Population divergence shaped by natural selection is central to evolutionary ecology research and has been in focus since Darwin formulated “The origin of species”. Still, the process of adaptive divergence among sympatric populations is poorly understood. In this thesis I studied patterns of adaptive divergence among subpopulations of pike (Esox lucius) that are sympatric in the Baltic Sea but become short-term allopatric during spawning and initial juvenile growth in freshwater streams. I also examined causes and consequences of phenotypic variation among individuals within subpopulations to evaluate the contribution of natural selection to population divergence.

I first investigated homing behaviour and population structures of pike to assess the potential for adaptive divergence among sympatric pike that migrate to spawn in different streams. Mark-recapture data suggested that migrating pike displayed homing behaviour and repeatedly returned to the same stream. Analyses of microsatellite data revealed partial reproductive isolation among subpopulations spawning in different streams. These subpopulations, however, were truly sympatric during the life-stage spent in the Baltic Sea.

To address whether short-term allopatry has resulted in adaptive divergence among sympatric subpopulations I combined observational, experimental and molecular approaches. Observational data showed that subpopulations differed in morphological and life-history traits and common-garden experiments suggested that differences were, at least in part, genetically based. Moreover, Q_ST-F_ST comparisons indicated that genetically based phenotypic differences has been driven by divergent selection, and a reciprocal translocation experiment showed that phenotypic variation represented local adaptations to spawning habitats. Finally, longitudinal and cross-sectional comparisons among individuals revealed associations between phenotypes, performance and fitness components.

In conclusion, my thesis illustrates how short-term allopatry due to migratory behaviour can result in adaptive divergence among sympatric subpopulations. These findings advance the understanding of evolutionary processes at the finest spatiotemporal scale and illustrate that local adaptations can arise in environments with high connectivity.  The results also emphasise that fine spatial scale population structures must be taken into consideration in management and conservation of biodiversity in the Baltic Sea.