Genomic Evidence of Widespread Admixture from Polar Bears into Brown Bears during the Last Ice Age

Abstract

Recent genomic analyses have provided substantial evidence for past periods of gene flow from polar bears (Ursus maritimus) into Alaskan brown bears (Ursus arctos), with some analyses suggesting a link between climate change and genomic introgression. However, because it has mainly been possible to sample bears from the present day, the timing, frequency, and evolutionary significance of this admixture remains unknown. Here, we analyze genomic DNA from three additional and geographically distinct brown bear populations, including two that lived temporally close to the peak of the last ice age. We find evidence of admixture in all three populations, suggesting that admixture between these species has been common in their recent evolutionary history. In addition, analyses of ten fossil bears from the now-extinct Irish population indicate that admixture peaked during the last ice age, whereas brown bear and polar bear ranges overlapped. Following this peak, the proportion of polar bear ancestry in Irish brown bears declined rapidly until their extinction. Our results support a model in which ice age climate change created geographically widespread conditions conducive to admixture between polar bears and brown bears, as is again occurring today. We postulate that this model will be informative for many admixing species pairs impacted by climate change. Our results highlight the power of paleogenomics to reveal patterns of evolutionary change that are otherwise masked in contemporary data.

Reticulate evolutionary history of a Western Palaearctic Bat Complex explained by multiple mtDNA introgressions in secondary contacts

“…It is likely that the admixture events which led to the observed patterns of introgressions occurred when climate was changing (see Cahill et al. () for an example). During such periods, distribution of habitats and accordingly the associated taxa reshape, often bringing together formerly isolated species (Currat et al., ; Hewitt, ).…”

Hybridization in human evolution: insights from other organisms

“…Preliminary analyses of the Irish brown bears' nuclear genomes suggest that a similar phenomenon may have taken place in those populations. 53 For closely related species that rarely come into contact because of their different ecological requirements, disruptive climatic events may temporarily create the opportunity for admixture that substantially exceeds the amount observed in more stable conditions. Again, this scenario is relevant to hominin evolution, and especially Late Pleistocene Europe, where climatic fluctuations would have affected ranges of ancient hominins, and potentially both the prevalence and intensity of contact.…”

Reticulate Evolutionary History of a Western Palaearctic Bat Complex Explained by Multiple mtDNA Introgressions in Secondary Contacts

“…During such periods, distribution of habitats and accordingly the associated taxa reshape, often bringing together formerly isolated species (Currat et al 2008;Hewitt 2000). Hybridization occurring at such periods can provide evolutionary novelty into gene pools of these expanding species, which in turn can affect their fitness (Cahill et al 2018). Nowadays, complex reticulate histories are emerging all across the tree of life (Cahill et al 2018;Palkopoulou et al 2018;Prüfer et al 2014).…”

“…Hybridization occurring at such periods can provide evolutionary novelty into gene pools of these expanding species, which in turn can affect their fitness (Cahill et al 2018). Nowadays, complex reticulate histories are emerging all across the tree of life (Cahill et al 2018;Palkopoulou et al 2018;Prüfer et al 2014). We think that understanding the biogeographical history of such species complexes will provide crucial insight about the underlying mechanisms of reticulated trees.…”