Competition is fact of life: all organisms are constantly engaged in competition for mates and other resources, and they often do so aggressively. These competitive interactions can have important consequences not just for the individuals that engage in them but also for the evolution and persistence of populations and species. For example, scientists have shown that new species can evolve when territory holders direct more aggression towards competitors that resemble themselves compared to those that are dissimilar. However, we know very little about the physiological and neurobiological processes that control this kind of aggression that is biased toward individuals that look more similar to oneself. The proposed research will use a combination of social learning experiments, genetic analysis, neural activity mapping, and brain gene expression analyses to investigate this question using two highly social cichlid fish species that differ in body coloration. This highly integrative approach will advance our understanding of how social competition influences evolutionary change and biodiversity. The multidisciplinary project will build collaborative relationships among institutions in the US (two R2 and one R1), Switzerland, and Tanzania. It will also provide research and training opportunities for undergraduate and graduate students. This project will promote STEM engagement through the development of an interactive exhibit on cichlid social behavior at a local children’s dis