Coexistence in Close Relatives: Beyond Competition and Reproductive Isolation in Sister Taxa

Understanding coexistence of closely related species lies at the nexus of disentangling how historical and ecological factors govern patterns of biodiversity. The criteria determining local coexistence in close relatives have typically been, for ecologists, whether these species meet conditions of stable coexistence when competing for resources; in contrast, evolutionists often consider coexistence of close relatives from the perspective of complete reproductive isolation. Clearly, both of these conditions must be met, but for coexistence in ecologically and phenotypically similar close relatives to occur, species must overcome a diverse suite of challenges beyond just these. The goal of this review is to present a more holistic, eco-evolutionary view of the factors governing successful coexistence of close relatives, expanding our consideration to recent clade mates, not just sister taxa, and drawing on new technologies and approaches to explore more deeply this classic conundrum. We review the major concepts explaining patterns of coexistence in close relatives, distinguishing between forces related to ( a ) history, speciation, and extinction; ( b ) divergence, dispersal, and drivers of range overlap; and ( c ) successful ecological coexistence of species once in contact. We end by highlighting major gaps and ways forward, including moving beyond the strict dichotomy of local and regional scales and scrutinizing non-native introductions as analogs of secondary contact to tease apart factors contributing to coexistence in real time. By reviewing literature from both ecological and evolutionary perspectives, we hope to illustrate the multifaceted factors that drive coexistence of close relatives and to highlight new questions and approaches that might expand this age-old topic to nonsister close relatives, which often face similar challenges to coexistence as those faced by sister taxa.