A debate continues on the relative importance of history, evolutionary constraints, and current ecological processes in organizing species into communities. The study of closest relatives is especially relevant to this debate and lies at the nexus of these processes. Closest relatives are, by definition, most genetically similar, and as a result of this shared ancestry are generally more similar in traits and environmental preferences (Darwin 1859). From an evolutionary perspective, closest relatives may co-occur because of dispersal limitation and niche conservatism (Wiens and Graham 2005), but may not co-occur if allopatric speciation is common (Jordan 1905). From an ecological perspective, limiting similarity suggests that closest relatives should not coexist due to their ecological similarity (Hutchinson 1959), but ecological facilitation could favor coexistence of closest relatives if they share mutualists (Boucher 1985). How evolutionary history affects the outcome of contemporary ecological interactions and species distributions represents the influence of the past on present and future communities (a theme of these meetings). Whether closest relatives coexist reflects the often opposing effects of limiting similarity, mode of speciation, reproductive isolation, niche conservatism, competition and facilitation, which may be strongest in sister taxa. Taking advantage of new resources in phylogenetics, species distribution and niche models, and experimental approaches, we address classic problems on coexistence of closest relatives. Our symposium will synthesize commonalities and differences in patterns of coexistence of closest relatives for plants, birds, mammals, and insects.