UBC Theses and Dissertations

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UBC Theses and Dissertations

Detecting the drivers of divergence : identifying and estimating natural selection in threespine stickleback Rennison, Diana Jessie


Differences in ecological factors between habitats drive evolutionary divergence and can lead to the generation of new species. While many studies provide evidence suggesting a given trait or genetic locus is adaptive few studies are able to elucidate the direct mechanisms responsible for differences in fitness. I use experimental and observational studies in threespine stickleback (Gasterosteus aculeatus) to identify factors that have driven divergence and to disentangle the mechanisms by which differential fitness arises. To disentangle direct selection on a phenotype from selection on correlated characters encoded by its underlying gene, I applied statistical methods for the estimation of selection on correlated characters (Chapter 2). These data provided the first evidence of direct selection on the lateral plate phenotype and suggested that pleiotropy at the Ectodysplasin locus is likely an important factor driving the rapid and repeated evolution of armour phenotypes. Spectral sensitivity is thought to evolve to match features of the local light environment. Marine and freshwater threespine stickleback inhabit divergent light environments and therefore provide an opportunity to test the hypothesis of spectral matching. I surveyed the opsin gene expression and spectral sensitivity of multiple marine and freshwater populations to test this hypothesis (Chapter 3). While I find weak evidence for spectral matching, I do find evidence suggesting adaptive divergence of spectral sensitivity between populations inhabiting different light environments. Competition is widely appreciated to play a direct role in driving trait divergence; however, it can also have indirect effects mediated through differential exposure to predators. To test for the contribution of species interactions to phenotypic and genetic divergence I conducted an experiment that manipulated exposure of threespine stickleback to a predator, coastal ii cutthroat trout (Chapter 4). After one generation of differential exposure to trout there was evidence of phenotypic and genetic divergence between treatments. These results suggest that cutthroat trout are an important source of divergent selection between populations of threespine stickleback. These studies suggest that bony armour and visual sensitivity are locally adapted and that pleiotropy and genetic architecture likely play an important role in determining evolutionary trajectory during the adaptation of threespine stickleback to freshwater habitats.

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