Intensity of sexual selection along the anisogamy-isogamy continuum

Adam Bjork, Scott Pitnick

Research output: Contribution to journalArticlepeer-review

105 Scopus citations


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 sperm. However, a decline in sperm numbers is predicted to weaken selection on males and increase selection on females. 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 anisogamyg-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 gradients decreases as sperm size increases. However, a resolution to the paradox is provided when this finding is interpreted in concert with the 'opportunity for selectiong' and the 'opportunity for sexual selection'. Furthermore, we show that most of the variation in measures of selection intensity is explained by sperm length and relative investment in sperm production.

Original languageEnglish (US)
Pages (from-to)742-745
Number of pages4
Issue number7094
StatePublished - Jun 8 2006

ASJC Scopus subject areas

  • General


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