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The genetics of adaptation in stickleback Barrett, Rowan Douglas Hilton


Understanding the process of adaptation requires elucidating the mechanisms through which natural selection alters the genetic variation underlying phenotypic traits. Here, I explore the genetics of adaptive evolution empirically, using lab and field experiments with threespine stickleback fish, and theoretically, using population genetic models. Freshwater stickleback populations are derived from ancestral marine populations that colonized lakes and streams at the end of the last ice age. These derived populations exhibit remarkable parallel divergence in a number of morphological and physiological traits. The parallel nature of these changes suggests the influence of natural selection, because genetic drift is unlikely to produce a strong correlation between phenotype and environment. Adaptive evolution in some of these traits is due to selection on standing genetic variation present in the ancestral marine population. I investigate the ecological, ontogenetic and behavioural mechanisms that contribute to the maintenance of this variation. I present evidence of extremely strong selection acting at phenotypic and genotypic levels over short time scales. Population genetic theory typically assumes much smaller selection coefficients than those measured in this work. I derive population genetic theory to describe the distribution of fitness effects of beneficial mutations without this restriction of weak selection, and test the analytical theory with numerical simulations. Collectively, this research is helping to identify some of the primary functional mechanisms that maintain genetic variation within and between natural populations.

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