• age at maturity;
  • divergent selection;
  • ecological theory of speciation;
  • life history;
  • Schistocephalus solidus

In the past decade, there has been a new effort to understand the ecology that drives population divergence and speciation. It is well established in theory that speciation is most likely to occur when a trait that is under divergent natural selection in different populations is also used in mate choice. Such traits have been dubbed ‘magic traits’ (Gavrilets, 2004) and, although there appears to be good evidence that they exist, the ecological mechanisms that underlie their divergence are not well understood. Size at maturity in three-spined sticklebacks is an archetypal example of a magic trait. The present study documents for the first time that differences in body size at maturity in sympatric species pairs of lacustrine three-spined sticklebacks in British Columbia, Canada, are caused by differences in age at maturity. It is also shown that there are differences between the sympatric species in the patterns of infection with a virulent cestode, Schistocephalus solidus. Although the evidence is circumstantial, these differences in infection are consistent with the hypothesis that they have contributed to the observed divergence in age and size at maturity in these populations. © 2009 The Linnean Society of London, Biological Journal of the Linnean Society, 2009, 96, 425–433.