Rivers as evolutionary and ecological racetracks: spatial sorting as a potent force in riverine systems

Rivers are among the most biodiverse ecosystems on Earth, and are often viewed as arenas of local adaptation driven by longitudinal environmental gradients. However, river networks are also highly structured, directional systems in which repeated movement is integral to the life history of many species. Here, we outline how spatial sorting-the non-random assembly of phenotypes and genotypes through space due to trait-dependent movement-represents an underappreciated process shaping evolutionary and ecological divergence in riverine systems. We synthesise theory and empirical evidence to show how directional flow can progressively filter movement-related traits, generating fine-scale genetic and phenotypic structuring even in the absence of natural selection. We review evidence from fishes and other aquatic taxa demonstrating trait-dependent movement, selective barrier passage, genetic differentiation within rivers, and rapid divergence over short spatial and temporal scales. We emphasise that spatial sorting operates alongside local adaptation by structuring populations in ways that facilitate assortative mating, effectively accelerating divergence. We highlight spatial sorting as a potent, yet overlooked, contributor to variation in riverine biodiversity and discuss its implications for the eco-evolutionary dynamics of populations and freshwater conservation.