Selection for increased reproductive isolation, termed reinforcement, was first proposed by Dobzhansky in 1937 and remains a controversial topic in the study of species formation. Reinforcement is increasingly being tested in many closely related organisms, however, only a single study has tested its role in plant species formation and no study has directly measured natural selection for increased reproductive isolation in the face of ongoing gene exchange. Comparative and experimental methods will be used to examine the role of reinforcement in the plant genus Mimulus, a model system in evolution and ecology. Increased floral divergence was observed between closely related species pairs with overlapping ranges, a pattern consistent with reinforcement. Research funded by this award will address whether the observed floral divergence is associated with mating system shifts and directly measure selection on floral traits that increase reproductive isolation in the field. Self-pollination rates of multiple populations of 16 species pairs will be quantified in the greenhouse, and molecular tools will be used to confirm self-pollination rates in natural populations in the field. Those studies will be followed by a field-based genotypic selection experiment to measure selection on floral traits that increase reproductive isolation. This research will constitute a strong test of a mechanism that is thought to play an important role in the formation of new species. It will have both strong intellectual merit and, because it will have the potential to improve public understanding of the evolutionary process, strong broader impacts. Other broader impacts of this project include mentoring of two undergraduate students who will participate in the research, and support for a doctoral student.
