Major climatic fluctuations throughout the Quaternary caused pronounced changes in plant species distributions, and shaped current geographical patterns in biodiversity on multiple continents. Population genetic theory and phylogeographic syntheses in Europe suggest that these glacial cycles should result in strong latitudinal patterns of genetic diversity: high haplotype endemism in southern populations and clines in genetic diversity northward. These patterns are thought to be ubiquitous across temperate forests like those in Eastern North America (ENA), and are therefore used to anticipate the potential genetic consequences of future warming. Using phylogeographic dataset and ancient DNA analyses, we tested for common genetic impacts of Quaternary climate swings across diverse taxa in ENA and between continents, exploring whether repeated population expansions and contractions through ice age cycles left diversity ‘hotspots’ in ENA as it did in Europe. We further explored the establishment and persistence of genetic diversity in ancient tree populations following initial population colonization in northern ranges using ancient DNA analyses (DNA from macrofossils preserved for thousands of years in lake sediments).
Results/Conclusions
Unlike their European counterparts, temperate ENA taxa do not show a strong latitudinal clines northward nor share common southern centres of haplotype endemism i.e. no genetic diversity ‘hotspots’. Most ENA taxa reached their northern range limits with most of their genetic diversity intact, suggesting that the suite of population processes proposed to structure gradients of genetic diversity across glacial-interglacial cycles, such as southern glacial refugia and bottlenecks along long migration routes away from those refugia did not structure gradients of genetic diversity in ENA as expected. Furthermore, ancient DNA analyses of ancient northern populations also suggest an early establishment of genetic diversity via a single wave of colonization event in northern ranges. These differences between the genetic impacts of Quaternary climate cycles across continents suggest refined lessons for managing genetic diversity in today’s warming world. Climate warming can reduce genetic richness, but not necessarily so. In the modern world, the rate and magnitude of warming may be comparable to post-glacial warming. This time, of course, migrating species will have to face both natural and anthropogenic biogeographic barriers and facilitations.