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Classical ecological theories emphasized species as the fundamental units at which ecologically relevant traits vary. Yet, within the species classification (and even within a single population) hides extraordinary variation in ecologically relevant traits, including diet, morphology, dispersal ability, habitat preferences, and behavior, to name a few. Ecologists are only beginning to scratch surface of what this variation means for population, community, and food web dynamics, and key knowledge gaps remain unfilled. In addition, explaining how this variation arises and is maintained presents a unique challenge in evolutionary biology. My research seeks to develop our understanding of the causes and consequences of intraspecific variation from both ecological and evolutionary perspectives. I study these topics in aquatic ecosystems of California’s eastern Sierra Nevada with a particular emphasis on fish. The eastern Sierra is a veritable natural laboratory where exceptional natural environmental heterogeneity coincides with severe anthropogenic disturbance and rapid environmental change. I use natural history to inform my research questions and design, and employ both experimental and observational methods in my work. In Chapter 1, I investigated how the shared evolutionary history between predator and prey assemblages affects the magnitude and pathways of top-down control in lake food webs using a mesocosm experiment. In Chapter 2, I identified the mechanisms by which introduced predators use the full suite of resources in their environment and used this information to show how individual specialization in diet arises. In Chapter 3, I studied how introgressive hybridization between an introduced and native subspecies of the minnow Tui Chub has affected fish body shape in lake versus stream habitats to test whether hybridization has facilitated or eroded local adaptation. In Chapter 4, I characterized the distribution of behavioral variation in Tui Chub both within and between populations and showed that nearly all the variation in behavioral traits is contained within single populations, and that individuals exhibit consistent behavioral differences over time. Finally, in Chapter 5, I used a mesocosm experiment to show that intrapopulation behavioral variation is likely maintained by frequency-dependent selection, and that the behavioral composition of mesopredators affects the strength, pathway, and spatial distribution of trophic cascades.