EFFECTIVE POPULATION SIZE AND CONNECTIVITY IN FISH

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A fundamental question in evolutionary and conservation biology is how habitat fragmentation influences the distribution of genetic diversity among populations and the ability of populations to adapt to local conditions. In my research, I use effective population size (Ne) as a measure of the genetic diversity of populations, and examine the mechanisms that determine the Ne of populations of fish found in complex landscapes. Effective population size is of importance to species conservation through its links to population persistence and evolutionary potential. Most species occur naturally as networks of populations interconnected by varying degrees of gene flow (meta-populations). Patterns of genetic diversity are thus a product of mechanisms that influence local population Ne’s (stochastic loss, local adaptive selection, gene flow), and the effective size of the meta-population (meta-Ne). Gene flow tends to increase Ne of local populations by countering the stochastic loss of genetic diversity, while directional selection is likely to reduce diversity. For meta-populations, theory predicts that the meta-Ne of a fragmented system will be smaller than that of a similar-sized panmictic system, if local populations contribute unevenly to the next generation. In complex, hierarchically arranged dendritic environments, however, the predicted meta- Ne can be greater than the sum of the local Ne’s. Dendritic watersheds, for example, have a hierarchical arrangement of local populations that usually leads to asymmetrical gene flow. Meta-Ne can also be affected by the environmental variation in the landscape that subjects populations to different selective pressures. My long term goals are to elucidate the genomic basis of local adaptation in fish within spatially complex meta-population systems, and to determine how gene flow and its asymmetries affect patterns of neutral and adaptive genetic diversity. I will be examining systems in several temperate post-glacial landscapes (Nova Scotia, Newfoundland, Labrador and Patagonia) that contrast in level of fragmentation and connectivity. These dendritic drainages differ in size and complexity, and thus are likely to differ in the relative importance of asymmetric gene flow and adaptive processes. I will also compare patterns of genetic diversity for sympatric species with very different life history traits in landscapes that contain lakes with contrasting selection pressures (water pH, transparency, productivity). The goal will be to disentangle the effects of gene flow, adaptive processes and species life histories on local Ne’s and meta-Ne. Some of this work will take place in the West River Sheet Harbour drainage in Nova Scotia, a highly complex dendritic system of nearly 20 interconnected lakes differing in environmental variables and containing three abundant sympatric species that vary in some key life history traits: banded killifish, yellow perch, and white sucker. My ongoing research addressing similar and related questions in salmonid species will continue focusing on the upper Humber river system in Newfoundland, while in Patagonia I will examine patterns of colonization and adaptation to salinity in the widespread Galaxias maculatus. At the other end of the connectivity spectrum I also work with marine species, including Atlantic herring, a highly abundant and widespread pelagic fish. This body of research will constitute some of the first empirical tests of the theoretical predictions regarding the effects of connectivity on the relationship between local population Ne’s and meta-Ne, and should give us some idea of the usefulness of the theory for understanding real systems as well help to indicate directions for further theoretical development.