Population Divergence and the Evolution of Reproductive Isolation in <i>Drosophila Subquinaria</i>

To better understand the sources of biological diversity we find in nature, we need insight on the mechanisms underlying the evolution of reproductive isolation and population divergence. Biological systems with patterns of naturally occurring variation in reproductive isolation can provide insight into mechanisms underlying its evolution, the genetic architecture of diverging traits, and the influence of genetic constraints on responses to selection. In this thesis, I used both experimental evolution and quantitative genetic approaches to study these questions in a system of reproductive character displacement in the North American mushroom-feeding fly <i>Drosophila subquinaria</i>. This species exhibits a pattern of stronger reproductive isolation against the closely related <i>Drosophila recens</i> in sympatry, where both female mating preferences and male chemical signaling traits have diverged from the ancestral allopatric populations. In chapter 2, we recreated this reproductive character displacement in the lab with replicate experimental populations of allopatric <i>D. subquinaria</i>. In chapter 3 I found divergence between allopatric and sympatric <i>D. subquinaria</i> in the genetic correlations among a set of these chemical signalling traits. In chapter 4, I found evidence indicating that this pattern of genetic correlations has played a key role in shaping the divergence of these traits, and that further adaptation in this system is likely constrained. Future research in this system would benefit from identifying the ancestral pattern of genetic correlations among these traits to better understand their stability over evolutionary time and from tracking the evolution of genetic correlations under controlled experimental conditions where the presence and absence of the sympatric species, <i>D. recens</i>, is manipulated.