Liu, L.; Borrell, J. S.; Wang, N. S.

The latitudinal gradient (LG) hypothesis predicts a poleward decrease of genetic diversity. Similarly, the central-marginal hypothesis (CMH) predicts higher genetic diversity in range centres than margins. We examined these patterns in Europe (EU), North America (NA) and East Asia (EA), which experienced contrasting patterns of landscape fragmentation. We compiled genetic variation data for 445 plant species and 8,530 populations. We calculated ecological metrics comprising distance to the range margin and native climatic niche margin, distance to refugia, latitude and altitude. We applied Bayesian inference to evaluate the relationships between genetic variation and ecological metrics. We found universal support for the CMH, with populations at the centres of species ranges and/or native climatic niches exhibiting higher genetic diversity than those at the margins and stronger patterns in woody versus herbaceous plants. Evidence for a LG in diversity was less consistent with latitude: negatively correlated with genetic diversity in EU, positively correlated in EA, and no correlation in NA. Our analysis supports the view that the LG is largely driven by latitudinal trends in glaciation, particularly in EU, but highlights that glaciation patterns and their resulting impacts on landscape genetic diversity were more heterogeneous in NA and EA.