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Abstract
Central to the diversification of life is the process of speciation. Speciation occurs when barriers arise that prevent interbreeding among populations, thus resulting in new evolutionary lineages. This dissertation investigates the formation of rapidly evolving barriers that can isolate populations and give rise to new lineages. I focus on the sister taxa Drosophila recens and D. subquinaria, generalist mushroom feeding flies found in boreal forests in North America. These species have come into secondary contact after the last glaciation, and they are partially reproductively isolated. Here, I study the reproductive isolating mechanisms that prevent gene flow between these species. First, I investigate levels of non-competitive gametic isolation and postmating prezygotic barriers between crosses of D. recens females and D. subquinaria males. Postmating prezygotic barriers are often under strong selection in Drosophila and include measures of sperm effectiveness and egg laying response. I found these barriers are strong and reduce the number of offspring produced in heterospecific crosses, and thus strengthen isolation between species. Second, I studied the earliest stages of incipient speciation between populations of D. subquinaria. Females from inland populations show premating isolation against males from coastal populations, and here I investigate whether there is evidence of post-mating reproductive isolation. I conducted crosses between conspecific coastal and inland D. subquinaria, and in both directions of the cross I find strong evidence of postmating barriers, including reduced number of eggs laid, reduced egg hatch, and reduced offspring number. These results support previous work that D. subquinaria populations are in the early stages of incipient speciation. Finally, I characterize and analyze epicuticular hydrocarbon composition across 24 taxa of Drosophila, including members of the Quinaria, Falleni, Acutilabella, and Testacea species groups. Epicuticular hydrocarbons play a role in ecological functions, such as water balance, and they have been co-opted as contact pheromones. I found substantial diversity in epicuticular hydrocarbons across species, with significant variation attributable to sexual dimorphism, host dietary state, and phylogenetic group. Additionally, evolutionary modeling of these compounds suggests a mix of phylogenetic and nonphylogenetic evolution, indicating rapid evolution on a trait often associated with both sexual selection and speciation.