Abstract: Traits showing elevated divergence across sister taxa are often hypothesized to drive speciation. However, comparative studies identifying such patterns cannot distinguish whether divergence of focal traits actively drove speciation, or merely occurred after the fact. In this study, we leveraged a tractable pair of incipient wildflower species to directly test how local adaptation and natural selection contribute to speciation. First, we transplanted the two subspecies into their own and each other’s microhabitats to test for small-scale local adaptation (tens of meters) in two secondary contact zones. Second, we planted >150 recombinant inbred lines into the same microhabitats to measure patterns of natural selection in secondary contact, testing whether the direction and/or phenotypic targets of selection vary across the subspecies’ microhabitats in secondary contact. Here, we show that fine-scale local adaptation may promote spatial segregation in sympatry and reduce the likelihood of hybridization between subspecies, promoting speciation. Furthermore, these data show that selection on both floral traits and physiological traits differ across subspecies' microhabitats in sympatry. Thus, it is likely that selection by both biotic and abiotic drivers (e.g. pollinators and drought) jointly contribute to subspecies divergence and maintenance of incipient species boundaries in this system.
