Evolutionary History of Polar and Brown Bears


Taxonomists have long recognised polar and brown bears as separate species with distinct ecological niches and largely nonoverlapping ranges. Surprisingly, phylogenetic studies of maternally inherited mitochondrial DNA (mtDNA) found polar bears nested within brown bears, with an estimated divergence time of <170 000 years. This indicated an unusually rapid speciation and adaptation of polar bears. However, several recent studies of autosomal and Y‐chromosomal DNA have revisited these findings, giving independent perspectives of bear evolutionary history. Results show that polar bears cluster separately from brown bears, and divergence time estimates are older than those based on mtDNA, ranging from >300 000 to 4–5 million years. These studies confirm uniqueness of the polar bear lineage, provide more time for speciation and adaptation, and have uncovered numerous candidate genes for evolutionary adaptations. Several instances of introgressive hybridisation between polar and brown bears have been inferred, revealing trans‐species transmission of mtDNA and some nuclear loci.

Key Concepts

  • DNA sequences in conjunction with a calibration of the mutation rate (e.g. inclusion of a previously estimated mutation rate, geological information or inclusion of a radiocarbon‐dated ancient sample) can be used to estimate the timing of speciation between species.
  • Differentially inherited loci can reveal different aspects of evolutionary history.
  • The genome of polar bears contains a wealth of alleles that are not found in brown bears, and vice versa.
  • Polar bears have passed through bottlenecks during their evolutionary history, leaving them much less genetically variable than brown bears.
  • When analysing DNA sequences that have passed through the species boundary due to introgressive hybridisation, studies will obtain information about the hybridisation event rather than the (earlier) speciation event.
  • Brown bears have acted as vectors for polar bear alleles, transporting introgressed genetic material far beyond the species' contact zones.
  • Incomplete lineage sorting (see ‘Glossary’) complicates phylogenetic inferences among rapidly and/or recently diverged taxa. Lineage sorting takes on average four Ne generations, which for many taxa can span at least several hundred thousand years (Ne being the effective population size). The time needed for lineages to be reciprocally monophyletic can therefore take very long, even under complete reproductive isolation.
  • Molecular studies have found signals of positive selection in polar bear genes that are involved in fat metabolism, energy production and cardiovascular function. These genes are exciting candidates that may help us better understand the genetic basis of polar bear adaptations to Arctic conditions.

Keywords: adaptation; arctic; genome sequencing; introgressive hybridization; mtDNA; Pleistocene; speciation; Ursus arctos; Ursus maritimus; Y chromosome

Figure 1. Approximate distribution ranges of polar and brown bears (shown in blue and brown, respectively). Yellow circles denote the Alaskan ABC Islands and Ireland, where genetic data indicate that introgressive hybridisation has occurred in the past. Range information is taken from the IUCN and additional sources.
Figure 2. Phylogenies of mtDNA, autosomes and Y chromosomal data from polar, brown and American black bears. (a) Phylogeny of partial mtDNA control region data, showing paraphyly. (b) Species tree based on 14 autosomal intron markers and (c) phylogeny of 5.2 kilobases of Y‐chromosomal sequence data, both showing polar and brown bears as reciprocally monophyletic lineages. Black circles in (a) show previously suggested instances of past mtDNA introgression that may explain the incongruence between mtDNA and nuclear loci. (a, b) Modified from Hailer et al. 2012 © The American Association for the Advancement of Science. (c) Modified from Bidon et al. 2014 © Oxford University Press. (b) Bear images: Reproduced with permission from Fauna, www.fauna.is. © Jón Baldur Hlíðberg.
Figure 3. Changes in effective population size through time estimated using PSMC (Li and Durbin,) from whole genome sequencing. Brown bears shown in brown (light brown: ABC Islands, dark brown: Alaskan mainland), black and polar bears are shown in black and blue, respectively. Time (shown in million years ago; Mya) was calibrated by assuming a mutation rate of 1 × 10−9 per site per year, similar to humans and a generation time of 10 years. Modified from Miller et al. 2012: © Proceedings of the National Academy of Sciences. Bear images: Reproduced with permission from Fauna, www.fauna.is. © Jón Baldur Hlíðberg.


Bidon T, Janke A, Fain SR, et al. (2014) Brown and polar bear Y chromosomes reveal extensive male‐biased gene flow within brother lineages. Molecular Biology and Evolution 31: 1353–1363.

Cahill JA, Green RE, Fulton TL, et al. (2013) Genomic evidence for island population conversion resolves conflicting theories of polar bear evolution. PLoS Genetics 9: e1003345.

Cahill JA, Stirling I, Kistler L, et al. (2015) Genomic evidence of geographically widespread effect of gene flow from polar bears into brown bears. Molecular Ecology 24: 1205–1217.

Campagna L, Gronau I, Silveira LF, Siepel A and Lovette IJ (2015) Distinguishing noise from signal in patterns of genomic divergence in a highly polymorphic avian radiation. Molecular Ecology 24: 4238–4251.

Cronin MA, Amstrup S, Garner G and Vyse ER (1991) Inter‐ and intraspecific mitochondrial DNA variation in North American bears (Ursus). Canadian Journal of Zoology 69: 8.

Cronin MA and MacNeil MD (2012) Genetic relationships of extant brown bears (Ursus arctos) and polar bears (Ursus maritimus). Journal of Heredity 103: 873–881.

Cronin MA, McDonough MM, Huynh HM and Baker RJ (2013) Genetic relationships of North American bears (Ursus) inferred from amplified fragment length polymorphisms and mitochondrial DNA sequences. Canadian Journal of Zoology 91: 626–634.

Cronin MA, Rincon G, Meredith RW, et al. (2014) Molecular phylogeny and SNP variation of polar bears (Ursus maritimus), brown bears (U. arctos), and black bears (U. americanus) derived from genome sequences. Journal of Heredity 105: 312–323.

Davison J, Ho SYW, Bray SC, et al. (2011) Late‐Quaternary biogeographic scenarios for the brown bear (Ursus arctos), a wild mammal model species. Quaternary Science Reviews 30: 418–430.

DeMaster DP and Stirling I (1981) Ursus maritimus. Mammalian Species 145: 1–7.

Derocher AE, Nelson RA, Stirling I and Ramsay MA (1990) Effects of feeding on serum urea and serum creatinine levels in polar bears. Marine Mammal Science 6: 196–203.

Drummond AJ, Nicholls GK, Rodrigo AG and Solomon W (2002) Estimating mutation parameters, population history and genealogy simultaneously from temporally spaced sequence data. Genetics 161: 1307–1320.

Durner GM, Douglas DC, Nielson RM, et al. (2009) Predicting 21st‐century polar bear habitat distribution from global climate models. Ecological Monographs 79: 25–58.

Edwards CJ, Suchard MA, Lemey P, et al. (2011) Ancient hybridization and an Irish origin for the modern polar bear matriline. Current Biology 21: 1251–1258.

Eyre‐Walker A and Keightley P (2007) The distribution of fitness effects of new mutations. Nature Reviews Genetics 8: 610–618.

Hailer F, Kutschera VE, Hallström BM, et al. (2012) Nuclear genomic sequences reveal that polar bears are an old and distinct bear lineage. Science 336: 344–347.

Hailer F, Kutschera VE, Hallström BM, et al. (2013) Response to comment on “Nuclear genomic sequences reveal that polar bears are an old and distinct bear lineage.” Science 339: 1522.

Hailer F (2015) Introgressive hybridization: brown bears as vectors for polar bear alleles. Molecular Ecology 24: 1161–1163.

Hardison RC and Taylor J (2012) Genomic approaches towards finding cis‐regulatory modules in animals. Nature Reviews Genetics 13: 469–483.

Hassanin A (2015) The role of Pleistocene glaciations in shaping the evolution of polar and brown bears. Evidence from a critical review of mitochondrial and nuclear genome analyses. Comptes Rendus Biologies 338: 494–501.

Ingólfsson Ó and Wiig Ø (2009) Late Pleistocene fossil find in Svalbard: the oldest remains of a polar bear (Ursus maritimus Phipps, 1744) ever discovered. Polar Research 28: 455–462.

Jarvis ED, Mirarab S, Aberer AJ, et al. (2014) Whole‐genome analyses resolve early branches in the tree of life of modern birds. Science 346: 1320–1331.

Jones FC, Grabherr MG, Chan YF, et al. (2012) The genomic basis of adaptive evolution in threespine sticklebacks. Nature 484: 55–61.

Kelly BP, Whiteley A and Tallmon D (2010) The Arctic melting pot. Nature 468: 891.

Kurtén B (1964) The evolution of the Polar Bear, Ursus maritimus Phipps. Acta Zoologica Fennica 108: 1–26.

Kutschera VE, Bidon T, Hailer F, et al. (2014) Bears in a forest of gene trees: phylogenetic inference is complicated by incomplete lineage sorting and gene flow. Molecular Biology and Evolution 31: 2004–2017.

Kutschera VE, Frosch C, Janke A, et al. (2016) High genetic variability of vagrant polar bears illustrates importance of population connectivity in fragmented sea ice habitats. Animal Conservation, Early online. http://dx.doi.org/10.1111/acv.12250.

Lennox AR and Goodship AE (2008) Polar bears (Ursus maritimus), the most evolutionary advanced hibernators, avoid significant bone loss during hibernation. Comparative Biochemistry and Physiology, Part A: Molecular & Integrative Physiology 149: 203–208.

Leonard JA, Wayne RK and Cooper A (2000) Population genetics of Ice Age brown bears. Proceedings of the National Academy of Sciences 97: 1651–1654.

Li H and Durbin R (2011) Inference of human population history from individual whole‐genome sequences. Nature 475: 493–496.

Lindqvist C, Schuster SC, Sun Y, et al. (2010) Complete mitochondrial genome of a Pleistocene jawbone unveils the origin of polar bear. Proceedings of the National Academy of Sciences 107: 5053–5057.

Liu S, Lorenzen ED, Fumagalli M, et al. (2014) Population genomics reveal recent speciation and rapid evolutionary adaptation in polar bears. Cell 157: 785–794.

Miller W, Schuster SC, Welch AJ, et al. (2012) Polar and brown bear genomes reveal ancient admixture and demographic footprints of past climate change. Proceedings of the National Academy of Sciences 109: E2382–E2390.

Nachman M, Hoekstra H and D'Agostino SL (2003) The genetic basis of adaptive melanism in pocket mice. Proceedings of the National Academy of Sciences of the United States of America 100: 5268–5273.

Nakagome S, Pecon‐Slattery J and Masuda R (2008) Unequal rates of Y chromosome gene divergence during speciation of the family Ursidae. Molecular Biology and Evolution 25: 1344–1356.

Nakagome S, Mano S and Hasegawa M (2013) Comment on “Nuclear genomic sequences reveal that polar bears are an old and distinct bear lineage.” Science (New York, N.Y.) 339: 1522.

Pagès M, Calvignac S, Klein C, et al. (2008) Combined analysis of fourteen nuclear genes refines the Ursidae phylogeny. Molecular Phylogenetics and Evolution 47: 73–83.

Peacock E, Sonsthagen SA, Obbard ME, et al. (2015) Implications of the circumpolar genetic structure of polar bears for their conservation in a rapidly warming Arctic. PLoS One 10: e112021.

Pollard DA, Iyer VN, Moses AM and Eisen MB (2006) Widespread discordance of gene trees with species tree in Drosophila: evidence for incomplete lineage sorting. PLoS Genetics 2: e173.

Pond CM, Mattacks CA and Colby RH (1992) The anatomy, chemical composition, and metabolism of adipose tissue in wild polar bears (Ursus maritimus). Canadian Journal of Zoology 70: 326–341.

Ritland K, Newton C and Marshall HD (2001) Inheritance and population structure of the white‐phased “Kermode” black bear. Current Biology 11: 1468–1472.

Sacco T and Van Valkenburgh B (2004) Ecomorphological indicators of feeding behaviour in the bears (Carnivora: Ursidae). Journal of Zoology 263: 41–54.

Slater GJ, Figueirido B, Louis L, Yang P and Van Valkenburgh B (2010) Biomechanical consequences of rapid evolution in the polar bear lineage. PLoS One 5: e13870.

Stirling I and McEwan EH (1975) The caloric value of whole ringed seals (Phoca hispida) in relation to polar bear (Ursus maritimus) ecology and hunting behavior. Canadian Journal of Zoology 53: 1021–1027.

Talbot SL and Shields GF (1996) Phylogeography of brown bears (Ursus arctos) of Alaska and paraphyly within the Ursidae. Molecular Phylogenetics and Evolution 5: 477–494.

Wagner J (2010) Pliocene to early Middle Pleistocene ursine bears in Europe: a taxonomic overview. Journal of the National Museum (Prague), Natural History Series 179: 197–215.

Waits LP, Sullivan J, O'Brien SJ and Ward RH (1999) Rapid radiation events in the family Ursidae indicated by likelihood phylogenetic estimation from multiple fragments of mtDNA. Molecular Phylogenetics and Evolution 13: 82–92.

Welch AJ, Bedoya‐Reina OC, Carretero‐Paulet L, et al. (2014) Polar bears exhibit genome‐wide signatures of bioenergetic adaptation to life in the Arctic environment. Genome Biology and Evolution 6: 433–450.

Yu L, Li Q, Ryder O and Zhang Y (2004) Phylogeny of the bears (Ursidae) based on nuclear and mitochondrial genes. Molecular Phylogenetics and Evolution 32: 480–494.

Further Reading

Avise JC (2004) Molecular Markers, Natural History, and Evolution, 2nd edn. Sinauer Associates: Sunderland, MA.

Good JM, Vanderpool D, Keeble S and Bi K (2015) Negligible nuclear introgression despite complete mitochondrial capture between two species of chipmunks. Evolution 69: 1961–1972.

Hall BG (2011) Phylogenetic Trees Made Easy: A How‐To Manual, 4th edn. Sinauer Associates: Sunderland, MA.

Harrison RG and Larson EL (2014) Hybridization, introgression, and the nature of species boundaries. Journal of Heredity 105: 795–809.

Primmer CR, Papakostas S, Leder EH, Davis MJ and Ragan MA (2013) Annotated genes and nonannotated genomes: cross‐species use of gene ontology in ecology and evolution research. Molecular Ecology 22: 3216–3241.

Toews DP and Brelsford A (2012) The biogeography of mitochondrial and nuclear discordance in animals. Molecular Ecology 21: 3907–3930.

Vitti JJ, Grossman SR and Sabeti PC (2013) Detecting natural selection in genomic data. Annual Review of Genetics 47: 97–120.

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Hailer, Frank, and Welch, Andreanna J(Jul 2016) Evolutionary History of Polar and Brown Bears. In: eLS. John Wiley & Sons Ltd, Chichester. http://www.els.net [doi: 10.1002/9780470015902.a0026303]