Mechanisms underlying speciation and adaptation processes in two closely related Drosophila virilis group species

Abstract

Speciation is a slow process that proceeds through populations’ ecological divergence and the development of reproductive barriers. Populations living in the same area (sympatry) are susceptible to the disruptive effects of gene flow and recombination, which can slow down or prevent their divergence. Accordingly, natural selection may favour genetic mechanisms, like chromosomal inversions, which protect divergent loci from the homogenising effects of gene exchange and promote speciation. In this dissertation, I investigated the central aspects of speciation and adaptation using two closely related fly species, Drosophila montana and D. flavomontana. In the first chapter, I found the reproductive barriers between these species to be strong, but not complete. In D. flavomontana, the prezygotic barriers showed signs of reinforcement in sympatric populations, the type of barriers varying according to the length of species coexistence and/or species abundancies. The second chapter showed ecological isolation between D. montana and D. flavomontana to be enhanced by multiple environmental variables and to be largely based on species differences in cold tolerance. The third chapter suggested that chromosomal inversions had originated already before the species’ split, where they may have played an important role in the development of early reproductive barriers and/or ecological differences between local populations of the ancestral form. The last chapter, where I performed repeated interspecific backcrosses, indicated that the X chromosomal inversions, together with an incompatibility locus residing within them, effectively prevent gene flow from D. montana to D. flavomontana. Overall, this dissertation gives a comprehensive overview of the mechanisms underlying speciation and adaptation processes in these two species. It supports the existing speciation theories, but also brings up new perspectives, and shows that finding the final answers in speciation research is extremely challenging.