Explaining the benefits of sex (beyond any instant personal gratification) presents a challenge to evolutionary biologists. In the December 12 Nature, Nick Colegrave, at the University of Edinburgh, Scotland, describes how sex has a marked effect on the rate of adaptation in large populations, but less effect in small populations (Nature, 420:664-666, December 12, 2002).

The effect of sex appears to be linked to the abundance of beneficial mutations, suggesting that large populations may have a larger number of mutations and therefore greater benefits from sex. Colegrave studied 20 populations of the facultatively sexual single-celled chlorophyte Chlamydomonas reinhardtii to examine adaptive evolution to novel growth medium conditions. Comparison of the growth rate of sexual and asexual populations, showed that in large populations sex appears to release the 'speed limit' on adaptation set by clonal interference.

Thus, in large populations, the benefits of sex extend well beyond...

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