Morphological Divergence Among Sympatric Demes of European Grayling

Divergent selection pressures, induced by variations in environmental conditions and resource availability, can be expected to favor different phenotypic expressions. Hence, different populations often show local adaptations that can be attributed to their respective environments. However, population differentiation may also be due to genotype-environment interactions (phenotypic plasticity). Observing and understanding the population divergence may shed more light on the speciation process. What is even more compelling is when such divergence is observed on a contemporary time scale. In this study, we have explored morphological variation between sympatric sub-populations of European grayling (Thymallus thymallus) that spawn in different streams running into the same lake, Lesjaskogsvatnet. The sub-populations can be considered part of a meta-population, and all the grayling inhabiting the lake shared common ancestors less than 25 generations ago. The lake is divided into two main basins; mark-recapture studies indicate limited between-basin migration. Long-term genetic data show that there is limited but varying levels of gene flow between sub populations. We sampled mature fish during spawning migration in several tributaries to the lake during two spawning seasons. The overall body morphology was quantified using a set of geometric morphometric methods.

We found differences in overall body morphology among populations as well as between years. Especially traits important for sexual selection differed among populations. These amongst population differences could be attributed to the degree of competition between females or between males for access to females in different spawning habitats. The large between-year variation in morphology indicates strong plastic effects on morphology. The evidence for among-popualtion polymorphism indicates variation in resources availability or resource use for the different populations in the lake. This is consistent with preliminary telemetry data indicating differential habitat use of fish belonging to different lake basins.