Sexual selection against hybrids between sympatric stickleback species: Evidence from a field experiment

Sexual selection against viable, fertile hybrids may contribute to reproduc tive isolation between recently diverged species. If so, then sexual select ion may be implicated in the speciation process. Laboratory measures of the mating success of hybrids may underestimate the amount of sexual selection against them if selection pressures are habitat specific. Male F-1 hybrids between sympatric benthic and limnetic sticklebacks (Gasterosteus aculeatu s complex) do not suffer a mating disadvantage when tested in the laborator y. However, in the wild males choose different microhabitats and parental f emales tend to be found in the same habitats as conspecific males. This set s up the opportunity for sexual selection against male hybrids because they must compete with parental males for access to parental females. To test f or sexual selection against adult F-1 hybrid males, we examined their matin g success in enclosures in their preferred habitat (open, unvegetated subst rate) where Limnetic males and females also predominate. We found significa ntly reduced mating success in F-1 hybrid males compared with limnetic male s. Thus, sexual selection, like other mechanisms of postzygotic isolation b etween young sister species, may be stronger in a wild setting than in the laboratory because of habitat-specific selection pressures. Our results are consistent with, but do not confirm, a role for sexual selection in stickl eback speciation.