Evolutionary History of Polar and Brown Bears

Abstract

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.
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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]