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Keywords:

  • natural selection;
  • poeciliid;
  • sexual selection

Natural and sexual selection can have either opposing or synergistic effects on the evolution of traits. In the green swordtail Xiphophorus helleri, sexual selection arising from female choice is known to favour larger males and males with  longer  swords.  We  examined  variation  in  male  and  female  size  and  fin  morphology  among  15  populations  that varied in their predation environments. Males and females from populations in which piscivorous fishes were present had longer and deeper bodies than did males and females from populations in which piscivorous fishes were absent.  Controlling  for  a  positive  effect  of  body  size  on  sword  length,  males  from  populations  in  which  piscivores were present had relatively shorter swords than did males from populations in which piscivores were absent. The associations between morphology and predation environment may be due to direct effects of predation, indirect effects of predation, other sources of selection that covary with predator presence, or other environmental effects on trait expression. These results suggest that while sexual selection favours longer swords, natural selection may have an opposing effect on sword length in populations with predators. Natural selection on body size, however, may act synergistically with sexual selection in populations with predators; both may favour the evolution of larger body size. The body size results for X. helleri contrast with related taxa that have become model systems for the study of life history  evolution. ©  2004  The  Linnean  Society  of  London, Biological  Journal  of  the  Linnean  Society,  2004, 83, 87–100.