Adaptation across a precipitation gradient from niche center to niche edge

Evaluating the potential for species to adapt to changing climate relies on an appreciation of current patterns of adaptive variation and selection, which might vary in intensity across a species niche, affecting our inference of where adaptation might be most important in the future. Here we investigate the genetic basis of adaptation in Lactuca serriola, the wild relative of the common lettuce, growing along a steep precipitation gradient in Israel approaching the species arid niche limit, and used candidate loci to inform predictions of its past and future adaptive evolution. Environmental association analyses combined with generalized dissimilarity models revealed 98 candidate genes showing non-linear shifts in allele frequencies across the gradient, with a large proportion under strong directional selection near the dry niche edge. Selection acts on genes with separate suites of biological functions, specifically related to osmotic stress and phenological adjustments at the dry edge, and related to biotic interactions at the niche center. The adaptive genetic composition of populations, as inferred through polygenic risk scores, indicates that intensified selection may be the key determinant of the dry niche edge. However, inference of past and future evolutionary change suggests larger adaptive shifts in the mesic part of the range, which is most affected by climate change. Our study highlights that non-linear spatial variation in selection has implications for predicting past and future responses to climate change.