“Local adaptation interacts with expansion load during range expansion: Maladaptation reduces expansion load”

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Kimberly J. Gilbert, Nathaniel P. Sharp, Amy L. Angert, Gina L. Conte, Jeremy A. Draghi, Frédéric Guillaume, Anna L. Hargreaves, Remi Matthey-Doret, and Michael C. Whitlock

Local adaptation to an environmental gradient reduces expansion load during species range expansions

We have become increasingly aware of the role that demographic history can play in contributing to varying levels of genetic diversity and fitness of modern populations. In particular, species range expansions create a unique scenario of repeated population bottlenecks that prolong the reduced efficacy of selection in small populations through time and space. This can result in reductions in population fitness due to the expansion process, termed expansion load. The degree to which expansion load contributes to the fitness of populations in the real world is debated, with contrasting results being found in human populations that have expanded out of Africa. To investigate other processes that might contribute to expansion load, researchers from the University of British Columbia, CUNY, and University of Zurich have conducted simulations that combine the presence of deleterious mutations with the presence of an environmental gradient during range expansion to understand the impact and interactions that local adaptation has on load accumulation.

They find that as the change in environment becomes greater over space, populations have more difficulty in locally adapting, leading to the slowing of range expansion. As this slowing occurs, less genetic drift acts at the range edges, reducing the potential for buildup of deleterious mutations through a process called allele surfing. Despite this reduction in expansion load, populations are still faced with the difficulty of locally adapting on steeper environmental gradients, so overall fitness is not improved.

This research uncovers an important interaction that may explain the absence or reduction of expansion load found in nature compared with predictions from theoretical work. This also highlights an important concern for species undergoing future expansion due to climate change or other anthropogenic forces, where we may expect fitness reduction due to several interacting factors. Read the Article