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Intensity of sexual selection along the anisogamy–isogamy continuum


Research into the evolution of giant sperm has uncovered a paradox within the foundations of sexual selection theory. Postcopulatory sexual selection on males (that is, sperm competition and cryptic female choice) can lead to decreased sperm numbers by favouring the production of larger sperm1. However, a decline in sperm numbers is predicted to weaken selection on males and increase selection on females2,3. As isogamy is approached (that is, as investment per gamete by males approaches that by females), sperm become less abundant, ova become relatively less rare, and competition between males for fertilization success is predicted to weaken. Sexual selection for longer sperm, therefore, is expected to be self limiting. Here we examine this paradox in Drosophila along the anisogamy–isogamy continuum using intraspecific experimental evolution techniques and interspecific comparative techniques. Our results confirm the big-sperm paradox by showing that the sex difference in sexual selection gradients4 decreases as sperm size increases. However, a resolution to the paradox is provided when this finding is interpreted in concert with the ‘opportunity for selection’ and the ‘opportunity for sexual selection’5,6. Furthermore, we show that most of the variation in measures of selection intensity is explained by sperm length and relative investment in sperm production.

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Figure 1: Drosophila bifurca sperm and egg.
Figure 3: Mating system measures in relation to investment in sperm production.
Figure 2: Intraspecific and interspecific sexual selection gradients in Drosophila.


  1. Miller, G. T. & Pitnick, S. Sperm–female coevolution in Drosophila. Science 298, 1230–1233 (2002)

    Article  ADS  CAS  Google Scholar 

  2. Bateman, A. J. Intrasexual selection in Drosophila. Heredity 2, 349–368 (1948)

    Article  CAS  Google Scholar 

  3. Trivers, R. L. in Sexual Selection and the Descent of Man (ed. Campbell, B.) 52–97 (Aldine Publishing, Chicago, 1972)

    Google Scholar 

  4. Arnold, S. J. Bateman's principals and the measurement of sexual selection in plants and animals. Am. Nat. 144, s126–s149 (1994)

    Article  Google Scholar 

  5. Wade, M. J. Sexual selection and variance in reproductive success. Am. Nat. 114, 742–747 (1979)

    Article  Google Scholar 

  6. Wade, M. J. & Arnold, S. J. The intensity of sexual selection in relation to male sexual behaviour, female choice, and sperm precedence. Anim. Behav. 28, 446–461 (1980)

    Article  Google Scholar 

  7. Emlen, S. & Oring, L. W. Ecology, sexual selection, and the evolution of mating systems. Science 197, 215–223 (1977)

    Article  ADS  CAS  Google Scholar 

  8. Clutton-Brock, T. H. & Parker, G. A. Potential reproductive rates and the operation of sexual selection. Q. Rev. Biol. 67, 437–456 (1992)

    Article  Google Scholar 

  9. Shuster, S. M. & Wade, M. J. Mating Systems and Strategies (Princeton Univ. Press, Princeton, New Jersey, 2003)

    Google Scholar 

  10. Jones, A. G., Rosenqvist, G., Anders, B., Arnold, S. J. & Avise, J. C. The Bateman gradient and the cause of sexual selection in a sex-role-reversed pipefish. Proc. R. Soc. Lond. B 267, 677–680 (2000)

    Article  CAS  Google Scholar 

  11. Jones, A. G., Arguello, J. R. & Arnold, S. J. Validation of Bateman's principles: a genetic study of mating patterns and sexual selection in newts. Proc. R. Soc. Lond. B 269, 2533–2539 (2002)

    Article  Google Scholar 

  12. Kokko, H. & Jennions, M. It takes two to tango. Trends Ecol. Evol. 18, 103–104 (2003)

    Article  Google Scholar 

  13. Lorch, P. D. Understanding reversals in the relative strength of sexual selection on males and females: a role for sperm competition? Am. Nat. 159, 645–657 (2002)

    Article  ADS  Google Scholar 

  14. Parker, G. A. Sperm competition and its evolutionary consequences in insects. Biol. Rev. 45, 525–567 (1970)

    Article  Google Scholar 

  15. Parker, G. A. Why are there so many tiny sperm? Sperm competition and the maintenance of two sexes. J. Theor. Biol. 96, 281–294 (1982)

    Article  CAS  Google Scholar 

  16. Parker, G. A., Baker, R. R. & Smith, V. G. F. The origin of evolution of gamete dimorphism and the male–female phenomenon. J. Theor. Biol. 36, 529–553 (1972)

    Article  CAS  Google Scholar 

  17. Briskie, J. V. & Montgomerie, R. Sperm size and sperm competition in birds. Proc. R. Soc. Lond. B 247, 89–95 (1992)

    Article  ADS  CAS  Google Scholar 

  18. Gage, M. J. G. Associations between body size, mating pattern, testis size, and sperm lengths across butterflies. Proc. R. Soc. Lond. B 258, 247–254 (1994)

    Article  ADS  Google Scholar 

  19. LaMunyon, C. W. & Ward, S. Evolution of larger sperm in response to experimentally increased sperm competition in Caenorhabditis elegans. Proc. R. Soc. Lond. B 269, 1125–1128 (2002)

    Article  Google Scholar 

  20. Pitnick, S., Markow, T. A. & Spicer, G. S. Delayed male maturity is a cost of producing large sperm in Drosophila. Proc. Natl Acad. Sci. USA 92, 10614–10618 (1995)

    Article  ADS  CAS  Google Scholar 

  21. Pitnick, S. Investment in testes and the cost of making long sperm in Drosophila. Am. Nat. 148, 57–80 (1996)

    Article  Google Scholar 

  22. Pitnick, S. & Markow, T. A. Male gametic strategies: sperm size, testes size, and the allocation of ejaculate among successive mates by the sperm-limited fly Drosophila pachea and its relatives. Am. Nat. 143, 785–819 (1994)

    Article  Google Scholar 

  23. Joly, D., Bressac, C., Devaux, J. & Lachaise, D. Sperm length diversity in Drosophilidae. Drosoph. Inf. Serv. 70, 104–108 (1991)

    Google Scholar 

  24. Pitnick, S., Spicer, G. S. & Markow, T. A. Phylogenetic examination of female incorporation of ejaculates in Drosophila. Evolution 51, 833–845 (1997)

    Article  Google Scholar 

  25. Pitnick, S., Spicer, G. S. & Markow, T. A. How long is a giant sperm? Nature 375, 109 (1995)

    Article  CAS  Google Scholar 

  26. Jones, A. G., Arguello, J. R. & Arnold, S. J. Molecular parentage analysis in experimental newt populations: the response of mating system measures to variation in the operational sex ratio. Am. Nat. 164, 444–456 (2004)

    Article  Google Scholar 

  27. Karr, T. L. & Pitnick, S. The ins and outs of fertilization. Nature 379, 405–406 (1996)

    Article  ADS  CAS  Google Scholar 

  28. Andersson, M. Sexual Selection (Princeton Univ. Press, Princeton, New Jersey, 1994)

    Google Scholar 

  29. Markow, T. A. & Sawka, S. Dynamics of mating success in experimental groups of Drosophila melanogaster (Diptera: Drosophilidae). J. Insect Behav. 5, 375–383 (1992)

    Article  Google Scholar 

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We thank R. Dallai for contributing the scanning electron micrographs of Drosophila bifurca for Fig. 1, W. T. Starmer for statistical advice, B. A. Byrnes for technical assistance, W. J. Etges for directions to the D. bifurca collection site in Mexico, and C. Hubbell and SUNY Upstate Medical University for providing access to the gamma irradiator. We are also indebted to S. M. Shuster, M. J. Wade, S. J. Arnold, M. Kirkpatrick, G. A. Parker, R. Lande, R. A. Schmedicke, L. L. Wolf, W. T. Starmer, W. D. Brown, J. A. C. Uy and G. T. Miller for discussion of our data and/or comments on an earlier draft of this manuscript. This work was supported by a National Science Foundation Grant to S.P. and A.B.

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Correspondence to Adam Bjork.

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Supplementary information

Supplementary Table 1

Summary of the slopes of the sexual selection gradients presented in Figure 2. (PDF 40 kb)

Supplementary Methods

Full description of methods and analyses used in this study. (PDF 66 kb)

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Bjork, A., Pitnick, S. Intensity of sexual selection along the anisogamy–isogamy continuum. Nature 441, 742–745 (2006).

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