The Effects Of Migration On Evolutionary Dynamics In Natural Populations Of Drosophila Melanogaster

Understanding how major mechanisms of evolution, i.e. selection, gene flow, mutations, and genetic drift, shape natural populations is a major goal of evolutionary biology. Due to climate change, the environment and ranges of species are changing rapidly. Through rapid adaptation, populations can potentially respond to rapid environmental changes. Yet, how gene flow and rapid adaptation work together to shape natural populations is still an open question in biology. North American populations of Drosophila melanogaster provide a strong model to investigate this relationship as they exhibit latitudinal clines, low levels of variation, and rapid adaptation to environmental parameters that change rapidly with season. In this dissertation, I investigate the relationship between migration and rapid adaptation using D. melanogaster as a model system. I first demonstrate that migration may facilitate adaptation due to epistatic interactions through highly replicated laboratory and field experiments where evolutionarily realistic populations are manipulated to simulate a single migration event. I exposed populations to different temperature regimes and used parallelism in responses to determine the direction of the adaptive response. By comparing the responses of populations that received migration to controlled gene flow events and simulated expectations, I showed that migration events might facilitate adaptive responses. Finally, to demonstrate the ecological relevance of migration, I used population genetics and Bayesian inference methods to estimate the connectivity between geographically separate natural populations of D. melanogaster and I showed that mid- and long-range connectivity is likely. These results together demonstrate that migration can be an ecologically realistic mechanism that facilitates adaptive response in nature. The adaptive outcome depends on the genetic architecture of the fitness-associated traits that respond to dynamic selection pressures. This work integrates the phenotypes and their genotypic architectures to explain the evolutionary responses through highly replicated experiments in the laboratory and field. Together, this work represents a novel contribution to the analysis of evolutionary impacts of migration and gene flow in natural populations.