Dispersal evolution in currents: spatial sorting promotes philopatry in upstream patches

Substantial literature is devoted to understanding dispersal evolution, but we lack theory on how dispersal evolves when populations inhabit currents. Such theory is required for understanding connectivity in freshwater and marine environments; moreover, many animals, fungi and plants rely on wind-based dispersal, but the effects of currents on dispersal evolution in these organisms is unknown. We develop an individual-based model for evolution of dispersal probability along a linear environment with a unidirectional current. Even a slight current substantially reduces overall emigration probability compared to no current. Under stronger currents, emigration can be drastically reduced, especially in the upstream patches. When introducing rare long-distance dispersal that is not subject to the current, higher emigration probabilities evolve and the spatial variability in emigration propensity along the stream is reduced. Our results provide an alternative solution to the long debated ‘drift paradox' concerning the loss of individuals from upstream populations due to advective forces. A combination of natural selection and spatial sorting generates and maintains downstream gradients in dispersal propensity, where individuals from upstream populations tend to be substantially more philopatric. This is likely to have major implications for ecological and genetic connectivity that will impact effective management strategies for populations inhabiting currents.