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Historic Evidence of Introgression in Two Ursid Species, the Polar Bear and Brown Bear

The phylogenetic relationships of bears (genus: Ursus) are complicated by incomplete lineage sorting (ILS), supported by evidence of gene flow between polar bears (Ursus maritimus) and brown bears (Ursus arctos) occurring as recently as the last ice age and correlating with range overlap. While studies have focused on levels of introgressions early during the speciation process, little work has been conducted on introgression in the past century due to historic range overlap. The polar bear is noted as an animal symbolic of climate change’s effects, forced to move further into the brown bear range and resulting in fertile hybrids informally known as ‘grolar bears’. Here we present preliminary results of phylogenetic analyses for one historic brown bear and five historic polar bear specimens collected from 1878 to 1953 and ranging across the Arctic circle in locality. This work allows us to understand the speciation and evolutionary history of these closely related species. With the black bear (Ursus americanus) as an outgroup, we aim to estimate levels of historic introgression between polar bears and grizzly bears over the past century through hybridization inferences analyses of mitochondrial DNA (mtDNA), which are used to construct a maximum likelihood network utilizing a coalescent framework to incorporate ILS and gene flow. The results of this study will contribute to the growing research focusing on studying past hybridization trends to understand current hybridization by serving as a reference source for historic introgression versus current introgression induced by anthropogenic climate change.

Alice Teele '22
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