Behavioral Ecology and Sociobiology

, Volume 65, Issue 3, pp 445–459 | Cite as

Two roads to two sexes: unifying gamete competition and gamete limitation in a single model of anisogamy evolution

  • Jussi LehtonenEmail author
  • Hanna Kokko
Original Paper


Recent studies have revealed the importance of self-consistency in evolutionary models, particularly in the context of male–female interactions. This has been largely ignored in models of the ancestral divergence of the sexes, i.e., the evolution of anisogamy. Here, we model the evolution of anisogamy in a Fisher-consistent context, explicitly taking into account the number of interacting individuals in a typical reproductive group. We reveal an interaction between the number of adult individuals in the local mating group and the selection pressures responsible for the divergence of the sexes. The same underlying model can produce anisogamy in two different ways. Gamete competition can lead to anisogamy when it is relatively easy for gametes to find each other, but when this is more difficult and gamete competition is absent, gamete limitation can provide another route for anisogamy to evolve. In line with earlier models, organismal complexity favors anisogamy. We argue that the early contributions of Kalmus and Scudo, largely dismissed as group selectionist, are valid under certain conditions. Linking their work with the contributions of Parker helps to explain why precisely males keep producing more sperm than can ever lead to offspring: sperm could evolve to provision zygotes but this brings little profit for the effort required, because sperm would have to be equipped with provisioning ability before it is known which sperm will make it to the fertilization stage. This insight creates a logical link between paternal care under uncertain paternity (where again investment is selected against when some investment never brings about genetic benefits) and gamete size evolution.


Anisogamy Sperm competition Parental care Sex roles 



We thank the organizers of the MMEE meeting in Bristol for their successful efforts to create a stimulating atmosphere and for inviting us to write this work. We also thank Michael Jennions and two anonymous reviewers for comments, and the Academy of Finland and the Finnish Cultural Foundation for funding; we declare no conflict of interest.


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Copyright information

© Springer-Verlag 2010

Authors and Affiliations

  1. 1.Laboratory of Ecological and Evolutionary Dynamics, Department of Biological and Environmental SciencesUniversity of HelsinkiHelsinkiFinland
  2. 2.Evolution, Ecology and Genetics, Research School of BiologyThe Australian National UniversityCanberraAustralia

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