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Abstract

While the role of competition is well documented from community assembly to ecological speciation, it is rarely studied in ways that quantify evolutionary change or predict demographic outcomes. My thesis makes progress through combining novel experiments with classical and modern competition theory to predict ecological persistence and answer evolutionary questions. First, I experimentally demonstrate how the demographic parameters that govern competitive ability, i.e. the ability to tolerate competition, of a native grass (Vulpia microstachys) have evolved in response to the invasion of a dominant species (Bromus hordeaceus). Competitive ability reflects a complex trait–yet our understanding of how each underlying parameter evolves remains incomplete. Here I find that competitive ability is not constrained by demographic trade-offs, and find invaded populations to increase competitive ability, in part, through lower within-species competition. Second, I experimentally apply coexistence theory to uncover the mechanisms of ecological persistence among diverging lineages of an aquatic flowering plant (Spirodela polyrhiza). Although divergence is key for speciation, little is known about when allopatric lineages evolve to ecologically coexist. I find that niche differentiation rapidly accumulates to promote coexistence well before speciation (<68,000 years), with continued but slower accumulation as species boundaries are crossed. Here I provide the first insights into the tempo and mode of lineage coexistence, tying ecological persistence to the origin of biodiversity. Third, I explore the evolutionary consequences of competitive asymmetries between nearby populations of an annual grass V. microstachys. Typically, fitness is rarely measured in ways that align with ecological theory of competition. Through field experiments, I find large differences in both the direction and strength of competitive interactions not predicted by assuming competition is uniform within a species, emphasizing the importance of ecological differences at genotypic scales. Finally, I align my previous three chapters to more mechanistic processes of competition. To do so, I focus on the modern-day utility of MacArthur’s consumer-resource model and review many tacit assumptions in coexistence theory. Here I demonstrate how this model can translate different currencies of competition (e.g., alpha coefficients, R*) and suggest a combined approach to a more unified ecological science.