Polar bear origins revised – they’re older and more distinct than we thought

By Ed Yong | April 19, 2012 4:00 pm

Doesn’t look a day older than 602,000

It looked like we had the polar bear’s origin story nailed down. Genetic studies suggested that between 111 and 166 thousand years ago, a group of brown bears, possibly from Ireland, split off from their kin. In a blink of geological time, they adapted to the cold of the Arctic, and became the polar bears we know and worry about. Fossils supported this story: the oldest polar bear bone is between 110 and 130 thousand years old.

But according to Frank Hailer at the Biodiversity and Climate Research Centre in Frankfurt, this story is wrong in two important ways. First, the polar bear aren’t just a branch of the brown bear family tree. They’re a separate lineage in their own right. Second, they around four times older than anyone had thought, arising around 600 thousand years ago.

If this new vision is right, the bear’s journey to polar dominance wasn’t a speedy sprint, but a more leisurely stroll. As a species, polar bears have seen many ice ages. Rather than being a symbol of extraordinarily fast evolution, they’ve actually had plenty of time to adapt to life in the freezer.

We have two separate origin stories for polar bears because, like all animals, they have two genomes. There’s the nuclear genome that hoards most of their DNA in the centre of their cells, and the smaller mitochondrial one, housed in small battery-like structures. The nuclear genome is inherited both parents, but the mitochondrial one is only inherited from mothers. That’s important, for reasons we’ll come to.

Until now, scientists had studied polar bear evolution by looking at their mitochondrial DNA. These studies produced a model in which the polar lineage is nested within a wider brown dynasty, meaning that some brown bears are more closely related to polar bears than to other brown bears. This called the polar bear’s status as a separate species into question.

Hailer changed that by sequencing the nuclear DNA of 45 brown, polar and black bears, at 14 different sites. His results showed that polar and brown bears are actually sister groups, that branched off from each other around 603 thousand years ago. Polar and brown bears are both distinct species, and they’re as genetically different from each other as they are from black bears.

It’s understandable why the fossils told a different story. Polar bears live in a world of shifting ice and freezing water, so many of their bones have been lost. But why did the mitochondrial DNA results differ so radically?

Hailer thinks that the two bears would often meet and mate with one another, long after they diverged. Indeed, they can still create hybrids today. If female brown bears mated with male polar bears, the hybrid (a pizzly bear) would carry a brown bear’s mitochondrial genome. This hitchhiking DNA could have invaded the wider polar population if the hybrids mated with other polar bears.

If you looked at the mitochondrial DNA, you’d see the footprint of these recent polar-brown liaisons, and mistake them for a split between the two groups. If you looked at the nuclear DNA, you’d see when the split actually happened. That’s exactly what Ceiridwen Edwards from Oxford University suggested when she analysed the bear’s mitochondrial DNA last year.

There are interesting parallels to our understanding of human evolution. For many years, scientists had been studying the mitochondrial DNA of Neanderthals, recovered from their bones. These sequences suggested that Neanderthals were a separate species to modern humans, and we never bred with one another. But the Neanderthal nuclear genome, unveiled in 2010, put paid to that idea. It revealed that Neanderthals and modern humans must have had sex, for 1 to 4 per cent of every European or Asian genome now comes from Neanderthals. Again, the nuclear genes uncovered a story that was invisible to the mitochondrial ones.

Charlotte Lindqvist, who led one of the earlier mitochondrial studies, isn’t surprised that polar and brown bears turn out to be distinct lineages. “These two species are clearly recognized as separate based on morphological and behavioral characteristics,” she says.

However, she thinks that it’s too early to say when the two species split from one another. Lindqvist says, “It doesn’t surprise me that it is older than previously suggested [but] I believe it is premature to settle on a date estimate based on relatively little molecular evidence.”After all, Hailer only looked at around 9,000 letters in each bear’s DNA – just a tiny fraction of their full genomes.

Edwards says that the study is a “welcome addition”. She’s now keen to analyse ancient DNA from bear fossils, to better understand how the two species diverged over time, and how they may have shared genes after their split.

Reference: Hailer, Kutschera, Hallstrom, Klassert, Fain, Leonard, Arnason & Janke. 2012. Nuclear Genomic Sequences Reveal that Polar Bears Are an Old and Distinct Bear Lineage. Science http://dx.doi.org/10.1126/science.1216424

More on polar bears:

CATEGORIZED UNDER: Animals, Carnivores, Evolution, Mammals

Comments (15)

  1. arctictern

    Well, if brown and polar bears can produce normal offspring, doesn’t that mean they belong to the same species?

  2. No. This is a common misconception about the species concept. To count as the same species, it is not enough that two animals *can* have offspring. They have to *actually* do that, and they have to produce viable and healthy offspring. So, for example, horses and donkeys can mate to make mules, but mules are infertile. Lions and tigers can mate, but again the hybrids aren’t great, and besides these two cats would almost never have the actual chance to mate. In the case of polar and brown bears, they can mate, but that’s still pretty rare because they occupy largely distinct habitats. And we don’t know about the viability of the offspring. True, this model suggests that the offspring must have been fit enough to mate back with the parental population, but it’s still unclear how often that happened or when it happened.

  3. I imagine by fossil bears he means cave bears and/or “short-faced” bears. Not sure of any other bears that would potentially preserve genetic information (too old). It would also be interesting to find out what population of black bears gave rise to the brown/polar split. AFAIK, black bears are pretty common in the northern hemisphere, occuring throughout North America and Eurasia.

  4. amphiox

    There are, what, 145+? species definitions, all in use in some context or another, out there. The definition of species is not as cut and dried as most laypeople think it is.

  5. Brian Too

    I still don’t understand the difference in results between m-DNA and n-DNA.

    Take the stated example. “Female brown bears mated with male polar bears…” OK, the offspring would get 100% of the maternal m-DNA, but they would get a roughly 50/50% split of the n-DNA.

    If this offspring then proceeded to great reproductive success, widely distributing it’s genetic legacy, and this happened even just occasionally, wouldn’t the result be the disappearance of the Polar Bear as a distinct species?

    Or are they saying that this happened rarely, and the different heritability of m-DNA and n-DNA caused the Polar Bears to look like brown bears genetically (m-DNA), while remaining distinct (n-DNA)?

  6. MattK

    Brian, mitochondria don’t crossover, they are inherited as unit. The mammalian Y-chromosome is the same – it only ever comes from the paternal side (rather than the maternal like mitochondria) and it doesn’t cross-over because there is now chromosome for it to pair with (X is too different). Anyway, if there was a hybridization event a couple hundred thousand years ago, the grizzly mtDNA would stay intact through generations while the nuclear DNA would get progressively broken up and diluted through each generation by crossing over and segregation. There may be other chunks of nDNA from grizzlies that also persist in polar bears but it would be only a tiny portion. Think of it this way, barring mutations your mitochondria are 100% identical to your great great great grandmother but only about 3% of your nDNA comes from her and that will be scattered across different chromosomes. As to why the mitochondrial genome from grizzlies spread through the polar bear population… I don’t know. Could have been drift.

  7. Does it mean that the 2 bears last interbred significantly around the time of the last interglacial? If so, it makes a lot of sense. Now with our warming world, we see polar bears and grizzlies coming more into contact…

    ?

    jules

  8. Eleanor

    psssst, Ceiridwen Edwards is female (and jolly nice, too). Women scientists are few enough without being reassigned!

    [I know! She let me know and proved her jolly niceness by accepting my grovelling apology – Ed]

    Seriously, the hybrid is a ‘pizzle’? Isn’t that a bit… rude?

  9. TJR

    If #2 is correct, that we don’t know whether polar and brown bears can produce viable offspring or not, then we don’t know whether they are the same or separate species, shirley?

    If the absence of opportunity to mate is allowed to define separate species then australian aborigines and south african bushmen will be separate species.

  10. Matt – I assume the brown bear n-DNA didn’t provide any advantages to polar bears but the m-DNA did, so the m-DNA spread while the n-DNA gradually got bred out.

    This topic raises the issue that when the species “really” diverged is somewhat plastic. As far as m-DNA is concerned, they really diverged 160k years ago.

    You could also argue that the polar bear as a m-DNA lineage went extinct at that time.

  11. MattK

    other Brian,

    I don’t think that the mtDNA must be adaptive to get fixed, especially if there were bottlenecks that would lessen the effect of selection and speed up drift. It’s also likely that the nuclear components were neutral and so wouldn’t have a huge effect if hybridization wasn’t common, and it’s also likely that any genes that had a large phenotypic effect would be selected against since the populations haven’t merged (as I think would happen if hybrids were more fit than either parent). The two species have quite different ecologies and natural history so I would predict that intermediate phenotypes would tend to have low fitness. Yes you are right that in most cases (excluding ploidy elevation and things like that) speciation is gradual and there is not usually an obvious threshold beyond which one becomes two.

    “As far as m-DNA is concerned, they really diverged 160k years ago. ” that depends on whether you treat mtDNA divergence as the actual phenomenon of interest or whether it is just a proxy for what we interested in (separation, reproductive isolation, and ecological divergence of populations into seperate species).

    “You could also argue that the polar bear as a m-DNA lineage went extinct at that time.” I suppose, but that strikes me as counterintuitive (the best kind of intuitive?) because it would be ignoring everything else about polar bears other than the mtDNA sequence. It’s an interesting way to play with the ideas.

  12. MrO

    Ed, I remember that in the las five years or so a few news have appeared on the press about some mixed polar and brown bears have been hunted or observed in the wild. Aparently the bears were mixed in different proportion, so indicating that the mix produce perfectly fertile offspring. Scientists are suggesting that the new climate scenario is putting both species into closer contact and making the interbreeding more usual (brown bears are wandering farther north and polar bears stay more in land due to the progresively lesser and thinner iced surface). Here there is thee links about this subject:

    http://www.sciencedaily.com/releases/2010/02/100223121439.htm

    http://news.nationalgeographic.com/news/2006/05/polar-bears.html

    http://en.wikipedia.org/wiki/Grizzly%E2%80%93polar_bear_hybrid

  13. Barbara

    Like mitochondria, chloroplasts have their own small genome. Several instances are now known of “chloroplast capture” – the transfer of chloroplasts from one plant lineage to another. Apparently there was a hybridization event, with the offspring breeding back to one of the parental species and thus transferring the chloroplast of species A to species B. Few if any of the nuclear genes of species A persist in the species B gene pool, and species B is still distinct. Rare hybridization is just one more source of variation, like mutation.

    How much hybridization is enough to cause us to change our minds and call two groups a single species? Hard to say, and it varies from one group to the next. In willows, for example, species can seem like evolutionary trends partly obscured by a haze of hybridization. In Carex sedges, most species are sharply distinct and reproductively isolated although some species are very similar. Interestingly, some closely related plant species introduced to North America are hybridizing so often here that the species are loosing their identities (on this continent); they’re undergoing “despeciation.”

    Polar Bears and Grizzly Bears remain distinct morphologically, behaviorally, and geographically (so far). We’d do best to consider them different species.

  14. “8. Eleanor Says:
    April 20th, 2012 at 6:52 am
    , , ,
    Seriously, the hybrid is a ‘pizzle’? Isn’t that a bit… rude?”

    Term is “pizzly”, sounded “peess-lee”, not “pitzl”. The word is a coinage from “polar”, and “grizzly”:

    p_{olar gr}_izzly . . . pizzly.

    To be sure, the moniker is not shy of comical hues. Other combinations don’t work so well: “pogri”, “polgrizz”, “grizzpo”, “grolar” . . . ? =)

  15. More complicated is black duck and mallard hybrids. Mallards are more common than black ducks. So if you’re hunting, you’re allowed to shoot more mallards than blacks. Hybrids are common enough that the law says you get to count a hybrid either way you want. And hybrids are called “black mallards”. If hybrids are so common, why aren’t they all hybrids? Because after a generation or three, you have either black ducks or mallards. It’s not at all like human skin color. We can define the word ‘species’ any way we want, and nature will continue doing whatever. More and more, i think of words like ‘species’ as a level zero approximation. Like the word ‘planet’.

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