Environmental heterogeneity determines evolutionary pathways to species coexistence

A cornerstone of biology is that species interactions drive evolution of niche divergence, thereby maintaining biodiversity. Yet controlled experiments consistently find evolution of competitive-ability differences instead, creating a fundamental discrepancy with patterns observed in nature. Here, we investigated the role of environmental heterogeneity, a major determinant on niche availability, on evolution of species interactions. Integrating theoretical models with experimental evolution in freshwater algal communities, we tracked the real-time evolution of coexistence mechanisms. In homogeneous environments, we found that although species consistently converged in resource use, the divergence in competitive-ability differences led to competitive exclusion. Conversely, in heterogeneous environments, species first evolved increased niche differences, which transiently stabilized coexistence before the communities converged to long-term, stable coexistence via both small niche and competitive-ability differences. Our study provides direct evidence that environmental heterogeneity determines whether evolution of species coexistence proceeds through niche or competitive-ability differences. It highlights that understanding the ecological context can help to understand and predict the evolutionary pathways that sustain biodiversity.