A fascinating challenge in biology is to understand how the evolution of molecular mechanisms leads to phenotypic variation. In plants, the morphology of flowers is subject to strong selective pressures, given their central role for reproduction. A common trend of flower evolution is the reduction of floral display following the transition from out-breeding via animal pollinators to selfing. This reduction of flower size is commonly assumed to reflect a conflict between pollination efficiency and energy cost; for a selfing species, pollinators become dispensable, and the presumed energetic cost to build large flowers for pollinator attraction would have losts its adaptive value. The genus Capsella is a genetically amenable model to dissect the genetic basis of this major trend in flower evolution. The selfing species C. rubella, with its dramatically reduced flowers, evolved from C. grandiflora, an outbreeding species with large flowers. The Lenhard group has identified and mendelized quantitative trait loci (QTL) underlying the change in flower size in the genus Capsella. This project proposes to use three complementary approaches to better understand evolutionary changes in flower morphology in relation to changes in the mating system. (1) A positional cloning approach will be used to identify the causal gene underlying the major flower size QTL. (2) Comparative QTL mapping and population genetic analyses will be employed to reconstitute the history of floral evolution in Capsella. (3) Isogenic lines differing only in flower size will be generated to rigorously determine the fitness value of flower size in a selfing species. This project will provide important novel insight into the genetic mechanisms and the forces driving evolutionary changes in morphology; at the same time, it will offer a highly relevant multidisciplinary training at the overlap of molecular biology and evolutionary genetics, which will be an increasingly sought-after combination.