Abstract
Every species experiences limits to its geographic distribution. Some evolutionary models predictthat populations at range edges are less well-adapted to their local environments due to drift, ex-pansion load, or swamping gene flow from the range interior. Alternatively, populations near rangeedges might be uniquely adapted to marginal environments. In this study, we use a database oftransplant studies that quantify performance at broad geographic scales to test how local adapta-tion, site quality, and population quality change from spatial and climatic range centers towardsedges. We find that populations from poleward edges perform relatively poorly, both on averageacross 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 (18% lower population quality)but, in contrast, outperform foreign populations (16% relative fitness advantage), suggesting thatpopulations from equatorial edges have strongly adapted to unique environments. Finally, we findthat populations from sites that are thermally extreme relative to the species’ niche demonstratestrong local adaptation, regardless of their geographic position. Our findings indicate that bothnonadaptive processes and adaptive evolution contribute to variation in adaptation across species’ranges.