Why Sex? To Keep Parasites at Bay, of Course

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    Oct 16, 2011 7:18 AM GMT

    Writing about science carries the risk of embarrassment. If you champion a theory and it gets disproved, you have some explaining to do. So it is nice when a theory you choose does win the race.

    In the early 1990s I wrote a book called "The Red Queen," which, among other things, came out strongly in favor of a particular explanation for why sex exists. The Red Queen theory—named after Lewis Carroll's monarch, who lives in a weird world "where it takes all the running you can do to stay in the same place"—holds that most creatures reproduce through sex rather than by cloning in order to keep a step ahead of threatening parasites.

    Although a cloning population can have twice as many babies as a sexual population, it becomes a sitting duck for parasites, which hone their genes to evade the immune system of the clone. A sexual population, by contrast, remixes its genes every generation, in effect changing the locks on its cells in order to outwit the parasites.

    I liked this theory partly because it fit the ecological evidence well, especially the fact that breeding between unrelated partners was more common in places where parasites are a problem (plants in tropical forests, for example) and rarer in places where parasites are less troublesome (plants living at high altitudes and latitudes).

    But in the years after my book came out, I had to watch a rival theory gain ground—namely, that sex flushes damaging mutations from the genome by remixing genes to expose bad ones. For much of the last two decades, the two theories have battled it out in the pages of journals, mainly in the form of rival mathematical models, on the whole demonstrating little but that models can support anything if you put the right assumptions in.

    Now, however, I am confident that the Red Queen holds the battlefield. She has vanquished her rivals largely because of a series of simple and rather beautiful experiments carried out in New Zealand by Curtis Lively of Indiana University and Jukka Jokela of the Federal Institute of Technology in Zurich and their colleagues.

    The subject of the experiments is a small freshwater snail that is often infected with a parasitic worm that it catches from, and passes back to, ducks. The snail can reproduce either sexually or asexually. It favors sex in lakes and parts of lakes where parasites are most common. That's the first hint of the presence of the Red Queen.

    Various other experiments now clinch the matter. Clones of asexual snails have a low parasite load when they first appear, but as they become more numerous, they become over-infected. The worm adjusts its genes to target the prevailing clone. Clones of snails that were common in the recent past prove to be more susceptible than rare clones to infection by the latest worms; and the recently common clones of snails are especially vulnerable to worms from their own lakes. In deeper water, asexual snails are more common; ducks do not feed there, so worms that reach these waters cannot evolve to attack the clones (the worm has sex inside the duck).

    For mammals like us, virgin birth is no longer an option (one alleged event aside). We are stuck with sex, presumably because of the need to keep outwitting short-lived, fast-evolving parasites.

    It is possible that the Red Queen also plays a role in the details of mammalian sex. One of the functions of male display seems to be to advertise relative health. By picking the most disease-resistant mates, females have bred males that display themselves in ways that would reveal any high parasite load.

    Does this apply to human beings? Perhaps. One seldom sees tapeworms or tuberculosis in tall, strong, high-achieving men with glossy hair and sonorous voices.
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    Oct 16, 2011 1:22 PM GMT
    Well that's just great for people who reproduce.... lol icon_smile.gif
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    Oct 16, 2011 1:47 PM GMT
    riddler78 saidWe are stuck with sex, presumably because of the need to keep outwitting short-lived, fast-evolving parasites.

    The life span of humans and many other animals is many times that of the snail that was studied. If parasites can evolve fast-enough during a single human lifetime, what good does sex do us in "outwitting" them, when many of us will fail to successfully reproduce due to disease & death caused by those parasites?

    True, if a given human generation does successfully reproduce despite parasitic disease, their offspring may indeed be able to better resist those same parasites. But for how many years, before the fast-evolving parasites overcome the defenses of these "new & improved" next-generation humans?

    I think the proposed "Red Queen" model could work with some species, but not with all. And I'm very dubious about its applicability to humans, where the mutation theory of sex might still retain the edge.
  • calibro

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    Oct 16, 2011 4:07 PM GMT
    There's actually a lot wrong with this article. First, the Red Queen Hypothesis is a hypothesis, not a theory. Second, it was formulated in 1973, not when this book came out in the 90s. Third, the Court Jester Hypothesis (the countering claim) is the actual counter to Red Queen, and it deals with abiotic factors. This dude is seriously just trying to trumpet a hypothesis he didn't even create.