the single-celled organisms in the world, like bacteria, reproduce asexually by making copies of themselves. So how did sex come to rule the animal kingdom? Scientists have been trying to figure out the origin of sex for hundreds of years, without much luck.

Asexual reproduction is more convenient and requires less effort: there’s no search for a partner and you get to pass all your genes along, from the U.K.’s National History Museum:

In many ways asexual reproduction is the better evolutionary strategy: only one parent is needed and all of their genes are passed on to the next generation.  All bacteria, most plants and even some animals reproduce asexually at least some of the time.
Sex is less efficient. Finding a mate can take time and energy, and any gametes that aren’t fertilised go to waste. Plus, each parent only passes half of its genes to the offspring.

But 99 percent of multicelled animals use sex to reproduce. They form gametes, mix those together and create progeny with an entirely new genome.  Scientists have long wondered what processes caused sex to evolve and become so incredibly prevalent in the animal world.

Most hypotheses about the evolution of sex point out that when genes are mixed between individuals sexually, bad genetic mutations can be eliminated more quickly than in asexual reproduction. But, since the 1880s, scientists have been unable to prove one hypothesis or another explains Megan Scudellari in Scientist magazine:

In 1886, German evolutionary biologist August Weismann proposed that sexual reproduction reshuffles genes to create “individual differences” upon which natural selection acts. Additional ideas have emerged since Weismann’s hypothesis: sex rids the genome of deleterious mutations; sex rapidly introduces beneficial mutations; sex helps organisms dodge parasitic infections. Yet these evolutionary justifications for sex have remained hypotheses because there is not enough evidence to suggest that any of them provide enough of a benefit to surmount the exquisitely high costs of sex, which include the time and energy it takes to find a mate, the passage of only half of one’s genes to the next generation, and the breaking apart of favorable gene combinations.

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Part of the problem, Sculdellari says, is where scientists look to try to test these ideas. Most of the organisms we know the most about sexually like flies, humans and bacteria only reproduce sexually or asexually. But there are some species that can do both depending on the environmental circumstances surrounding them. Yeast, snails and rotifers, microscopic freshwater animals reproduce both sexually and asexually. By studying these organisms, scientists can compare the relative health of their asexually and sexually produced offspring.

The Red Queen hypothesis, named after the Alice in Wonderland character, suggests that sex is really about eliminating the chance of disease through the exchange of cell surface genes that alter the proteins where diseases try to invade:

The Red Queen hypothesis for sex is simple: Sex is needed to fight disease. Diseases specialize in breaking into cells, either to eat them, as fungi and bacteria do, or, like viruses, to subvert their genetic machinery for the purpose of making new viruses. To do that they use protein molecules that bind to other molecules on cell surfaces. The arms races between parasites and their hosts are all about these binding proteins. Parasites invent new keys; hosts change the locks. For if one lock is common in one generation, the key that fits it will spread like wildfire. So you can be sure that it is the very lock not to have a few generations later.

Evidence in a species of New Zealand lake snail shows that those that produced sexually were much less likely to be infected by a common parasite than those that were the product of asexual production. But in subsequent generations that pattern flips, which may mean that for organisms that go both ways, reproductively, the ability to switch itself conveys a benefit.

Looking for evidence explaining why sex persevered is even more complicated. We may someday know why sex started in the first place, but figuring out why asexual reproduction was never resurrected in the animal kingdom is trickier.

Sex allows species to adapt to the loss of food sources, the arrival of parasites, rising temperatures, and more. There is some doubt, however, whether the environment fluctuates fast enough to warrant the prevalence and persistence of sex in the eukaryotic kingdom. “Is the force favoring sex large enough in the face of the costs?” asks theoretical biologist Sally Otto. “The niggling doubt in the back of my head is that it is not.”

A version of this story originally ran at the GLP on November 28, 2016.

Meredith Knight is a freelance science and health writer. Follow her on Twitter @meremereknight

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