The role of local adaptation in the evolution of reproductive isolation in <i>Diodia teres</i>
Numerous studies of local adaptation have shown that populations can adapt quickly to local environmental conditions. Other studies have shown that sister species tend to occur in different environments. Recent work has gone a long way toward showing that adaptation can directly result in reproductive isolation under some circumstances, but few studies have attempted to measure the effects of local adaptation on the degree of reproductive isolation. Here I have attempted to bring together the research on local adaptation and studies of reproductive isolation to test hypotheses of the ultimate causes and mechanisms of local adaptation, and their consequences for the evolution of reproductive isolation between locally adapted populations. In the first chapter I describe a test of the underlying mechanism of local adaptation between populations of the annual plant Diodia teres. In the second chapter (written in collaboration with Alice A. Winn), I test the hypothesis that the degree of adaptive divergence between six populations of this species is correlated with the degree of environmental variation between populations. The third chapter is a test of hypotheses about the differences between the strength of prezygotic and postzygotic reproductive isolation. In the fourth chapter I test hypotheses that reproductive isolation is correlated with the degree of adaptive and nonadaptive divergence between populations. I found evidence of local adaptation between some populations, but was not able to identify traits that were responsible for that adaptation. The degree of adaptation was not correlated with environmental differences, suggesting that divergent selection is not the only force acting on reproductive isolation. I found that postmating/prezygotic isolation was stronger than postzygotic isolation, and that the degree of divergence was not correlated with any measure of reproductive isolation. Overall this study shows that adaptation to local conditions can be associated with the evolution of reproductive isolation, but it also shows that divergent selection alone does not account for all adaptive divergence and isolation. It is one of the few studies to quantify reproductive isolation early in divergence, and to examine the relationships between divergence and reproductive isolation over a range of variation, not limited to a single pair of nearly completely isolated populations.