Consequences of Population Divergence in Threespine Stickleback (Gasterosteus aculeatus)

Abstract

Population divergence, changes in allele frequencies or mean phenotype between populations of the same species, is a key process generating biodiversity within and between lineages. Divergence reflects the aggregate effects of genetic drift, migration, mutation and/or selection. In this thesis, I characterized the origin, patterns and maintenance of genetic diversity in a putatively young radiation of threespine stickleback (Gasterosteus aculeatus) near its northern range limit in and around Katmai National Park and Preserve and Aniakchak National Preserve, Alaska. Freshwater populations in this region are assumed to have evolved from marine ancestors after the last glacial recession. To characterize the population structure and colonization history of this radiation and evaluate the influences of gene flow, drift and selection on the genomic architecture of divergence I used >4000 genetic markers (single-nucleotide polymorphisms) throughout the whole genome for fish collected from 1 anadromous, 1 marine and 9 freshwater sites. Results suggest that marine/anadromous fish colonized the new freshwater environments, except for one lake population that appeared to be refugial. Subsequent to colonization, these populations diverged genetically, creating a hierarchical population structure that reflects their colonization history, rather than isolation by distance. Contemporary immigration estimates reveal moderate to low gene exchange between environments on the whole, and gene flow due to immigration has not had a predictable affect on the genomic architecture of divergence in these populations. Divergent selection on loci distributed throughout the genome is driving population differentiation. To determine whether phenotypic differences in body length at reproductive maturity between fish from two freshwater populations and an anadromous population is genetically based I used a common environment experiment. Despite differing genetic divergence (FST) between the two freshwater populations, they exhibit similar size divergence from the anadromous fish, indicating that this trait is genetically influenced. Parallel genetically-based divergence in a phenotype important for fitness suggests a contribution of natural selection in driving divergence between these populations. This thesis provides insight into how evolutionary processes are acting within the genome towards diversification, both specifically for stickleback and in a broader context for all species, and it also adds to a body of literature characterizing colonization of new environments and genetic basis of phenotypic traits.