Adaptive capacities in Arctic charr (): The importance of phenotypic and genetic variation

Abstract

Intraspecific diversity remains largely overlooked by the International Union for Conservation of Nature (IUCN) but is vital to the future viability of species, which are under increasing threat from climate change and other anthropogenic pressures. It is expected that species existing in isolated populations at the edge of their thermal
tolerance may be the most vulnerable. Post-glacial fishes display extensive intraspecific diversity, demonstrating repeated patterns of resource-driven ecotype assemblages. Many populations inhabit genetically isolated lakes at the limits of their known biogeographic range with small effective population sizes (Ne) and restricted gene flow. In this thesis, I analyse phenotypic and genetic diversity among and within populations of the postglacial fish Arctic charr (Salvelinus alpinus) across England and Norway. I investigate the roles of plasticity and evolutionary adaptation in generating and maintaining within-lake phenotypic and genetic variation, and its importance for adaptive capacity. Utilising ^C and ^N isotopes and morphological analysis, I identify phenotypically divergent ecotypes within lakes. With genome-wide ddRAD sequencing, I asses the relative roles of plasticity, drift, and selection in the development and retention of divergent phenotypes. Across the sites, I find a lack of repeatability among ecotype assemblages and the processes driving intraspecific diversity, likely to be a result of differing ecosystem size and complexity. I find high genetic drift among populations (F_ 0.151–0.490) and low Ne within them (maximum Ne of 1000), characteristic of at-risk populations. Signals of selection relating to phenotype and thermal tolerance indicate that populations and ecotypes will not respond uniformly to climate change. The work supports management recommendations for preserving the environments and processes that lead to intraspecific phenotypic and genotypic patterns. It raises concerns about small, fragmented populations at the edge of their physiological and thermal tolerance, and about whether they can adapt in situ to rapid climate change.