Study title

The evolution of threespine sticklebacks in freshwater lakes constitutes a well-studied example of a phenotypic radiation that has produced numerous instances of parallel evolution, but the exact selective agents that drive these changes are not yet fully understood. This project aims to study the adaptive diversification in the polymorphic/euryhaline threespine stickleback at genomic and phenotypic scales during colonization from the ocean to freshwater. Four lateral plate morphs were found, where a completely plated morph dominates in the ocean, a partially plated morph in brackish water, and a low plated morph in freshwater. In a few lakes, a fourth morph lacking lateral plates was found. This salinity gradient is viewed as an evolutionary temporal transect where populations adapt to local selection pressures, and where hybridization and adaptive introgression occur in contact zones. This project aims to detect selection on phenotypes in the wild, search for signatures of selection, adaptive introgression and drift along the genome, and illuminate how phenotypes are linked to genomic divergence, studying mechanisms behind heterogenous genomic divergence in a framework of evolutionary parallelism. Only weak indications of adaptation were found. Instead, populations seem to have diversified in phenotypic directions consistent with allometric scaling relationships. This indicates that evolutionary constraints may have played a role in structuring phenotypic variation across freshwater populations of stickleback.