All living organisms require nitrogen to grow, but atmospheric nitrogen is not directly available to most species and must first be converted into a form suitable for use. Legumes, such as peas, lentils, and soybeans, and several other flowering plant groups can form symbiotic relationships with nitrogen-fixing bacteria and house them in their roots in return for access to usable nitrogen. This symbiotic relationship is essential to ecosystem functioning and agricultural productivity. Despite the strong value in understanding this interaction, scientists have yet to unravel how this symbiosis evolved and its long-term evolutionary consequences for the global diversity of flowering plants. This project will close this gap by resolving the evolutionary relationships of 15,000 nitrogen-fixing species of flowering plants, analyzing the genes associated with their bacterial symbioses, and linking this new knowledge to a comprehensive database of species' habitat and morphological traits. Researchers will build scientific capacity through workshops on cutting-edge data science approaches in biology, targeting high school, undergraduate, and post-graduate levels. Outreach to the general public will increase awareness of the importance of organismal symbioses across the tree of life and how knowledge about these relationships can enhance food security and human well-being. <br/><br/>Researchers will evaluate the macroevolutionary consequences of angiosperms' symbiotic relationships with nitrogen-fixing bacteria by testing four overarching hypotheses. Did bacterial symbiosis enable plant species to invade new, harsher soil environments low in nitrogen? If so, were other plant traits gained that allowed the plants to cope with these extreme habitats? Did the gain of bacterial symbioses allow these plant groups to evolve new species more rapidly than those without this relationship? Did global climate change over geologic time, including the spread of colder and drier habitats and falling atmospheric carbon dioxide levels, drive recent evolution of bacterial symbiosis? Researchers will use sequence data from ca. 230 nuclear loci to reconstruct a time-calibrated phylogenetic framework for the nitrogen-fixing clade of angiosperms and develop a suite of biodiversity informatics methods to generate species-distribution models and functional trait matrices at scale. Researchers will then analyze these data using comparative methods. Outcomes of the research will provide a rigorous understanding of the ecological and evolutionary factors that drove the gains and losses of this symbiosis through time and of how nodulation has itself impacted the diversification and global distribution of angiosperms. Methods developed by the project will facilitate future synthetic analyses utilizing rich, curated data resources across broad phylogenetic, spatial, and temporal scales.<br/><br/>This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.