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The diversity among species, populations, and individuals in nature is astonishing. Genetic and phenotypic variation constitutes the raw material for evolution. It influences how populations respond to changing environments, and can impact long-term survival of species and ecosystem functioning. In this thesis I used field observations, laboratory experiments, and population genetic studies to explore how environmental conditions and eco-evolutionary processes influence genetic and phenotypic variation within and among populations of three ecotypes of pike (Esox lucius).

The results indicate that a complex interplay of geography, divergent selection, gene flow, developmental plasticity, stochastic events, and consequences of admixture has influenced patterns of diversity. Results further suggest that the importance of different processes differ for neutral and adaptive genetic variation, and within and among ecotypes. Neutral differentiation mainly varied according to gene flow and time since divergence, whilst adaptive differentiation appeared to be explained by latitude, likely in part reflecting adaptations to salinity and temperature.

The role of salinity and temperature was further evidenced by population-specific adaptations detected in the laboratory experiments, and by the identification of candidate genes previously shown to be associated with these environmental variables. The experiments also uncovered differences in within-population phenotypic responses to salinity and temperature, indicating that some populations might be predisposed to cope with environmental changes. Because of local adaptations, spawning in foreign habitats likely incur costs for individuals. This, in combination with the finding that none of the populations responded positively to admixture, might explain the persistence of the natal homing behaviour in anadromous pike.

These studies illustrate how fundamental ecological and evolutionary processes influence genetic and phenotypic diversity in pike, and exemplify how the effects can vary depending on spatiotemporal heterogeneity, level of organization (within and among ecotypes), and differ between neutral and adaptive genetic variation. Perhaps the most novel realization was that the effects of admixture for a population combination can vary depending on the origin of the male and female, respectively, which further complicates conservation measures. Overall, these findings illustrate the intricacy of the mechanisms that shape patterns of biological diversity, and highlight the importance of considering adaptive variation in management.