AbstractUnderstanding the dynamics of biological invasions is crucial for managing numerous phenomena, from invasive species to tumours. While Allee effects (where individuals in low-density populations suffer lowered fitness) are known to influence both the ecological and evolutionary dynamics of invasions, the possibility that an invader’s susceptibility to the Allee effect might itself evolve on an invasion front has received almost no attention. Since invasion fronts are regions of perpetually low population density, selection should favour vanguard invaders that are resistant to Allee effects. Evolution in response to this pressure could cause invasions to transition from pushed waves, propelled by dispersal from behind the invasion front, to pulled waves, driven by the invasion vanguard. To examine this possibility, we construct an individual-based model in which a trait that governs resistance to the Allee effect is allowed to evolve during an invasion. We find that vanguard invaders rapidly evolve resistance to the Allee effect, causing invasions to accelerate. This also results in invasions transforming from pushed waves into pulled waves, an outcome with consequences for predictions of invasion speed, the population’s genetic structure, and other important behaviours. These findings underscore the importance of accounting or evolution in invasion forecasts, and suggest that evolution has the capacity to fundamentally alter invasion dynamics.
Evolution transforms pushed waves into pulled waves
