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Population density drives the local evolution of a threshold dimorphism

Nature volume 431, pages 1099–1103 (2004)Cite this article

Abstract

Evolution can favour more than one reproductive tactic among conspecifics of the same sex1,2. Under the conditional evolutionarily stable strategy, individuals adopt the tactic that generates the highest fitness return for their status: large males guard females, whereas small males sneak copulations3,4. Tactics change at the status at which fitness benefits switch from favouring one tactic to favouring the alternative1,5. This ‘switchpoint’ is expressed in many species as a threshold between divergent morphologies3. Environmental and demographic parameters that influence the relative fitness of male tactics are predicted to determine a population's switchpoint1,5 and consequently whether the population is monomorphic or dimorphic. Here we show threshold evolution in the forceps dimorphism of the European earwig Forficula auricularia and document the transition from completely monomorphic to classical male-dimorphic populations over a distance of only 40 km. Because the superior fighting ability of the dominant morph6 will be more frequently rewarded at high encounter rates, population density is likely to be a key determinant of the relative fitness of the alternative tactics, and consequently the threshold. We show that, as predicted, population density correlates strongly with the shift in threshold, and that this factor drives the local evolution of the male dimorphism in these island populations. Our data provide evidence for the origin of phenotypic diversity within populations7,8,9, through the evolution of a switchpoint in a conditional strategy that has responded to local population density.

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Figure 1: The status-dependent ESS and threshold variation in F. auricularia.
Figure 2: Morph ratio variation among populations of F. auricularia.
Figure 3: Variation in threshold among island populations of F. auricularia.
Figure 4

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References

  1. Gross, M. R. Alternative reproductive tactics: diversity within sexes. Trends Ecol. Evol. 11, 92–98 (1996)

    Article  CAS  Google Scholar 

  2. Maynard Smith, J. Evolution and the Theory of Games (Cambridge Univ. Press, Cambridge, 1982)

    Book  Google Scholar 

  3. Hunt, J. & Simmons, L. W. Status-dependent selection in the dimorphic beetle Onthophagus taurus. Proc. R. Soc. Lond. B 268, 2409–2414 (2001)

    Article  CAS  Google Scholar 

  4. Emlen, D. J. Alternative reproductive tactics and male dimorphism in the horned beetle Onthophagus acuminatus (Coleoptera: Scarabaeidae). Behav. Ecol. Sociobiol. 41, 335–341 (1997)

    Article  Google Scholar 

  5. Hazel, W. N., Smock, R. & Johnson, M. D. A polygenic model for the evolution and maintenance of conditional strategies. Proc. R. Soc. Lond. B 242, 181–187 (1990)

    Article  ADS  CAS  Google Scholar 

  6. Radesäter, T. & Halldórsdóttir, H. Two male types of the common earwig: male-male competition and mating success. Ethology 95, 89–96 (1993)

    Article  Google Scholar 

  7. West-Eberhard, M. J. Alternative adaptations, speciation and phylogeny (a review). Proc. Natl Acad. Sci. USA 83, 1388–1392 (1986)

    Article  ADS  CAS  Google Scholar 

  8. West-Eberhard, M. J. Phenotypic plasticity and the origins of diversity. Annu. Rev. Ecol. Syst. 20, 249–278 (1989)

    Article  Google Scholar 

  9. West-Eberhard, M. J. Developmental Plasticity and Evolution (Oxford, New York, 2003)

    Google Scholar 

  10. Huxley, J. S. Studies in heteronic growth (III). Discontinuous variation and heterogeny in Forficula. J. Genet. 17, 309–327 (1927)

    Article  Google Scholar 

  11. Bateson, W. & Brindley, H. H. On some cases of variation in secondary sexual characters, statistically examined. Proc. R. Soc. Lond. 1892, 585–594 (1892)

    Google Scholar 

  12. Tomkins, J. L. Environmental and genetic determinants of the male forceps length dimorphism in the European earwig Forficula auricularia L. Behav. Ecol. Sociobiol. 47, 1–8 (1999)

    Article  Google Scholar 

  13. Tomkins, J. L. & Simmons, L. W. Female choice and manipulations of forceps size and symmetry in the European earwig Forficula auricularia L. Anim. Behav. 56, 347–356 (1998)

    Article  CAS  Google Scholar 

  14. Forslund, P. Male competition and large size mating advantage in the European earwigs Forficula auricularia. Anim. Behav. 59, 753–762 (2000)

    Article  CAS  Google Scholar 

  15. Tomkins, J. L., LeBas, N. R., Unrug, J. & Radwan, J. Testing the status-dependent ESS: population variation in fighter expression in the mite Sancassania berlesei. J. Evol. Biol. (in the press)

  16. Moczek, A. P., Hunt, J., Emlen, D. J. & Simmons, L. W. Threshold evolution in exotic populations of a polyphenic beetle. Evol. Ecol. Res. 4, 587–601 (2002)

    Google Scholar 

  17. Unrug, J., Tomkins, J. L. & Radwan, J. Alternative phenotypes and sexual selection: can dichotomous handicaps honestly signal quality? Proc. R. Soc. Lond. B 271, 1401–1406 (2004)

    Article  Google Scholar 

  18. Roff, D. A. Evolutionary Quantitative Genetics (Chapman and Hall, New York, 1997)

    Book  Google Scholar 

  19. Emlen, D. J. Artificial selection on horn body-length size allometry in the horned beetle Onthophagus acuminatus (Coleoptera: Scarabaeidae). Evolution 50, 1219–1230 (1996)

    Article  Google Scholar 

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

    Article  ADS  CAS  Google Scholar 

  21. Shuster, S. M. & Wade, M. J. Mating Systems and Strategies (Princeton, Princeton, 2003)

    Google Scholar 

  22. Moczek, A. P. The behavioral ecology of threshold evolution in a polyphenic beetle. Behav. Ecol. 14, 841–854 (2004)

    Article  Google Scholar 

  23. Roff, D. A. The evolution of threshold traits in animals. Q. Rev. Biol. 71, 3–35 (1996)

    Article  Google Scholar 

  24. Eberhard, W. G. & Gutierrez, E. E. Male dimorphisms in beetles and earwigs and the question of developmental constraints. Evolution 45, 18–28 (1991)

    Article  Google Scholar 

  25. Zar, J. H. Biostatistical Analysis (Prentice-Hall, New Jersey, 1984)

    Google Scholar 

  26. Kotiaho, J. S. & Tomkins, J. L. The discrimination of alternative male morphologies. Behav. Ecol. 12, 553–557 (2001)

    Article  Google Scholar 

  27. Schluter, D. Estimating the form of natural selection on a quantitative trait. Evolution 42, 846–861 (1988)

    Article  Google Scholar 

  28. Snow, D. W. & Perrins, C. M. Birds of the Western Palearctic Concise edition (OUP, Oxford, 1998)

    Google Scholar 

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Acknowledgements

We thank J. Walton, J. Stewart-Clark, H. Hamilton Dalrymple, C. Agasim-Pereira, P. Barry, K. Smith, C. Gallagher, R. Selley, B. Sampson, J. Love, D. Mawer, M. Gurr, R. Dorrien-Smith, I. Bullock, S. Avery, J. Brown and G. Thompson for permission to visit the islands; R. Mavor and D. Jones for data on seabirds; island wardens, R. Summers, J. Wilson, I. Parkinson, A. Shrieve, R. Harvey, O. Gabb, M. Steele, B. Teunis and J. Thompson for logistical support; the boat crews, B. McConnell, A. Hall (SMRU1), S. Moss, J. Dale and N. Quick (SMRU2), B. Sheil and crew (Gladtidings) and F. Mar (Sula); research assistants, A. Arthur, C. Benskin and J. Wernham; A. Arthur for artwork; K. Wilson for Splus 2000 code; N. LeBas for field assistance, discussion of the manuscript and insights into the data; and J. Alcock, N. Colegrave, D. Hosken, R. Knell, J. Kotiaho, S. Tomkins, T. Tregenza, J. Radwan, M. Ritchie, L. Simmons and K. Wilson for comments. J.L.T. is supported by a BBSRC David Phillips Research Fellowship. G.S.B. is supported by a NERC PhD studentship.

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  1. Division of Environmental and Evolutionary Biology, Sir Harold Mitchell Building, University of St Andrews, St Andrews, Fife, KY16 9TH, UK

    Joseph L. Tomkins & Gordon S. Brown

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Correspondence to Joseph L. Tomkins.

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Tomkins, J., Brown, G. Population density drives the local evolution of a threshold dimorphism. Nature 431, 1099–1103 (2004). https://doi.org/10.1038/nature02918

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