The Ecological and Evolutionary Drivers of Speciation in Walnut-Infesting Flies

Speciation is the evolutionary process by which previously interbreeding populations become separate species and is thus responsible for the vast diversity of taxa on earth. Investigating the biogeographic context of speciation is important to understand how speciation occurred in the past and how species are maintained at present. However, ongoing climate change is presently shifting species distributions and changing interspecific interactions, which may in turn influence the future existence of species. In my dissertation, I investigated the factors driving speciation in the walnut-infesting Rhagoletis suavis species group to understand 1) how and where species diverged in the past, 2) how climate change is affecting present species distributions and interactions, and 3) what evolutionary and ecological mechanisms are involved in reproductively isolating species. First, I explored how biogeography shaped the past speciation history of walnut flies through a genetic sequence analysis of mitochondrial DNA (mtDNA). Walnut flies diverged across the eastern Sierra Madre Oriental mountain range in Mexico, similar to other Rhagoletis species groups. However, I show that the timing of divergence based on mtDNA sequences of walnut-infesting R. suavis differs substantially from the other species groups. Second, I modeled the role of climate change in shifting present-day species distributions, comparing past and present distributions and predicting the change in distributions with species distribution models. Walnut fly species have moved predictably in the United States over the past 40 years, shifting to higher elevations in the Southwest United States likely in response to increasing temperature, and moving eastward in the Midwest United Statesin response to decreasing precipitation. Third, I investigated ecological and evolutionary mechanisms of reproductive isolation to understand barriers to gene flow in areas where species co-occur. Gene flow is restricted by temporal, sexual, and post-mating isolation. In nature, hybridization was only found between R. juglandis and R. completa in the Southwest United States. Comparing these results in the R. suavis species group to other Rhagoletis species groups with different modes of speciation can help to uncover patterns in the evolution of reproductive isolation.