Explaining the evolutionary maintenance of sex remains a key problem in evolutionary biology1,2,3. One potential benefit of sex is that it may allow a more rapid adaptive response when environmental conditions change, by increasing the efficiency with which selection can fix beneficial mutations4,5,6,7. Here I show that sex can increase the rate of adaptation in the facultatively sexual single-celled chlorophyte Chlamydomonas reinhardtii, but that the benefits of sex depend crucially on the size of the population that is adapting: sex has a marked effect in large populations but little effect in small populations. Several mechanisms have been proposed to explain the benefits of sex in a novel environment, including stochastic effects in small populations, clonal interference and epistasis between beneficial alleles. These results indicate that clonal interference is important in this system.
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I thank N. Barton, G. Bell, T. Johnson and S. Nee for discussion of this experiment and for comments on the manuscript; A. Poon for suggestions for improvement to earlier versions; S. Otto and A. Read for advice at early stages; and D. Haydon, L. Kruuk and M. Spencer for lengthy discussions on curve fitting. This work was supported by a NERC postdoctoral fellowship.
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