Dispersal evolution as a driver of island biodiversity

Island biodiversity reflects a tension between isolation and connectivity. Dispersal lies at the heart of this paradox: it enables colonization and ecological opportunity, yet excessive gene flow can constrain divergence and speciation. We use a spatially explicit, individual-based model to ask how dispersal evolution influences speciation and long-term diversity. Across archipelagos, rare long-distance dispersal events trigger colonization and divergence, after which dispersal traits evolve along two trajectories: increasing or decreasing range. These outcomes correlate with island size and isolation. Structured landscapes stabilize species richness through low turnover, whereas continuous landscapes remain species-poor despite high diversification rates. Our results reveal that spatial structure and dispersal evolution jointly govern biodiversity, underscoring the need to integrate movement into theories of speciation.