New genetic study confirms polar bears survived several warm Interglacials

There is a new polar bear genetics paper out in the Journal of Heredity, by University of Alaska Fairbanks genetics professor Matt Cronin and colleagues. Matt Cronin, in case you didn’t know, was the first to pick up the close genetic relationship between polar bears and grizzlies, as a result of research he and colleagues did back in the early 1990s (Cronin et al. 1991).

Figure1 from Cronin et al. 2014 (in press) showing the locations of bear samples used in their genetic study. MT, Montana; AK, Alaska; Polar bear samples were from the Chukchi, Beaufort and Barents Sea populations.

Figure 1 from Cronin et al. 2014 (in press) showing the locations of bear samples used in their genetic study. MT, Montana; AK, Alaska; Polar bear samples were taken from the Chukchi, Beaufort and Barents Sea populations.

While no earth-shattering new information was revealed in this new study, reported over the weekend by the Alaska paper SitNews (March 15), it used a more detailed method to confirm the results of previous work – that polar bears have been around long enough to have survived several past Interglacial periods that were warmer than today (with less ice in the Arctic) and are genetically distinct from grizzlies.

A feature that really set this work apart was how it was promoted.

Cronin says this about the results in the UAF press release (pdf here) :

“The ramifications are that if the polar bear was an independent species for about 1 million years it survived previous cold and warm periods,” Cronin said. “This means the polar bear has been an independent lineage a long time through glacial and interglacial and warm periods.”

The last glacial period was at maximum extent about 22,000 years ago, and was preceded by a warm interglacial period about 130,000 years ago. Other warm and cold periods preceded that. Cronin thinks that if polar bears survived previous warm periods in which there was little or no arctic summer sea ice, this should be used in models predicting the species’ response to current climate change.

It seems logical that if polar bears survived previous warm, ice-free periods, they could survive another. This is of course speculation, but so is predicting they will not survive, as the proponents of the endangered species act listing of polar bears have done.” [my bold]

Compare these reasoned comments made by Cronin with the statements made about this time last year by polar bear geneticists Beth Shapiro and Ed Green, who spun tales of catastrophic global warming right over the top in order to promote their new paper (Cahill et al. 2013), which I discussed in this post: Polar bear spin reaches new heights as geneticists promote their work.

A world of difference – which one sounds like a rational scientist to you?

[my comments on the paper referred to above (Cahill et al. 2013) are in this post, Polar bears cavorting with ABC brown bears not supported by geological and fossil evidence”]

More about the Cronin et al. 2014 research (from the UAF press release):

“The 2014 paper replicates other research on bear genomes but includes analysis of genetic variation in more than 300 bears from Alaska and genetic elements not assessed previously in bears. These are known as ultra-conserved elements, and show the polar and brown bears to be more closely related than either is to black bears.

The data was used in a “molecular clock” that uses the numbers of differences (mutations) in DNA sequences to estimate when the sequences, and hence the species, diverged. The data suggest that polar bears and brown bears diverged as different species 1.2 million years ago, and black bears diverged from the polar/brown bear lineage 2.3 million years ago. These estimates are within the ranges in other studies.

Utilizing labs at the University of California Davis, Cronin and technology experts pored over huge datasets. He also analyzed tissue samples from Montana and from Alaska’s Admiralty, Baranof and Chichagof islands, obtained from state and federal wildlife agencies. In recent years DNA science has improved so much that Cronin is able to study billions of nucleotides of DNA rather than the thousands he used to be limited to. “It’s very advanced because of the applications in medicine and agriculture,” Cronin said.”

Figure 2 from Cronin et al. 2014 (in press) showing the overlap in genetic elements (“SNPs” between black, brown (grizzly) and polar bear: there is more overlap between brown and polar bear than between either of them and black bears, but there are also elements unique to each species. Other results were “consistent with introgressive hybridization involving male brown bears and female polar bears” as occurs today in the wild.

Figure 2 from Cronin et al. 2014 (in press) showing the overlap in genetic elements (“SNPs”) between black, brown (grizzly) and polar bear. You can see that there was more overlap found between brown and polar bear than between either of them and black bears, but there are also elements unique to each species. Other results were “consistent with introgressive hybridization involving male brown bears and female polar bears” as occurs today in the wild.

Related posts:
Eemian excuses: the warm was different then, polar bears were fine
November 12 2013

More evidence that the polar bear is a distinct species
December 6, 2012

Is it plausible that Polar bears are 4-5 million years old? Part 1.
August 19, 2012.

Is it plausible that polar bears are 4-5 million years old? Part 2, Hybridization
August 22, 2012

Is it plausible that polar bears are 4-5 million years old? Part 3, sea ice.
August 23, 2012

References
Cahill JA, Green RE, Fulton TL, Stiller M, Jay F, et al. 2013. Genomic evidence for island population conversion resolves conflicting theories of polar bear evolution. PLoS Genetics 9(3): e1003345.

Cronin, M.A., Amstrup, S.C. and Garner, G.W. 1991. Interspecific and intraspecific mitochondrial DNA variation in North American bears (Ursus). Canadian Journal of Zoology 69:2985-2992. http://www.nrcresearchpress.com/doi/abs/10.1139/z91-421#.UyZhLoVa9lo Pdf here.

Cronin, M.A., Rincon, G., Meredith, R.W., MacNeil, M.D., Islas-Trejo, A., Cánovas, A. and Medrano, J. F. 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, in press. http://jhered.oxfordjournals.org/content/early/2014/01/28/jhered.est133.abstract

Abstract

We assessed the relationships of polar bears (Ursus maritimus), brown bears (U. arctos), and black bears (U. americanus) with high throughput genomic sequencing data with an average coverage of 25× for each species. A total of 1.4 billion 100-bp paired-end reads were assembled using the polar bear and annotated giant panda (Ailuropoda melanoleuca) genome sequences as references. We identified 13.8 million single nucleotide polymorphisms (SNP) in the 3 species aligned to the polar bear genome. These data indicate that polar bears and brown bears share more SNP with each other than either does with black bears. Concatenation and coalescence-based analysis of consensus sequences of approximately 1 million base pairs of ultraconserved elements in the nuclear genome resulted in a phylogeny with black bears as the sister group to brown and polar bears, and all brown bears are in a separate clade from polar bears. Genotypes for 162 SNP loci of 336 bears from Alaska and Montana showed that the species are genetically differentiated and there is geographic population structure of brown and black bears but not polar bears.

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