Here's my latest Mind and Matter column from the Wall
Street Journal:
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.
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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.
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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.
