Abstract
Species ranges are frequently constrained by local adaptation near their boundaries. Yet, range expansions typically occur along continuous environmental gradients, where conditions change gradually without abrupt transitions. This general observation raises a fundamental question: how do limitations to local adaptation and range expansion arise when abrupt transitions are absent? In this dissertation, I used the wildflower Campanula americana to explore how environmental gradients of differing steepness interact with microevolutionary factors, like genetic drift and natural selection, to influence local adaptation and range expansion. I investigated these patterns using natural populations of C. americana found along a steep environmental gradient, driven by elevation in the Appalachian Mountains, and along a shallow environmental gradient, driven by latitude, in the North American Midwest. In Chapter 1, I tested patterns of local adaptation by planting common gardens and modeling adaptive genetic differentiation over the steep elevational and shallow latitudinal gradient. I found that local adaptation is limited along both gradients, though patterns of fitness differ among gradients. Along the steep gradient, fitness in range-edge populations is poor, while fitness is generally high among populations along the shallow gradient. In Chapter 2, I explored patterns of phenotypic and genetic selection along gradients. I found that differences in the strength and direction of selection on reproductive phenology were strong along the steep gradient. Additionally, adaptive alleles shifted in frequency more slowly along the shallower latitudinal gradient than along the steep elevational gradient. In Chapter 3, I evaluated genetic load and found it tended to increase toward range limits. Furthermore, along a shallow environmental gradient, fitness costs associated with genetic load were increased in stressful environments. In Chapter 4, I investigated patterns of gene flow and drivers of genetic differentiation along each gradient. I found asymmetric gene flow along the steep environmental gradient and strong signatures of isolation-by-distance. Along the shallow environmental gradient, gene flow among populations was symmetric and genetic differentiation among populations was not associated with either geographic (i.e., drift) or environment distance (i.e., selection). Along both gradients, effective population size declined toward range limits. Together, these results paint a picture of ecological specialization and local adaptation constrained by genetic drift and gene flow along steep environmental gradients; and ecological generalization along a shallow environmental gradient, where further range expansion is likely constrained by interactions of genetic drift and environmental stress. Finally, in Chapter 5, I assessed how dynamics of postglacial range expansion influence outcomes of contact between intraspecific lineages. I found that all contact zones arose under a model of lineage divergence in parapatry, but gene flow was significantly greater in a southern contact zone near the species’ rear edge. At the northern and mid-latitude contact zones, gene flow was minimal and strongly asymmetric. These results suggest that historic range dynamics can strongly influence outcomes of contact across a species range. Together, my dissertation presents a cohesive narrative demonstrating how the rate of environmental change along gradients influences patterns of genetic drift and natural selection. In turn, these patterns shape interactions among populations, influencing local adaptation, range expansion, and speciation potential.
