Dense seasonal sampling of an orchard population uncovers population turnover, adaptive tracking, and structure in multiple Drosophila species

Organisms require genetic variation in order to evolve, and balancing selection is one way by which genetic variation is maintained in genomes. Adaptation to temporally varying selection is a mechanism of balancing selection that is not well characterized in natural populations. In my dissertation, I utilize the fruit fly Drosophila melanogaster, which adapts to seasonally varying selection, to better understand balancing selection through adaptation to temporally varying selection. In Chapter 1, I utilize frequent sampling across three years to identify the kinds of loci involved in adaptation to seasonal selection as well as the environmental drivers of selection. I find that coding loci and previously identified seasonally adaptive loci are involved in adaptive differentiation through time. I also identify extreme hot and extreme cold temperatures to have the strongest selective force on allele frequency changes through time. In Chapter 2, I investigate the concept that spatial population structure can bolster the ability of temporally varying selection to maintain large amounts of genetic variation across long periods of time by asking if natural populations of fruit flies have population structure. I identify signals of population structure in natural fruit fly populations, which contradicts long standing assumptions about fruit fly migration behavior. In Chapter 3, I ask if seasonal adaptation is a unique phenomenon, or if other species of fruit flies also exhibit signals of adaptation to seasonally varying selection. I find that other species of fruit flies look like established, overwintering populations, indicating that they are likely also adapting to seasonal change. Overall, my dissertation deepens our understanding of this model system of balancing selection.