Limits to adaptation along environmental gradients

Abstract

Brief SummaryWhy do species ranges often end when no obvious change in the environment suggests they should? Our theory explains that there is an inherent limit to adaptation arising in any (finite) natural population, and identifies the key parameters that determine this limit to a species range. Two observable parameters describe the threshold when adaptation fails: i) the loss of fitness due to dispersal to a different environment, and ii) the efficacy of selection relative to stochastic effects in finite populations.\n\nAbstractWhy do species not adapt to ever-wider ranges of conditions, gradually expanding their ecological niche and geographic range? Gene flow across environments has two conflicting effects: while it increases genetic variation, which is a prerequisite for adaptation, gene flow may swamp adaptation to local conditions. In 1956, Haldane proposed that when the environment varies across space, \"swamping\" by gene flow creates a positive feedback between low population size and maladaptation, leading to a sharp range margin. Yet, current deterministic theory shows that when variance can evolve, there is no such limit. Using simple analytical tools and simulations, we show that genetic drift can generate a sharp margin to a species range, by reducing genetic variance below the level needed for adaptation to spatially variable conditions. Aided by separation of ecological and evolutionary time scales, the identified effective dimensionless parameters reveal a simple threshold that predicts when adaptation at the range margin fails. Two observable parameters determine the threshold: i) the effective environmental gradient, which can be measured by the loss of fitness due to dispersal to a different environment, and ii) the efficacy of selection relative to genetic drift. The theory predicts sharp range margins even in the absence of abrupt changes in the environment. Furthermore, it implies that gradual worsening of conditions across a species habitat may lead to a sudden range fragmentation, when adaptation to a wide span of conditions within a single species becomes impossible.