<p>  </p> <p>Evolution is the foundation for all of biology. However, our approaches and understanding of evolution—simply, the change of allele frequencies from one generation to the next—have themselves evolved over time. In this dissertation I explore multiple approaches to understand evolution and the consequences of evolution across variable scales and study organisms. First, I use meta-analytic techniques and Bayesian hierarchical models to investigate the phenotypic consequences of two forms of cryptic local adaptation, co- and countergradient variation, by leveraging a decades-old quantitative genetics approach (Chapter 1). I find large effects for both co- and countergradient variation, however they are obscured in natural settings by concurrent large environmental effects. I also show that these large effects are ubiquitous across phenotypic traits, organisms, and environmental gradients, suggesting that while similar phenotypes may be the evolutionary end point, the mechanisms to achieve those phenotypes likely vary. In the following chapter I explore the rapid evolution of a unique and understudied species introduction, pink salmon (<em>Oncorhynchus gorbuscha</em>) in the Great Lakes. Pink salmon were introduced into Lake Superior in a single introduction event and have broken two obligate life histories, anadromy (though they treat the Great Lakes like surrogate oceans) and their fixed two-year life cycle, making them ripe subjects for contemporary evolution. Using whole-genome sequence data, I first investigate the effects of a genetic drift in the form of a bottleneck at introduction and characterize the subsequent loss of genetic diversity (Chapter 2). I show that despite a large loss of genetic diversity, pink salmon also rapidly adapted to their novel environment based on signals of putative selection across numerous regions of the genome, particularly in a period gene associated with their daily circadian clock (<em>per2</em>). Next, I explore how genome structure likely aided adaptation by pink salmon to the Great Lakes, providing evidence that a supergene (~29 Mbp) containing an inversion on chromosome 10 swept to near fixation in the Great Lakes (Chapter 3) and likely aided in osmoregulatory adaptation to this novel environment. Finally, I end with a short perspective chapter (Chapter 4) where I highlight potential future research directions for each of the previous chapters. Together, this research investigates the drivers and consequences of evolution across multiple scales and shows the powerful effect of genetic drift and genetic adaptation in shaping the genomic and phenotypic attributes of populations.</p>

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