LAUSR

Significance Microscopic plankton form the ecological and biogeochemical foundation of almost all marine ecosystems. In the fluid ocean environment, biodiversity and community structure are determined by the poorly constrained balance of local selection and global dispersal. While ocean currents have the capacity to rapidly connect distant locations, we use numerical simulations to show that extremely high rates of adaptation are required for populations to traverse large-scale gradients in environmental variables such as temperature. Changing the assumed balance of selection, adaptation, and dispersal in our simulations has pronounced effects on the simulated community structure, accounting for emergent patterns in the global ocean microbiome and emphasizing the importance of evolutionary history in global marine biodiversity and biogeography. Marine microbial communities are highly interconnected assemblages of organisms shaped by ecological drift, natural selection, and dispersal. The relative strength of these forces determines how ecosystems respond to environmental gradients, how much diversity is resident in a community or population at any given time, and how populations reorganize and evolve in response to environmental perturbations. In this study, we introduce a globally resolved population–genetic ocean model in order to examine the interplay of dispersal, selection, and adaptive evolution and their effects on community assembly and global biogeography. We find that environmental selection places strong constraints on global dispersal, even in the face of extremely high assumed rates of adaptation. Changing the relative strengths of dispersal, selection, and adaptation has pronounced effects on community assembly in the model and suggests that barriers to dispersal play a key role in the structuring of marine communities, enhancing global biodiversity and the importance of local historical contingencies.