The evolutionary paradox of sex

Megan Scudellar has a fascinating article up at The Scientist Magazine on the mystery of why sexual reproduction evolved to be so pervasive: The Sex Paradox | The Scientist Magazine®.

Sarah “Sally” Otto was sitting in a lab meeting of evolutionary biologist Marcus Feldman’s group at Stanford University in 1988 when she overheard a graduate student describe sex as “such a big puzzle.” Otto, an undergraduate at the time, didn’t agree. Sexual reproduction—genetic recombination between individuals—“obviously” promotes variation, she thought, allowing species to adapt to changing environments.

WHY SEX?: See full infographic: JPG | PDF

Otto’s reaction makes sense, and echoes one of the oldest formal explanations for why sex evolved to be ubiquitous in the animal kingdom. 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. (See “Why Sex?”)

I have to say that when I first heard about this paradox, I had pretty much the same reaction as Otto.  It seemed like sexual reproduction had so many “obvious” benefits, that asking why it existed felt pointless.  But apparently demonstrating those benefits scientifically is problematic.

The article describes experiments on various microscopic species who can reproduce both asexually and sexually, trying to isolate what exactly the benefit of the sexual reproduction might be.  The results sometimes showed a benefit in variable adaptability, in the rapid propagation of beneficial mutations, and in resistance to parasitic infections.

Due to the lack of consistent results, doubts remain among biologists.

In a static world, sex is likely unnecessary. The ever-changing environments of Earth call for a different scenario, however. 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 Otto. “The niggling doubt in the back of my head is that it is not.”

And so the paradox of sex lives on. “We still really don’t know the answer to this very most basic question,” says Mark Welch. “We don’t know why sex exists.”

It seems clear that sex provides some kind of benefit.  The question is what that benefit is.

As someone who is interested in what extraterrestrial life might look like, I find these kinds of speculations fascinating.  How “necessary” was sexual reproduction?  Given time, would it evolve again, or was it an evolutionary accident?  How likely would the evolution of complex life have been if sex had never developed?

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4 Responses to The evolutionary paradox of sex

  1. Brett says:

    Maybe it offered a bigger advantage in the past, such that it spread rapidly once it appeared and then got “locked-in” in terms of evolutionary path dependency.

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    • You could be right. Although if that were true, I’d think asexual species would proliferate more since they’d have the advantage of not having to exert energy finding a reproductive partner.

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  2. I wonder if the environment might not change pretty rapidly in nature after all, particularly when you consider that it may not be a case of global conditions changing but organisms rapidly colonising new niches. Those organisms which occupy and successfully claim new niches are those which can colonise first, and these are likely to be the fastest to adapt, which probably means sexual reproduction.

    Asexually reproducing species are still abundant, and I believe make up the majority of the biomass on earth. The asexual reproducers are doing just fine. They’re not losing out in terms of survival, so I’m not surprised that tests fail to demonstrate such an effect. But they have not managed to adapt fast enough to occupy the niches currently claimed by the more complex sexual reproducers.

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    • Good points. It might simply be that sexual reproduction leads to more rapid evolution. I still wonder if something like complex animal life is possible without it, but given enough time, say on a planet orbiting a red dwarf star, it might be that an asexual animal species could arise. It might just take longer than it did here.

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