Editing Polar bear (section)

==Taxonomy==
[[Carl Linnaeus]] classified the polar bear as a type of [[brown bear]] (''Ursus arctos''), labelling it as ''Ursus maritimus albus-major, arcticus'' ('mostly-white sea bear, arctic') in the [[10th edition of Systema Naturae|1758 edition]] of his work ''[[Systema Naturae]]''.{{sfn|Fee|2019|p=48}} [[Constantine John Phipps]] formally described the polar bear as a distinct species, ''Ursus maritimus'' in 1774, following his [[1773 Phipps expedition towards the North Pole|1773 voyage towards the North Pole]].<ref name="Phipps1774">{{cite book |last=Phipps |first=John |author-link=Constantine John Phipps |title=A voyage towards the North Pole undertaken by His Majesty's command, 1773 |location=London |publisher=W. Bowyer and J. Nicols, for J. Nourse |year=1774 |url=https://www.biodiversitylibrary.org/page/628763 |page=185 |access-date=13 January 2018 |archive-url=https://web.archive.org/web/20170707094547/http://www.biodiversitylibrary.org/page/628763 |archive-date=7 July 2017 |url-status=dead}}</ref>{{sfn|Fee|2019|p=41}} Because of its adaptations to a marine environment, some taxonomists, such as Theodore Knottnerus-Meyer, have placed the polar bear in its own genus, ''Thalarctos''.<ref name="DeMaster1981"/>{{sfn|Ellis|2009|p=67}} However ''Ursus'' is widely considered to be the valid genus for the species on the basis of the fossil record and the fact that it [[Hybrid (biology)|can breed]] with the brown bear.{{sfn|Ellis|2009|p=67}}<ref name="ReferenceC"/>

Different [[subspecies]] have been proposed including ''Ursus maritimus maritimus'' and ''U. m. marinus''.{{efn|Phipps (1774) and [[Peter Simon Pallas|Pallas]] (1776) respectively.<ref name="Wilson1976"/>}}<ref name="Wilson1976">{{cite journal|last1=Wilson|first1=Don E.|title=Cranial variation in polar bears|journal=Bears: Their Biology and Management|date=1976|volume=3|pages=447–453|doi=10.2307/3872793|jstor=3872793 | issn = 1936-0614 }}</ref> However, these are not supported, and the polar bear is considered to be [[monotypic taxon|monotypic]].<ref name=":0">{{Cite web|url=https://www.marinemammalscience.org/species-information/list-of-marine-mammal-species-subspecies/|title=List of Marine Mammal Species & Subspecies|author=Committee on Taxonomy|date=October 2014|publisher=The Society for Marine Mammalogy|archive-url=https://web.archive.org/web/20150106152733/https://www.marinemammalscience.org/species-information/list-of-marine-mammal-species-subspecies/|archive-date=6 January 2015}}</ref> One possible fossil subspecies, ''[[Ursus maritimus tyrannus|U. m. tyrannus]]'', was posited in 1964 by [[Björn Kurtén]], who reconstructed the subspecies from a single fragment of an ulna which was approximately 20 percent larger than expected for a polar bear.<ref name="ReferenceC"/> However, re-evaluation in the 21st century has indicated that the fragment likely comes from a giant brown bear.<ref name=Harington>{{cite journal|last=Harington|first=C. R.|year=2008|title=The evolution of Arctic marine mammals|journal= Ecological Applications|volume=18|issue=sp2|pages=S23–S40|doi=10.1890/06-0624.1|pmid=18494361 |doi-access=free|bibcode=2008EcoAp..18S..23H }}</ref>{{sfn|Derocher|2012|p=37}}

===Evolution===
The polar bear is one of eight extant species in the [[bear]] family, Ursidae, and of six extant species in the subfamily [[Ursinae]]. {{Phylogeny/Ursidae}}

Fossils of polar bears are uncommon.<ref name="ReferenceC">{{cite journal|last1=Kurtén|first1=B.|title=The evolution of the polar bear, ''Ursus maritimus'' Phipps|journal=Acta Zoologica Fennica|date=1964|volume=108|pages=1–30|url=https://helda.helsinki.fi/handle/10138/37762}}</ref><ref name=Harington/> The oldest known fossil is a 130,000- to 110,000-year-old jaw bone, found on [[Prince Charles Foreland]], Norway, in 2004.<ref name="Lindqvist">{{cite journal|doi=10.1073/pnas.0914266107|pmid=20194737|pmc=2841953 |title=Complete mitochondrial genome of a Pleistocene jawbone unveils the origin of polar bear |journal=Proceedings of the National Academy of Sciences |volume=107 |issue=11 |pages=5053–5057 |year=2010 |last1=Lindqvist |first1=C. |last2=Schuster |first2=S. C. |last3=Sun |first3=Y. |last4=Talbot |first4=S. L. |last5=Qi |first5=J. |last6=Ratan |first6=A. |last7=Tomsho |first7=L. P. |last8=Kasson |first8=L. |last9=Zeyl |first9=E. |last10=Aars |first10=J. |last11=Miller |first11=W. |last12=Ingolfsson |first12=O. |last13=Bachmann |first13=L. |last14=Wiig |first14=O. |bibcode=2010PNAS..107.5053L|doi-access=free}}</ref><ref name="polarresearch">{{cite journal|last1=Ingólfsson|first1=Ólafur|last2=Wiig|first2=Øystein|title=Late Pleistocene fossil find in Svalbard: the oldest remains of a polar bear (''Ursus maritimus'' Phipps, 1744) ever discovered|journal=Polar Research|date=2009|volume=28|issue=3|pages=455–462|doi=10.3402/polar.v28i3.6131|doi-access=free}}</ref> Scientists in the 20th century surmised that polar bears directly descended from a population of brown bears, possibly in eastern [[Siberia]] or [[Alaska]].<ref name="ReferenceC"/><ref name=Harington/> [[Mitochondrial DNA]] studies in the 1990s and 2000s supported the status of the polar bear as a derivative of the brown bear, finding that some brown bear populations were more closely related to polar bears than to other brown bears, particularly the [[ABC Islands bear]]s of [[Southeast Alaska]].<ref name="Lindqvist"/><ref>{{cite journal|last1=Talbot|first1=S. L.|last2=Shields|first2=G. F.|year=1996|title=Phylogeography of brown bears (''Ursus arctos'') of Alaska and paraphyly within the Ursidae|journal=Molecular Phylogenetics and Evolution|volume=5|issue=3|pages=477–494|doi=10.1006/mpev.1996.0044|pmid=8744762 |bibcode=1996MolPE...5..477T }}</ref><ref>{{cite journal|last1=Shields|first1=G. F.|last2=Adams|first2=D.|last3=Garner|first3=G.| last4=Labelle|first4=M.|last5=Pietsch|first5=J.|last6=Ramsay|first6=M.| last7=Schwartz|first7=C.|last8=Titus|first8=K.|last9=Williamson|first9=S.|year=2000|title=Phylogeography of mitochondrial DNA variation in brown bears and polar bears|journal=Molecular Phylogenetics and Evolution|volume=15|issue=2|pages=19–26|doi=10.1006/mpev.1999.0730|pmid=10837161 |bibcode=2000MolPE..15..319S }}</ref> A 2010 study estimated that the polar bear lineage split from other brown bears around 150,000 years ago.<ref name="Lindqvist"/>
[[File:polarbrown-1.jpg|thumb|[[Grizzly–polar bear hybrid|Polar/brown bear hybrid]] taxidermy specimen on display at [[Natural History Museum at Tring]] in [[Hertfordshire]], England]]
More extensive genetic studies have refuted the idea that polar bears are directly descended from brown bears and found that the two species are separate [[sister group|sister lineages]]. The genetic similarities between polar bears and some brown bears were found to be the result of interbreeding.<ref name="Hailer">{{cite journal |doi=10.1126/science.1216424 |pmid=22517859 |title=Nuclear genomic sequences reveal that polar bears are an old and distinct bear lineage |journal=Science |volume=336 |issue=6079 |pages=344–347 |year=2012 |last1=Hailer |first1=F. |last2=Kutschera |first2=V. E. |last3=Hallstrom |first3=B. M. |last4=Klassert |first4=D. |last5=Fain |first5=S. R. |last6=Leonard |first6=J. A. |last7=Arnason |first7=U. |last8=Janke |first8=A. |bibcode=2012Sci...336..344H|hdl=10261/58578 |s2cid=12671275|hdl-access=free }}</ref><ref name=Lan>{{cite journal|last1=Lan|first1=T.|last2=Leppälä|first2=K.|last3=Tomlin|first3=C.|last4=Talbot|first4=S. L.|last5=Sage|first5=G. K.|last6=Farley|first6=S. D.|last7=Shideler|first7=R. T.|last8=Bachmann|first8=L.|last9=Wiig|first9=Ø|last10=Albert|first10=V. A.|last11=Salojärvi|first11=J.|last12=Mailund|first12=T.|last13=Drautz-Moses|first13=D. I.|last14=Schuster|first14=S. C.|last15=Herrera-Estrella|first15=L.|last16=Lindqvist|first16=C.|year=2022|title=Insights into bear evolution from a Pleistocene polar bear genome|journal=Proceedings of the National Academy of Sciences|volume=119|issue=14|page=e2200016119|doi=10.1073/pnas.2200016119|doi-access=free |pmid=35666863|pmc=9214488|bibcode=2022PNAS..11900016L}}</ref> A 2012 study estimated the split between polar and brown bears as occurring around 600,000 years ago.<ref name="Hailer"/> A 2022 study estimated the divergence as occurring even earlier at over one million years ago.<ref name=Lan/> [[Glaciation]] events over hundreds of thousands of years led to both the origin of polar bears and their subsequent interactions and hybridizations with brown bears.<ref name="Hassanin">{{Cite journal |last=Hassanin |first=Alexandre |year=2015 |title=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 |url=https://comptes-rendus.academie-sciences.fr/biologies/item/10.1016/j.crvi.2015.04.008.pdf |journal=Comptes Rendus Biologies |volume=338 |issue=7 |pages=494–501 |doi=10.1016/j.crvi.2015.04.008 |pmid=26026577}}</ref>

Studies in 2011 and 2012 concluded that [[gene flow]] went from brown bears to polar bears during hybridization.<ref name="Hailer"/><ref name=Edwards>{{cite journal|last1=Edwards|first1=C. J.|last2=Suchard|first2=M. A.|last3=Lemey|first3=P.|last4= Welch|first4=J. J.|last5=Barnes|first5=I.|last6=Fulton|first6=T. L.|last7=Barnett|first7=R.|last8=O'Connell|first8=T.|last9=Coxon|first9=P.|last10= Monaghan|first10=N.|last11=Valdiosera|first11=C. E.|last12=Lorenzen|first12=E. D.|last13=Willerslev|first13=E.|last14=Baryshnikov|first14=G. F.|last15=Rambaut|first15=A.|last16=Thomas|first16=M. G.|last17=Bradley|first17=D. G.|last18=Shapiro|first18=B.|year=2011|title=Ancient hybridization and an Irish origin for the modern polar bear matriline |journal=Current Biology|volume=21|issue=15|pages=1251–1258|doi=10.1016/j.cub.2011.05.058|pmid=21737280 |pmc=4677796 |bibcode=2011CBio...21.1251E }}</ref> In particular, a 2011 study concluded that living polar bear populations derived their [[matriline|maternal line]]s from now-extinct Irish brown bears.<ref name=Edwards/> Later studies have clarified that gene flow went from polar to brown bears rather than the reverse.<ref name=Hassanin/><ref name=Cahill>{{cite journal|last1=Cahill|first1=J. A.|last2=Heintzman|first2=P. D.|last3=Harris|first3=K.|last4=Teasdale|first4=M. D.|last5=Kapp|first5=M. D.|last6=Soares|first6=A. E. R.|last7=Stirling|first7=I.|last8=Bradley|first8=D.|last9=Edward|first9=C. J.|last10=Graim|first10=K.|last11=Kisleika|first11=A. A.|last12=Malev|first12=A. V.|last13=Monaghan|first13=N.|last14=Green|first14=R. E.|last15=Shapiro|first15=B.|year=2018|title=Genomic evidence of widespread admixture from polar bears into brown bears during the last ice age|journal=Molecular Biology and Evolution|volume=35|issue=5|pages=1120–1129|doi=10.1093/molbev/msy018|pmid=29471451 |doi-access=free|hdl=10037/19512|hdl-access=free}}</ref><ref>{{cite journal|last1=Wang|first1=M-S|last2=Murray|first2=G. G. R.|last3=Mann|first3=D.|last4=Groves|first4=P.|last5=Vershinina|first5=A. O.|last6=Supple|first6=M. A.|last7=Kapp|first7=J. D.|last8=Corbett-Detig|first8=R.|last9=Crump|first9=S. E.|last10=Stirling|first10=I.|last11=Laidre|first11=K. L.|last12=Kunz|first12=M.|last13=Dalén|first13=L.|last14=Green|first14=R. E.|last15=Shapiro|first15=B.|year=2022|title=A polar bear paleogenome reveals extensive ancient gene flow from polar bears into brown bears|journal=Nature Ecology & Evolution|volume=6|issue=7|pages=936–944|doi=10.1038/s41559-022-01753-8|pmid=35711062 |bibcode=2022NatEE...6..936W |s2cid=249747066 }}</ref> Up to 9 percent of the genome of ABC bears was transferred from polar bears,<ref>{{cite journal|last1=Cahill|first1=J. A.|last2=Stirling|first2=I.|last3=Kistler|first3=L.|last4=Salamzade|first4=R.|last5=Ersmark|first5=E.|last6=Fulton|first6=T. L.|last7=Stiller|first7=M.|last8=Green|first8=R. E.|last9=Shapiro|first9=B.|year=2015|title=Genomic evidence of geographically widespread effect of gene flow from polar bears into brown bears|journal=Molecular Phylogenetics and Evolution|volume=24|issue=6|pages=1205–1217|doi=10.1111/mec.13038|pmid=25490862 |pmc=4409089 |bibcode=2015MolEc..24.1205C }}</ref> while Irish bears had up to 21.5 percent polar bear origin.<ref name=Cahill/> Mass hybridization between the two species appears to have stopped around 200,000 years ago. [[Grizzly–polar bear hybrid|Modern hybrids]] are relatively rare in the wild.<ref name=Lan/>

Analysis of the [[copy number variation|number of variations of gene copies]] in polar bears compared with brown bears and [[American black bears]] shows distinct adaptions. Polar bears have a less diverse array of [[olfactory receptor]] genes, a result of there being fewer odours in their Arctic habitat. With its carnivorous, high-fat diet the species has fewer copies of the gene involved in making [[amylase]], an enzyme that breaks down [[starch]], and more selection for genes for fatty acid breakdown and a more efficient [[circulatory system]]. The polar bear's thicker coat is the result of more copies of genes involved in [[keratin]]-creating proteins.<ref>{{cite journal|last1=Rink|first1=D. C.|last2=Specian|first2=N. K.|last3=Zhao|first3=S.|last4=Gibbons|first4=J. G.|year=2019|title=Polar bear evolution is marked by rapid changes in gene copy number in response to dietary shift|journal=Proceedings of the National Academy of Sciences|volume=116|issue=27|pages=13446–13451|doi=10.1073/pnas.1901093116 |doi-access=free |pmid=31209046 |pmc=6613075|bibcode=2019PNAS..11613446R }}</ref>