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

These are some of the points regarding this new paper by Miller et al. (2012, in press), on genetic evidence for the origin of polar bears, that I think deserve a critical look. See part 1, for my comments on Claim #1 (the fossil evidence)(see Doug Hoffman summary of the paper here).

In this post, I’ll elaborate on Claim #2. I’ve added one more to the original three listed.

Claim #1 Polar bears and brown bears (aka grizzlies) arose 4-5 million years old. [I countered with the fossil evidence]

Claim #2 Hybridization in both directions occurred repeatedly throughout the evolutionary history of polar bears and brown bears.

Claim #3 Svalbard may have been an important refugium for polar bears during warm interglacial periods.

Claim #4 Polar bear population numbers (population size estimates) over the last one million years tracked changes in climate (warmer/colder periods).

Claim #2 Hybridization in both directions occurred repeatedly throughout the evolutionary history of polar bears and brown bears [a claim also made in two other recent papers on polar bear evolution (Edwards et al. 2011; Hailer et al. 2012)].

Here’s the problem with this claim: so far, all known examples of hybridization in the wild are in one direction only: male grizzly X female polar bear. All known examples in the other direction (male polar bear X female grizzly) are known only from animals held in captivity.

See the story on this grizzly (male) X polar bear (female) from 2006 in the National Post. The specifics on this cross (which species was the female partner) have been confirmed by DNA analysis. By convention, naming of hybrids puts the male partner first, which would make this a ‘grolar bear.’

Earlier this year, I submitted an official comment, now available online, to both the Edwards et al. 2011 paper (published in Current Biology) and Hailer et al. 2012 (published in Science), both of which assumed that hybridization between grizzlies and polar bears was occurred in both directions during their evolutionary history. I also re-posted the comment to Edwards et al. in a blogpost by filmmaker Arthur C. Smith III at Pierre Gosselin’s blog, No Tricks Zone. I’ve copied the longer of the two below.

The take-home message is this: Although hybridization with brown bears may indeed have occurred during the evolutionary history of polar bears, the proposed direction of gene-flow between species (i.e. introgression) must be based on documented examples of mating between species in the wild. All recent papers on polar bear evolution – including the most recent Miller et al. 2012 paper – assume that either cross (grizzly male X polar bear female vs. polar bear male X grizzly female) is equally likely to have occurred in the wild, a conclusion which not currently supported by evidence.

So far, the evidence shows that only the grizzly bear male X polar bear female cross occurs in the wild, with compelling behavioral reasons for this being the case. Even in 2nd generation crosses, involving a mature hybrid animal, a grizzly was the male partner.

Directionality in polar bear hybridization
(comment to Edwards et al. 2011)

Hybridization between polar bears and brown bears has recently been discussed in two influential papers on the genetic evidence for polar bear evolution [1,2]: this one (Edwards et al.), which appeared last year in this journal, and another one published earlier this year in Science Magazine. Citing Kelly et al. [3] as evidence for the existence of confirmed brown bear/polar bear hybrids, the authors of these two genetic papers suggest that gene flow between polar bears and brown bears is just as likely in one direction as the other when hybridization occurs in the wild.

Both of these papers miss an important consideration. The only hybrid example mentioned by Kelly et al. [3] was the 2nd generation offspring of a brown bear male and a hybrid female shot in 2010 in the western Canadian Arctic. Analysis of nuclear and mtDNA indicated that this animal’s hybrid mother was the offspring of a male brown bear and a female polar bear.

The only other hybrid documented in the wild was the offspring of a brown bear male and a female polar bear in 2006 (details on both hybrids were reported widely in newspapers around the world, with the details of the genetic testing provided in government-issued press releases and follow-up interviews. Recently, I confirmed the accuracy of these reports with the wildlife officials involved.

Therefore, only male brown bears x female polar bear crosses have been documented in the wild. The small number of verified hybridizations does not allow quantification of past introgression, but the examples from extant wild populations indicate only male brown bears are involved in inter-species matings.

Two instances of hybridization involving polar bear males and brown bear females (the cross proposed by Hailer et al. [1]) have been documented, but both involved captive animals. In the most recent case [4], mating occurred for the first time after the animals had been together for 24 years.

Brown bears mate from mid-May to July while polar bears mate April to May, leaving a brief period in late May when an early-breeding brown bear male and a late-breeding polar bear female might get together. Brown bear males also tend to emerge from their winter dens before females, increasing the chances that a brown bear male might encounter and accept a polar bear female as a mate.

In addition, although polar bears are often larger than brown bears, polar bears are less aggressive than brown bears during interactions between them [6, see pg. 16, 69]. This behavioral difference suggests another reason why only brown bears have been documented as the male partner in all inter-species mating with polar bears in the wild.

It appears that for brown bears (as for wolves and domestic dogs, among many other examples), the less aggressive species is usually the derived [newer] species and also the female partner in inter-species crosses [6-9].

Taken together, this evidence suggests that hybridization between brown bears and polar bears may be largely unidirectional in the wild and predictable based on life histories, behavior and evolutionary relationship. Although hybridization with brown bears may indeed have occurred during the evolutionary history of the polar bear, the direction of introgression must be critically assessed with documented examples of inter-species matings.

References [originals were truncated, these are expanded]
1. Hailer, F., Kutschera, V.E., Hallstrom, B.M., Klassert, D., Fain, S.R., Leonard, J.A., Arnason, U., and Janke, A. 2012. Nuclear genomic sequences reveal that polar bears are an old and distinct bear lineage. Science 336:344-347.

2. Edwards, C.J., Suchard, M.A., Lemey, P., Welch, J.J., Barnes, I., Fulton, T.L., Barnett, R., O’Connell, T.C., Coxon, P., Monoghan, N., Valdiosera, C.E., Lorenzen, E.D., Willerslev, E., Baryshnikov, G.F., Rambaut, A., Thomas, M.G., Bradley, D.G. and Shapiro, B. 2011. Ancient hybridization and an Irish origin for the modern polar bear matriline. Current Biology 21:1251-1258.

3. Kelly, B., Whiteley, A., and Tallmon, D. 2010. Comment: The Arctic melting pot. Nature 468:891. [the comments below this short article, visible even without a subscription, are worth reading]

4. Walker, M. 2009. Polar bear plus grizzly equals? BBC News Online, Friday 30 October, 2009. accessed 2 Nov. 2011.

5. Aars, J., Lunn, N. J. and Derocher, A.E. 2006. Polar Bears: Proceedings of the 14th Working Meeting of the IUCN/SSC Polar Bear Specialist Group, 20-24 June 2005, Seattle, Washington, USA. Occasional Paper of the IUCN Species Survival Commission 32. IUCN, Gland (Switzerland) and Cambridge (UK).

6. Bradley, R.D., Davis, S.K., and Baker, R.J. 1991. Genetic control of premating-isolating behavior: Kaneshiro’s hypothesis and asymmetrical sexual selection in pocket gophers. Journal of Heredity 82:192-196.

7. Crockford, S.J. 2006. Rhythms of Life: Thyroid Hormone and the Origin of Species. Victoria: Trafford.

8. Kaneshiro, K.Y. 1980. Sexual isolation, speciation and the direction of evolution. Evolution 34:437-444.

9. Roca, A.L., Georgiadis, N. and O’Brien, S.J. 2005. Cytonuclear genomic dissociation in African elephant species. Nature Genetics 37:96-100.

To quote this comment, use this citation:
Crockford, S.J. 2012. Directionality in polar bear hybridization. Comment, with references (posted May 1) in response to Edwards et al. 2011. “Ancient hybridization and an Irish origin for the modern polar bear matriline.” Current Biology 21: 1251-1258.

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