Most species are formed when geographic barriers physically separate populations for long periods of time. For species to remain distinct once they are in contact with one another they must evolve reproductive isolation often through ecological differentiation. Without reproductive isolation incipient species will hybridize and fuse back together into a single species. Without ecological differentiation intense competition will often cause one species to go locally extinct. Species that presently co-occur may have evolved ecological differences while physically separated, or they may have evolved differences in the same area through selection to minimize hybridization or competition. Little is known about the ecological trait differences that allow species to coexist in the same area or the genetic loci responsible for ecological differences between species. This study will identify the traits and genetic loci underlying ecological differences that enable coexistence between two western North American wildflower species. The broadly distributed Mimulus guttatus, and its relatively rare sister species Mimulus nudatus co-occur in some areas but live in distinct microhabitats that differ in the onset of summer drought. Mimulus nudatus grows on serpentine slopes that dry out early in the spring, while M. guttatus inhabits ephemeral streams where drought imposed death is later than in the M. nudatus habitat. The rare endemic possesses several traits that are likely adaptations to earlier drought, including a shorter critical photoperiod to induce flowering resulting in an earlier production of a flowering shoot (bolting) and an earlier flowering time than the more common M. guttatus. Understanding how rare endemic species are maintained with more widespread species has important implications for conservation and the preservation of biodiversity. Determining the underlying genetic changes to enable species coexistence will shed light on the interaction among genetic, ecological, and evolutionary processes that contribute to the maintenance of species biodiversity. This project will maintain and develop new mentoring opportunities targeting underrepresented groups at the middle school, high school, and undergraduate level. The results of the proposed research will be broadly disseminated though talks and publications to the public and scientific community.
This project will explicitly test the ecological and evolutionary forces maintaining spatial, phenotypic and genetic differences between two closely related species of Mimulus. Ecological similarity between sympatric species may lead to competitive exclusion. Consequently, ecological divergence is a necessary prerequisite for stable coexistence. Differences in flowering time between the common M. guttatus and the rare endemic M. nudatus are hypothesized to allow these species to occur in sympatry by inhabiting distinct microhabitats that differ in the onset of summer drought. Three quantitative trait loci (QTL) have been identified that profoundly influence differences in critical photoperiod and bolting and have already been genetically mapped, yet we do not know how these loci contribute to microhabitat differentiation and species interactions in nature. This project will utilize QTL mapping, reciprocal transplant and competition experiments to test: (1) whether flowering time differences in the field have the same or different genetic basis from photoperiod and bolting in the lab, (2) whether photoperiod and bolting loci are adaptive and enable coexistence by contributing to niche differentiation, and (3) whether ecological or reproductive character displacement are responsible for flowering time differences in the two closely related sympatric species.
