Genetic diversity decreases toward species range edges

Chloé Schmidt , Jussi Mäkinen, Jean-Philippe Lessard , Colin Garroway

Changing species distributions amidst global change underscores a pressing need to better understand the processes that cause range limits. However, despite over a century of work, there is no consensus regarding whether there are generalizable contributors to the emergence of species range limits. Species’ geographic distributions frequently taper without obvious physical or environmental barriers, raising a fundamental question about whether evolutionary processes contribute to the emergence of species range limits species. Theoretical models suggest that spatial variation in the strength of genetic drift can reduce the efficiency of natural selection to the point where range limits form. The extent to which this theory holds in real-world populations is uncertain. With data comprising 37,397 genotypes sampled across 59 mammal species’ ranges from 1,271 sample sites, we show that effective population size and genetic diversity decline toward range edges, while genetic differentiation increases, indicating stronger drift and reduced gene flow in peripheral populations. These results demonstrate that limits to range expansion arise predictably as populations become small and isolated, which reduces the efficiency of natural selection. Our findings point to genetic drift as a general mechanism constraining range expansion, helping to explain the widespread structure of species’ distributions and help explain commonly observed idiosyncratic patterns of range configuration and the organization of biodiversity.