Climate Drives Polar Bear Origins

Authors: GALBREATH, KURT - Northern Michigan University; COOK, JOSEPH - University Of New Mexico; Hoberg, Eric

Submitted to: Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/8/2012
Publication Date: 6/8/2012
Citation: Galbreath, K.I., Cook, J.A., Hoberg, E.P. 2012. Climate Drives Polar Bear Origins. Science. 336:1230.

Interpretive Summary: Climate has been postulated as a critical driver in evolution, and climatological events including glacial-intergalcial cycles over the past 2-3 million years have been viewed as important determinants in the origin and distributions of organisms and ecosystems. Although the association of evolution and climate has been recognized, there are few powerful examples of the role of climate in influencing trends in evolution. Following the publication of a provocative analysis of northern bears (“Nuclear genomic sequences reveal that polar bears are an old and distinct bear lineage,” Reports, 20 April, p. 344), F. Hailer et al. which showed that polar bears originated during the middle Pleistocene, rather than during the late Pleistocene, we identified a putative mechanism for this event. Although these authors discuss the possible role of climate warming in creating discord between nuclear and mitochondrial genetic signatures, the authors do not address climate’s critical role in driving the evolution of polar bears in the first place. A reliance on perennial sea ice and a strongly pagophilic (i.e., ice-dependent) prey base, including seals, suggests polar bears could not have evolved in a world in which the Arctic Ocean remained unfrozen for large portions of the year, as it did most recently during the warm period of the middle Pliocene. Cooling of the Arctic Ocean commenced during the late Pliocene, driving a transition from predominantly seasonal to perennial sea ice that was largely complete by the middle Pleistocene about 700 thousand years ago. We identify a striking temporal concordance between the new date for divergence of polar bears and persistent freezing of the Arctic Ocean. This observation suggests that this may be one of relatively few instances in which a specific paleoclimatological episode can be convincingly linked to a specific evolutionary event, and it provides vivid demonstration of climatic forcing as a determinant of diversification in biological systems. Our conclusions will be of importance for biologists, including disease specialists in developing a robust understanding of the role of climate in evolutionary change and as a factor that influences ecological structure and geographic distributions for complex faunas.

Technical Abstract: In their provocative analysis of northern bears (“Nuclear genomic sequences reveal that polar bears are an old and distinct bear lineage,” Reports, 20 April, p. 344), F. Hailer et al. use independent nuclear loci to show that polar bears originated during the middle Pleistocene, rather than during the late Pleistocene as previously inferred from mitochondrial data. Although they discuss the possible role of climate warming in creating discord between nuclear and mitochondrial genetic signatures, the authors do not address climate’s critical role in driving the evolution of polar bears in the first place. A reliance on perennial sea ice and a strongly pagophilic (i.e., ice-dependent) prey base, including seals, suggests polar bears could not have evolved in a world in which the Arctic Ocean remained unfrozen for large portions of the year, as it did most recently during the warm period of the middle Pliocene. Cooling of the Arctic Ocean commenced during the late Pliocene, driving a transition from predominantly seasonal to perennial sea ice that was largely complete by the middle Pleistocene about 700 thousand years ago. Striking temporal concordance between the new date for divergence of polar bears and persistent freezing of the Arctic Ocean suggests that this may be one of relatively few instances in which a specific paleoclimatological episode can be convincingly linked to a specific evolutionary event, and it provides vivid demonstration of climatic forcing as a determinant of diversification in biological systems.