Phylogeography of introgression: Spatial and temporal analyses identify two introgression events between brown and American black bears.

Brown bears (Ursus arctos) colonized North America from Eurasia in two distinct and temporally separated waves. Once in North America they encountered endemic American black bears (U. americanus) during range expansions from eastern Beringia southwards into the interior of the continent. The establishment of sympatry between these species provided the opportunity for hybridization and introgression, which was previously identified at the species level using D-statistics. Both species have broad spatial ranges that should limit the extent of introgression, such that it is found primarily between sympatric populations. Here, we used range-wide sampling and whole genome sequencing of both bear species to test for spatial variability in introgression. We identified two pulses of introgression between brown and American black bears, and demonstrate the introgressed segments occur across spatially structured lineages in both species. The first pulse occurred 270-120 kya, near the initiation of intraspecific divergence, approximately 99-93 kya, within each species. This pulse occurred as sympatry was established in western North America. The second pulse occurred between western American black bears and North American brown bears and lasted to 9 kya. Introgression was bidirectional and sympatric lineages had more introgressed tracts and a larger proportion of the genome introgressed from the other species. This study advances our phylogeographic understanding of both iconic bear species through investigating the timing of divergence and gene flow as bears expanded and contracted their ranges across North America.