Letter | Published:

Sex releases the speed limit on evolution


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.

Access optionsAccess options

Rent or Buy article

Get time limited or full article access on ReadCube.


All prices are NET prices.


  1. 1

    Maynard Smith, J. The Evolution of Sex (Cambridge Univ. Press, Cambridge, 1978)

  2. 2

    Bell, G. The Masterpiece of Nature (Croom Helm, London, 1982)

  3. 3

    West, S. A., Lively, C. M. & Read, A. F. A pluralist approach to sex and recombination. J. Evol. Biol. 12, 1003–1012 (1999)

  4. 4

    Fisher, R. A. The Genetical Theory of Natural Selection (Clarendon, Oxford, 1958)

  5. 5

    Muller, H. J. Some genetic aspects of sex. Am. Nat. 66, 118–138 (1932)

  6. 6

    Otto, S. P. & Barton, N. H. The evolution of recombination: removing the limits to natural selection. Genetics 147, 879–906 (1997)

  7. 7

    Burt, A. Perspective: Sex, recombination, and the efficacy of selection—was Weismann right? Evolution 54, 337–351 (2000)

  8. 8

    Gerrish, P. J. & Lenski, R. E. The fate of competing mutations in an asexual population. Genetica 102/103, 127–144 (1998)

  9. 9

    de Visser, J. A. G. M., Zeyl, C. W., Gerrish, P. J., Blanchard, J. L. & Lenski, R. E. Diminishing returns from mutation supply rates in asexual populations. Science 283, 404–406 (1999)

  10. 10

    Miralles, R., Gerrish, P. J., Moya, A. & Elena, S. F. Clonal interference and the evolution of RNA viruses. Science 285, 1745–1747 (1999)

  11. 11

    Otto, S. P. & Barton, N. H. Selection for recombination in small populations. Evolution 55, 1921–1931 (2001)

  12. 12

    Colegrave, N., Kaltz, O. & Bell, G. The ecology and genetics of fitness in Chlamydomonas. VIII. The dynamics of adaptation to novel environments after a single episode of sex. Evolution 56, 14–21 (2002)

  13. 13

    Vasi, F., Travisano, M. & Lenski, R. E. Long-term experimental evolution in Escherichia coli. 2. Changes in life-history traits during adaptation to a seasonal environment. Am. Nat. 144, 432–456 (1994)

  14. 14

    Goho, S. & Bell, G. Mild environmental stress elicits mutations affecting fitness in Chlamydomonas. Proc. R. Soc. Lond. B 267, 123–129 (2000)

  15. 15

    Lenski, R. E., Rose, M. R., Simpson, S. C. & Tadler, S. C. Long-term experimental evolution in Escherichia coli. I. Adaptation and divergence during 2,000 generations. Am. Nat. 138, 1315–1341 (1991)

Download references


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.

Author information

Competing interests

The author declares that he has no competing financial interests.

Correspondence to Nick Colegrave.

Rights and permissions

Reprints and Permissions

About this article

Further reading

Figure 1: Effect of bottleneck size on adaptation in asexual populations.
Figure 2: Interaction between sex and bottleneck size.


By submitting a comment you agree to abide by our Terms and Community Guidelines. If you find something abusive or that does not comply with our terms or guidelines please flag it as inappropriate.