Competition between species shapes the ecological communities all around us, determining which species are common and which are rare, which coexist, and which exclude one another. Rarely do ecologists recognize that as species compete over multiple generations, they are simultaneously evolving, and that this evolution involves traits that determine species’ competitive outcome and their ability to coexist. Importantly, such rapid evolution may also determine the effectiveness of biocontrol agents targeting crop pests, the resistance of gut bacterial communities to invading pathogens, and the persistence of species threatened by biological invasions. Thus, better understanding of how competition plays out as species evolve to one another is important for both our basic understanding of ecological communities and the applications of this knowledge in agriculture, nature conservation, and health. In this project, researchers will measure how rapid evolution of orchard flies in response to their competitors determines (1) how traits and genetic factors seasonally evolve over summer and fall, (2) their winning and losing in competition, and (3) their ability to coexist. The project will train a postdoctoral scientist, graduate and undergraduate students and form the basis of outreach efforts to nearby community, and a 4-year college, high school students and the public. The research will integrate theory and field experiments in the northeastern United States to address thr