Morphological divergence among sympatric demes of European grayling (Thymallus thymallus).

Abstract

Divergent selection pressures, induced by variations in environmental conditions, can be expected to favour different phenotypic expressions. Different populations show thus often local adaptations that could be attributed to their respective environments. Such local adaptations have even been demonstrated to occur at a contemporary time scale. Observing rapid local adaptations in the wild may shed more light on speciation and fundamental evolutionary processes. In this study, I have explored morphological variation among demes of European grayling (Thymallus thymallus) within a Norwegian lake, Lesjaskogsvatnet. The morphological variation has been quantified using geometric morphometric methods. The grayling inhabiting the lake shared common ancestors less than 25 generations ago. I hypothesised there to be a divergence in morphological traits affecting survival and reproduction due to variations in sexual and natural selection within the system. I found differences in sexual dimorphic traits among demes. Further, I found a small difference in the degree of sexual dimorphism between demes. This morphological pattern may be due to among-tributary differences in environmental conditions during spawning, and thus different natural and sexual selection gradients. The among-deme difference may also have occurred due to founder effects and genetic drift. I also found evidence of polymorphism, which might indicate there to be variations in resources or resource competition between different basins in the lake. All this morphological variations might have occurred due to either adaptive divergence or phenotypic plasticity.