The Arctic tundra biome is undergoing rapid shrub expansion (“shrubification”) in response to anthropogenic climate change. During the previous ~2.6 million years, glacial cycles caused substantial shifts in Arctic vegetation, leading to changes in species’ distributions, abundance, and connectivity, which have left lasting impacts on the genetic structure of modern populations. Examining how shrubs responded to past climate change using genetic data can inform the ecological and evolutionary consequences of shrub expansion today. Here we test scenarios of Quaternary population history of dwarf birch species (Betula nana L. and Betula Glandulosa Michx.) using SNP markers obtained from RAD sequencing and approximate Bayesian computation. We compare the timings of population events with ice sheet reconstructions and other paleoenvironmental information to untangle the impacts of alternating cold and warm periods on the phylogeography of dwarf birch. Our best supported model suggested that the species diverged in the Mid-Pleistocene Transition as glaciations intensified, and ice sheets expanded. We found support for a complex history of inter- and intraspecific divergences and gene flow, with secondary contact occurring during periods of both expanding and retreating ice sheets. Our spatiotemporal analysis suggests that the modern genetic structure of dwarf birch was shaped by transitions in climate between glacials and interglacials, with ice sheets acting alternatively as a barrier or an enabler of population mixing. Tundra shrubs may have had more nuanced responses to past climatic changes than phylogeographic analyses have often suggested, with implications for future eco-evolutionary responses to anthropogenic climate change.

Aim: Rangifer tarandus L. play a key role in Arctic ecosystems as the most numerous and widespread large herbivore. Sea ice is vital for maintaining genetic connectivity in Arctic islands, yet the historical role of sea ice in shaping Rangifer biogeography is unknown. We study the timing of island dispersal and the role of sea ice changes and ice sheet retreat since the last glacial period. Methods: We compiled published datasets of mitochondrial DNA sequences which informed population history scenarios, evaluated in a coalescent-based approximate Bayesian computation (ABC) modelling framework to test hypotheses of island (re)colonisation and to estimate timings of divergence and admixture. Population events were compared with modelled and proxy-based paleo-sea ice cover and published ice sheet chronologies. Results: Our analysis supports Holocene dispersal onto deglaciated Arctic islands rather than High Arctic glacial refugia. The degree of population admixture and the effect of sea ice was dependent on regional geography and climate history. North American initial island population divergence occurred as sea ice cover was declining. A lack of strong genetic structure and late Holocene admixture suggest that island populations were somewhat connected by sea ice during the Holocene. The Svalbard and West Greenland divergence times lagged deglaciation but broadly align with fossil-based estimates of colonisation, suggesting dispersal limitation due to sea ice conditions, potentially modulated by ocean currents and sea ice drift. Main conclusions: Our study sheds light on the Late Quaternary (~60 ka - present) history of Arctic island Rangifer and suggests that ice sheet retreat, sea ice cover, and ocean currents were important in shaping present-day genetic patterns. Regional differences in postglacial dynamics suggest that dispersal during contemporary climate change may vary regionally and depend upon decreasing connectivity provided by sea ice.