Evol Ecol Res 6: 857-871 (2004)     Full PDF if your library subscribes.

Sexual dimorphism or evolutionary branching?

Tom J.M. Van Dooren,1* M. Durinx1 and I. Demon1,2

1Institute of Biology, University of Leiden, Kaiserstraat 63, 2311 GP Leiden, The Netherlands and  2Rothamstead Research, Harpenden, Herts AL5 2JQ, UK

Author to whom all correspondence should be addressed.
e-mail: vdooren@rulsfb.leidenuniv.nl


Disruptive selection due to ecological causes can lead to different types of phenotypic polymorphism. For a broad range of ecological scenarios, we investigate the odds that disruptive selection leads to sexual dimorphism relative to polymorphisms that appear after evolutionary branching. These involve genetic polymorphism, such as sympatric species or Mendelian genes with strong dominance-recessivity. When models that allow for sexual dimorphism are compared with constrained models with equal phenotypes in males and females, a sexual dimorphism is expected to evolve instead of the evolutionary branching in the constrained model. This is an important general result on the odds of different types of ecological polymorphism. It implies that the possibility for sympatric speciation caused by ecological selection pressures can be removed by the evolution of ecological differences between the sexes. Evolutionary branching becomes more likely when: (1) there is a strong constraint on sex differentiation; (2) secondary branching events occur after sexual dimorphism has already evolved; (3) assortative mate choice occurs before trait divergence starts. The possibility of sexual selection driving sympatric speciation is not affected by our conclusions.

Keywords: dominance evolution, ecological polymorphism, evolutionary branching, sexual dimorphism, sympatric speciation.

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