Adaptation across geographic ranges is consistent with strong selection in marginal climates and legacies of range expansion

Abstract

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. In this study, we use a database of transplant studies that quantify performance at broad geographic scales to test how local adaptation, site quality, and population quality change from spatial and climatic range centers toward 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 (18% lower population quality) but, in contrast, outperform foreign populations (16% relative fitness advantage), suggesting that populations from equatorial edges have strongly adapted to unique environments. Finally, we find that populations from sites that are thermally extreme relative to the species' niche demonstrate strong local adaptation, regardless of their geographic position. Our findings indicate that both nonadaptive processes and adaptive evolution contribute to variation in adaptation across species' ranges.

Climate biogeography ofArabidopsis thaliana:linking distribution models and individual variation

“…Where populations inhabit harsh environments (e.g. at range margins), local adaptations can emerge, such as life history changes to tolerate or escape harsh periods (Bontrager et al, 2021). In Arabidopsis, there is evidence that local adaptation to environment involves genetic changes in life history (e.g.…”

Understanding local plant extinctions before it’s too late: bridging evolutionary genomics with global ecology.

“…Here, (mal)adaptation refers to relative fitness differences among genotypes, which may help explain absolute fitness (total viable offspring) and population decline (Brady et al , 2019). To understand large-scale patterns of (mal)adaptation across species and their geographic ranges, Bontrager et al (2021) summarized published studies of 130 plant species in which multiple populations were grown in common gardens at different locations within their range. This analysis confirmed the well-known pattern that adaptation decays as populations are transplanted further from their environment of origin and supported two long-theorized hypotheses of species' geographic range evolution (Angert et al , 2020): (i) cold (poleward) range edge populations, which were likely established during post-glacial expansions, show signs of general maladaptation with a ~18% fitness disadvantage across all gardens compared to all other populations; (ii) warm (equatorial) range edge populations had strong local adaptation with a fitness advantage of ~16% compared to all other populations, but only in common gardens in extreme environments.…”

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