Understanding the expansion of populations across a landscape is essential for predicting the speeds at which introduced species invade their new ranges and native species invade habitats that are made newly suitable by climate change. Eco-evolutionary theory suggests that rapid evolution can accelerate range expansion speed. In addition to average speed, recent experimental studies reveal that evolution can also influence the amount of variability across replicates of spreading populations, and in contrasting ways, with some studies showing increased variability and others decreased. Variability is a key measure of spread dynamics because it affects how precisely we can predict the trajectories of expanding populations, something ecologists are increasingly called upon to do. We developed a framework for understanding how evolution may affect the variability, and thus predictability, of range expansion. We then evaluated several hypotheses within this framework to help resolve current, conflicting results from recent empirical studies.
Results/Conclusions
Our predictive framework draws on ideas from population genetics and spread theory, to understand when, why, and in what direction evolution will modify the variability of range expansion. Our framework revolves around the balance of variance-generating and variance-reducing evolutionary processes, and the factors that may tip this balance, including population size, mating system and genetic architecture. We developed several new metrics to quantify population size at the leading edge. None are sufficient to fully capture the contrasting effects of evolution on variability across replicate range expansions from recent experiments, suggesting that multiple factors must contribute to the conflicting results. Despite the complexity, resolving when, why, and in what direction evolution may influence the variability of population spread is an urgent challenge, and we will conclude by highlighting a way forward for studying eco-evolutionary dynamics of range expansion.