Colonizing populations often adapt to novel environments, evolving higher growth rate and carrying capacity, which can promote range expansion. In addition, dispersal ability of individuals residing at range edges can evolve rapidly, further contributing to range expansions. Although rapid evolution of colonizing populations is common, it is less clear whether rapid evolution is truly a driver of range expansion, or if it is merely a by-product of the population encountering a novel environment. Understanding the importance of evolution in driving the growth and spread of colonizing populations is important in predicting the speed and spatial extent of invasions and guiding effective management of invasive species. To assess the role of rapid evolution during colonization, we founded replicate populations of Tribolium beetles in a novel environment and tracked their growth and spread across six generations. Populations were either allowed to evolve or restricted from evolving by replacing beetles one-for-one with beetles from a stock population each generation, thus inhibiting adaptation to the novel environment and spatial selection across the landscape. After six generations, we performed a common garden experiment to evaluate whether populations evolved higher growth rates or dispersal ability (at core vs. range edges) compared to populations that were restricted from evolving.
Rapid evolution was a significant driver of colonization. Evolving populations grew nearly three times larger and spread 40% further than non-evolving populations over the course of six generations. The increased speed of range expansion for evolving populations was largely driven by adaptation to the novel environment that doubled growth rates of evolving compared to non-evolving populations. Adaptation also increased the carrying capacity of evolving populations, and the associated increase in population density likely contributed to speed of range expansions, as dispersal increases in crowded conditions. We found small and variable evidence for evolution of increased dispersal tendencies: individuals from the edge of evolving populations exhibited a small increase in dispersal as compared to those from the core when dispersing at low density, a pattern consistent with other studies. We hypothesize that adaptation to the novel environment could have overwhelmed the subtler effect of dispersal evolution at range edges. This experiment provides valuable empirical evidence that rapid evolution is a driver rather than merely a by-product of range expansion. Our results suggest that understanding characteristics of founding populations that promote rapid evolution (e.g. high genetic diversity) may be crucial in predicting the speed and spatial extent of invasions.