How multiple species coexist in the face of limiting resources remains one of the central questions in ecology. Recent theoretical and empirical studies have documented the importance of evolutionary forces in species coexistence. However, there remains a disconnect between these two approaches, as empirical studies are generally too short to explore long-term coexistence and theoretical studies are rarely specific enough to allow for meaningful comparisons with natural systems. Here I combine field data with simulation modeling to test how a genetic trade-off between intra- and interspecific competitive ability alters the long-term coexistence of plant species. In two of the three species combinations tested, coexistence was possible only in models that included evolutionary processes. Additionally, genetic variation and the resultant evolutionary change allowed for coexistence under a much wider range of ecological conditions by both increasing equalizing (neutral) effects and providing a novel evolutionary stabilizing (niche) effect. Biodiversity is declining at both the species and the genetic levels. These results suggest that conserving species diversity may depend critically on our ability to conserve the genetic diversity within species.