Organisms regularly meet members of their own and other species. Whenever these interactions affect survival and reproduction - the currencies of Darwin's natural selection - they are "social" in evolutionary terms. We study how social interactions affect and define biological systems. We have an interest in animal groups and interactions within genomes, including transposon evolution. But our primary focus is the microbes who commonly live in dense and diverse communities. We are also interested in host-symbionts systems, particularly the mammalian microbiome where we are applying our ideas to engineer beneficial communities for improved health . Our work combines theory and experiments, and we write reviews of key concepts and debates in the study of interacting organisms.