An Examination of Polar Bear and Brown Bear Phylogeny Using Nuclear Genes as Molecular Markers

Open Access
Moncla, Louise Hillier
Area of Honors:
Bachelor of Science
Document Type:
Thesis Supervisors:
  • Dr Beth Shapiro, Thesis Supervisor
  • James Harold Marden, Honors Advisor
  • phylogeny
  • polar bear
  • brown bear
  • evolution
  • biology
  • gene
Recent discussion of global climate change has brought polar bears (Ursus maritimus) into the spotlight. Although their close relationship to the brown bear (Ursus arctos) has long been accepted, ambiguity still remains regarding their divergence time and the role of hybridization in polar bears’ species history. Past phylogenies built on mitochondrial DNA have elucidated that polar bears and brown bears’ mitochondrial lineages are reciprocally monophyletic, and that polar bear mitochondria are most closely related to a clade of extant brown bears inhabiting the Admiralty, Baranof, and Chichagof Islands (ABC Islands) Alaska. However, mitochondrial data only reflect the maternal lineage of a species. Here I use nuclear gene sequences obtained from multiple polar bear and brown bear individuals to construct species phylogenies with aims of determining both the phylogenetic relationship of polar bears and brown bears, and the role of hybridization in their evolutionary histories. I targeted four nuclear genes in 31 polar bear and brown bear tissue samples obtained from the Alaska area, and constructed both single gene and multiple gene trees. Different analyses of my data are consistent with two proposed hypotheses of polar bear evolution. My data also show possible evidence of recent polar bear-brown bear hybridization, and group ABC Island brown bears with brown bears rather than with polar bears.