Testing for mating isolation between ecotypes: laboratory experiments with lake, stream and hybrid stickleback

Mating isolation is a frequent contributor to ecological speciation – but howconsistently does it evolve as a result of divergent selection? We tested forgenetically based mating isolation between lake and stream threespinestickleback (Gasterosteus aculeatus L.) from the Misty watershed, VancouverIsland, British Columbia. We combined several design elements that areuncommon in the studies of stickleback mate choice: (i) we used secondgenerationlaboratory-reared fish (to reduce environmental and maternaleffects), (ii) we allowed for male–male competitive interactions (instead of thetypical no-choice trials) and (iii) we included hybrids along with pure types.Males of different types (Lake, Inlet, hybrid) were paired in aquaria, allowed tobuild nests and then exposed sequentially to females of all three types. Wefound that Lake and Inlet males differed in behaviours thought to influencestickleback mate choice (inter- and intra-sexual aggression, display and nestactivities), whereas hybrids were either intermediate or apparently ‘inferior’ inthese behaviours. Despite these differences, Lake and Inlet fish did not mateassortatively and hybrid males did not have a mating disadvantage. Our studyreinforces the noninevitability of mating isolation evolving in response toecological differences and highlights the need to further investigate the factorspromoting and constraining progress towards ecological speciation.