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Abstract
Species exist across heterogenous environments and encounter various agents of selection, which can result in the evolution of plasticity and genetic differentiation. Under climate change, agents of selection may shift, resulting in novel fitness landscapes that species will need to traverse to avoid extinction. Here, I evaluated the role of three agents of selection related to global change in shaping phenotypic variation and fitness landscapes across elevational gradients. In field and greenhouse studies, I examined trait expression, fitness consequences, and selection in diverse accessions of the montane forb, Boechera stricta. This species is found across broad climatic gradients and demonstrates extensive local adaptation to abiotic and biotic conditions.In montane systems, climate change is accelerating the timing of snowmelt, potentially exposing germinating seeds and seedlings to freezing conditions. In chapter 2, I found early snowmelt was indeed associated with an increase in freezing events, suppressed early life stages. In addition, I detected a genetic cline in freezing tolerance in a complementary freezing assay.
Herbivore abundance and water availability co-vary across elevational gradients, and both could influence clinal trait evolution. In chapter 3, I found water availability and herbivore load contributed to expression of and selection on traits and contribute to local adaptation in this system.
Phenotypic variation in a population can be divided into components of evolutionary history, plasticity, and maternal effects. In chapter 4, I manipulated herbivory across generations in the greenhouse over multiple simulated growing seasons. I found that plasticity determined most of the variation in trait expression and fitness, with evolutionary history playing a minor role in shaping herbivore resistance. I also documented patterns of divergent selection dependent on the presence/absence of herbivores in the maternal environment.
My results characterize plant responses to multiple agents of selection that structure genetic variation across an elevational gradient. Under climate change, populations are likely to encounter novel selective regimes. Plasticity could allow populations to rapidly respond to shifts in historical agents of selection, but local genetic differentiation could hinder adaptation as climate change progresses.