Contrasting Drivers of Genetic Diversity in Plants Across Previously Glaciated Northern Hemisphere Landscapes

ABSTRACT Aim Understanding how global biogeographical and evolutionary processes have shaped continental scale patterns of plant genetic diversity is increasingly tractable, given the proliferation of population genetic data. Predominant theories are the geographical central‐marginal hypothesis (CMH), ecological CMH and historical CMH which predict decreasing genetic diversity from range centres to margins, from suitable to marginal environments, and from refugia to newly colonised areas, respectively. In addition, the latitudinal gradient (LG) and longitudinal gradient (LoG) hypotheses predict a decrease of within‐population genetic diversity along dispersal routes across species ranges. Here we test these hypotheses across North America (NA), Europe (EU), and East Asia (EA), which experienced contrasting patterns of post‐glacial landscape fragmentation. Location North America, Europe and East Asia. Time Period The last glacial maximum to present. Major Taxa Studied Plants. Methods We collated data from 8333 populations, representing 435 plant species and calculated distances from populations to the range margin, climatic niche margin and refugia, latitude and longitude. We applied Bayesian phylogenetic mixed‐effects models to assess the relationship between these variables and genetic diversity. All possible combinations of the five variables were constructed, yielding 31 candidate models, and the best‐supported model was identified through model comparison. Results We found that the geographical CMH, ecological CMH, historical CMH, LG and LoG have shaped patterns of genetic diversity across the Northern Hemisphere, but varied substantially by region and between woody and herbaceous species. Specifically, the geographical CMH has mainly shaped genetic diversity of herbaceous species in the EU and EA whereas the ecological CMH mainly impacted that of woody species in NA. The historical CMH has limited impacts on patterns of genetic diversity in EA. Main Conclusions Our analyses illustrate contrasting drivers of spatial genetic diversity across continents, reflecting heterogeneous impacts of historical glaciation interacting with life history traits. Our results underscore the value of geographical, historical and ecological variables as proxies for within population diversity, enabling identification of populations or regions that could be prioritised for conservation in previously glaciated Northern hemisphere.