Expansion dynamics and marginal climates drive adaptation across geographic ranges

Every species experiences limits to its geographic distribution. Some evolutionary models predict that populations at range edges are less well-adapted to their local environments due to drift, expansion load, or swamping gene flow from the range interior. Alternatively, populations near range edges might be uniquely adapted to marginal environments if they experience strong selection. In this study, we use a database of transplant studies that quantify relative fitness at broad geographic scales to test how local adaptation, site quality, and population quality change from spatial and climatic range centers towards their range edges. We find that populations from poleward edges perform relatively poorly, both on average across all sites (15% lower population quality) and when compared to other populations at home (31% relative fitness disadvantage), consistent with these populations harboring high genetic load. Populations from equatorial edges also perform poorly on average across sites (18% lower population quality) but, in contrast, outperform foreign populations at home (16% relative fitness advantage), suggesting that populations from equatorial edges are specialized to unique environments. Finally, we find that populations from sites that are thermally extreme relative to the species9 niche demonstrate strong local adaptation, regardless of their geographic range position. Our findings indicate that both nonadaptive processes and adaptive evolution contribute to variation in adaptation across species9 ranges.