For more than half a century nearly all attempts to understand how species diversity is maintained have focused on ecological processes, while largely ignoring the role of evolution. Evolution was assumed to be fundamentally slow, and therefore could not possibly influence the maintenance of species diversity in contemporary time. Here we test this assumption by leveraging a powerful new empirical system for studying eco-evolutionary dynamics based on a guild of globally distributed, floating, freshwater plants. Using experiments that disrupt the ability of species to evolve to competitors, we show that interspecific competition causes rapid evolution, and that this evolutionary change immediately feeds back to change the population trajectories of the competing species. While classic theory suggests these changes should occur via the evolution of niche differentiation, we show that evolution influences species coexistence by reversing the competitive hierarchy. Our study suggests that species diversity maintenance in contemporary time cannot be understood without considering the effects of rapid evolution.