Phylogeography of a western North American white oak shaped by introgression and post‐Pleistocene climate change

Abstract Aim We examined range‐wide genetic variation in a widespread white oak species in western North America to determine phylogeographic patterns (including possible latitudinal gradients) and investigate their underlying causes. Location Western North America. Taxon Quercus garryana (Fagaceae). Methods We sampled Q . garryana at 117 locations throughout its range and related species of white oaks in 8 additional populations. We sequenced DNA from four variable intergenic plastid spacers and the ribosomal nuclear ITS region. We constructed haplotype networks and phylogenetic trees, mapped the geographical distributions of plastid haplotypes, and analysed genetic diversity patterns. Results We identified 25 plastid haplotypes that clustered in two major groups, each showing pronounced genetic variation with latitude. In contrast, ITS showed little or no geographical or phylogenetic variation. Quercus garryana shared several plastid haplotypes with related white oaks and was not differentiated from these species by ITS. Plastid haplotype diversity in Q . garryana was highest in the central part of its range and sharply reduced to the north. Two haplotypes (one from each haplotype group) occurred in the northern third of the range, with only one at the northern range limit. Populations at the southern range limit were characterised by distinct haplotypes. Main Conclusions Genetic patterns of Q . garryana have been shaped both by post‐Pleistocene climate change, interacting with features of the landscape, and by hybridisation with other white oaks. The species contains substantial genetic variation with strong spatial structuring of plastid haplotypes, indicating (1) early divergence of southern (Sierra Nevada) and northern populations, and (2) northward dispersal during the Holocene resulting in greatly reduced genetic diversity in its northern range. Species that can adjust their ranges with changing climates may remain susceptible to loss of genetic variation, potentially affecting their ability to persist in novel conditions at northern range margins.