Genetic differentiation across a steep and narrow environmental gradient: Quantitative genetic and genomic insights into Lake Superior populations of Quercus rubra

Adaptive differentiation of traits and underlying loci can occur at a small geographic scale if natural selection is stronger than gene flow and drift. We investigated this hypothesis using coupled quantitative genetic and genomic approaches for a wind-pollinated tree species, Quercus rubra , along the steep, narrow gradient of the Lake Superior coast that encompasses four USDA Hardiness Zones within 100 km. For the quantitative genetic component, we examined phenotypic differentiation among eight populations in a common garden. For the genomic component, we quantified genetic differentiation for 26 populations using RAD-seq. Because hybridization with the congener Quercus ellipsoidalis has been documented elsewhere, we also included two populations of this species for comparison. In the common garden study, we found a strong signal of population differentiation that was significantly associated with at least one climate factor for nine of ten measured traits. In contrast, we found no evidence of genomic differentiation among populations based on FST or other measures. However, distance-based, and genotype-environment association analyses identified loci showing the signature of selection, with one locus in common across five analyses. This locus was associated with the minimum temperature of the coldest month, a climate factor was also significant in the common garden analyses. In addition, we documented introgression from Q. ellipsoidalis into Q. rubra , with rates of introgression correlated with the climate gradient. In sum, this study reveals signatures of selection at the quantitative trait and genomic level consistent with climate adaptation occurring at a small geographic scale.