WHY did sex evolve? Experiments suggest that this kind of reproduction has
different advantages for an organism, depending on how challenging its
environment becomes.
Sex is expensive. Besides expending energy in the generation and use of the
necessary organs, sex permits an individual to pass on only half of its genetic
material to its offspring. Theoretically, evolution should favour asexual
organisms that pass on their entire genetic complement.
There are two main theories about why such a costly strategy would evolve.
One says that sex creates more variability in each generation and so improves a
species鈥� chances of survival in an ever-changing environment. The other suggests
that sex maintains the overall fitness of a species by clearing out harmful
mutations鈥攖he least healthy offspring die, taking the damaged DNA with
them.
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鈥淭here鈥檚 a lot of theory and virtually no data,鈥� says Clifford Zeyl of Wake
Forest University in North Carolina. But in this week鈥檚 Nature (vol
388, p 465), Zeyl and Graham Bell at McGill University in Montreal say they have
got to the bottom of the mystery experimentally鈥攁t least for yeast. They
found that sexually reproducing yeast clear out bad mutations more efficiently
than yeast that don鈥檛 have sex, but are no better at adapting to a mildly
challenging environment.
Zeyl and Bell grew brewer鈥檚 yeast, Saccharomyces cerevisiae, in two
sorts of growth medium. One contained glucose, a sugar which the yeast finds
easy to eat; the other contained galactose, a sugar to which it is poorly
adapted. They allowed the yeast in some of the lab cultures to reproduce
sexually, while the others reproduced asexually.
At first, all the cultures in the galactose medium grew more slowly, a
reflection of the less suitable conditions. But by the end of the experiment,
eight months later, both the sexual and asexual yeast cultures were doing much
better on the unfamiliar sugar, showing that sex was not an important factor in
their ability to adapt.
In glucose, on the other hand, sex made a difference. Whereas the asexual
yeast was still growing at the same rate after eight months, about 600
generations, the sexually reproducing populations were healthier on both glucose
and galactose, growing 22 per cent faster than they had at the start of the
experiment. Since they were fitter than the asexual cultures on both types of
food, Zeyl attributes the improvement to the loss of harmful mutations rather
than to more rapid adaptation to glucose.
鈥淪election can pick out the few that were lucky and had fewer deleterious
mutations,鈥� Zeyl says, while the 鈥済enetic scapegoats鈥� die off.
However, recent experiments have shown that a species of green algae adapts
more rapidly to a very tough new environment with sex than without it. 鈥淲e鈥檝e
found that both these mechanisms work,鈥� Bell says.
Commenting on the yeast experiments, evolutionary theorist Alexey Kondrashov
of Cornell University in Ithaca, New York, says 鈥渢hat鈥檚 pretty good evidence鈥�.
But he cautions that Zeyl and Bell only infer the clearing of deleterious
mutations from the relative fitness of the yeast cultures鈥攖hey do not
demonstrate it directly. 鈥淚 would like to see the deleterious mutation rate
reliably measured for at least one organism,鈥� he says.