Archive for the ‘Taxonomy’ Category

"Kiss me. I'm Irish."

From the CBC:

A female grizzly [brown] bear who lived in Ireland less than 50,000 years ago was an ancestor of all modern polar bears, suggesting “grolar” hybrids were an important part of polar bear history.

The unexpected DNA evidence suggests that interbreeding between polar bears and brown bears — usually known as grizzly bears in North America — is not unusual at times when climate change caused the range of the two species to overlap, such as around the beginning of the last ice age.

“It’s something that’s part of the history of the polar bear,” said Daniel Bradley, a genetics researcher at Trinity College Dublin who co-authored the study published Thursday in the journal Current Biology.

Grizzly-polar bear hybrids are rare, but a number have been seen in Canada in recent years. A bear in the Northwest Territories in April 2010 is believed to be the first second-generation hybrid — the offspring of a female grizzly-polar bear hybrid and a male grizzly bear — ever found in the wild.

Bradley and his colleagues wrote that their results suggest that hybrid bears should be protected, as both polar bears and grizzly bears currently are.

“They may play an underappreciated role in the survival of species,” the paper said.

Previous DNA studies had suggested that polar bears became a separate species about 800,000 to 150,000 years ago, and were most closely related to brown bears in the Admiralty, Baranof and Chichagof Islands near Alaska.

Bradley and his colleagues uncovered the evidence about the bears’ ancestry by looking at mitochondrial DNA extracted from the bones of museum specimens of Irish brown bears, which became extinct around 3,000 years ago. They compared it to mitochondrial DNA from other brown bears around the world as well as two ancient polar bear specimens from over 100,000 years ago.

“What was interesting was that the sequences from our brown bears were closer to modern polar bears than those ancient polar bears were,” he said in a phone interview Thursday.

In fact, the results, which Bradley said were unexpected, suggest that all polar bears share a female Irish brown bear ancestor from within the last 50,000 to 20,000 years — not that long ago on the evolutionary time-scale.

Based on the similarity between modern polar bear DNA and that of the Irish brown bear around the last ice age (between 100,000 and 10,000 years ago), they may have interbred multiple times.

About 22,000 years ago, an ice sheet covering most of Britain and Ireland was at its largest. That would have provided habitat for polar bears and may have forced brown bears into lowlands near the sea, where they could have encountered and mated with polar bears, the researchers suggested.

Bradley said he’s not sure how polar bears would have arrived in Ireland, but they may have crossed over the ice from Scandinavia.

The large differences between the mitochondrial DNA of ancient and modern polar bears is due to the fact that many polar bear lineages have died out, Bradley said. Modern polar bears are all very closely related to one another and may belong to a lineage that is different from that of the ancient specimens in the study.

The research team involved in the study included scientists from Ireland, the U.S., Belgium, England, Spain, Denmark, Russia, Scotland and Sweden.

When I first heard about the grolar bears, I thought that this could possibly the future for the polar bear species.  The polar bear really isn’t a good species. It is really just a modified form of brown bear, and this study strongly indicates that there has always been some gene flow between the two animals.

Of course, they don’t normally live in the same areas, so opportunities for cross-breeding are not as common as one might suppose.

This study found that after the polar bear population evolved fromn the ancient brown bear species, the only surviving lineage of polar bear still in existence is derived from hybridization with the Irish brown bear.

Hybridization between very closely related species is much more common in the wild that we have realized, and in the case of the polar bear, the only surviving form is derived from a brown bear ancestor.

I certainly think that grolar bears should be protected, but we’re going to have to come up with some way of classifying brown and polar bears that recognizes their close relationship.

I don’t know whether we should call them a single species or if we should call it the “brown and polar bear species complex.”

But we have such a narrow typological definition of species that it may be impossible to find some way to legally protect grolars.

This is one of those weird situations, which we humans have a very hard time understanding. Being the only surviving member of your genus means that we have a hard time realizing how two members of the same genus can have a gene flow. And just because one species has the genes of another doesn’t mean that one species is “polluting” the other. These gene flows are not necessarily bad things. They maintain genetic diversity, and they have the potential to spread beneficial traits from one species to another.

But when we blind ourselves to narrow typologies, we can’t see how crossbreeding can have some merit.

The world for the polar bear won’t end if we find  a polar bear with a grizzly bear ancestor three generations away.  It didn’t end when a polar bear mated with an Irish brown bear all those years ago. Indeed, that crossbreeding may have conferred certain advantages onto the ancestors of modern polar bears, allowing that lineage to survive while all the other polar bear lineages died out.

Polar and brown bears exist within a dynamic that includes some gene flow between populations. It isn’t as extensive as it is between wolves and dogs or between wolves and coyotes. But it isn’t insignificant.

We just like to think that species are unchanging. We like to think that once they have evolved, nothing more will happen.

But there is no “once they have evolved.”  Species will continue to evolve and adapt to new conditions.

One way they are able to adapt is to receive new genes from hybridizing with related species– if we want to think of polar and brown bears as distinct species.

Hybridization isn’t always species killer.  In this case, it might be a species saver. And it may have been one in the past.

But this is so difficult to grasp.

We are the species that does all the classifying. But we, as a species, are an unusually genetically depauperate species of ape, which is also the only surviving member of our genus.

We just don’t understand that other species can have greater genetic diversity than we do and can augment their genetic diversity through crossbreeding with a related species.

So we like to split species.

But we really ought to be lumpers.

Read Full Post »

« Newer Posts

%d bloggers like this: