Brown Bears and Polar Bears are two separate species. They each have their own characteristic set of DNA that belongs only to their respective species. But it wasn’t always this way. Long ago, a common ancestor of both polar and brown bears existed. Somewhere along the line, the ancestor species adapted to different environments and evolved into two different species.
Determining when exactly this point in time occurred–when polar bears and brown bears diverged–has given paleontologists and biologists quite the headache. Their trouble results mainly from a tricky population of bears in Alaska; on three islands known as the ABC Islands: Admiralty, Baranof, and Chichagof. These bears were believed to be brown bears due to their morphology, behaviors, and habitats; but had DNA very similar to that of polar bears. Scientists thus concluded that polar bears had diverged from brown bears about 150,000 years ago, when the DNA started changing to look more like a composite of brown and polar bear DNA. This didn’t jive with other bear genetics studies, which came up with estimates for divergence around 4 million years ago.
A new study shows the mixed DNA is not evidence for divergence; it is evidence of distinctly different brown and polar bears mating. Beth Shapiro, from the University of California Santa Cruz, working with other scientists, conducted an analysis of brown and polar bears, and compared it to the DNA of an ABC island bear. The results were unexpected: Polar bears have very pure DNA—that is, their DNA is consistent across the species and distinct from that of other species. The ABC Islands bear DNA came out to be 1% similar to that of polar bears, lending credence to the postulate that these bears were brown bears. But when the researchers compared only the X chromosomal DNA from the ABC Islands bears and polar bears, they found a 6.5% degree of similarity.
What could account for this? Like humans, bears have an X chromosome from their mother and a Y chromosome from their dad. The research team concluded that the ABC bears must have been the result of female polar bears mating with male brown bears. We know this is possible—in Northwest Canada, male brown bears and female polar bears mate regularly.
The researchers then ran a series of test simulations to determine what could have occurred demographically to cause these hybrid bears. The most likely scenario? When the earth warmed and glaciers started melting, male brown bears probably swam across from the mainland to the islands. It is a regular behavior for male brown bears to go searching for new habitats (they do this today). There they found female polar bears living on the islands and mated with them, producing hybrid offspring. The bear demographic eventually shifted from polar bears to brown bears, which are more suited to warmer weather. (Read more about today’s bear behaviors and how they helped bring light to the ABC bear population at usc.edu)
Though they did discover why the estimates for divergence between brown and polar bears was so varied by determining the most likely migration pattern of the brown bears, scientists still don’t know when exactly brown and polar bears diverged and what the common predecessor was. Due to the freezing and melting characteristic of the Arctic, remains don’t fossilize easily, and scientists are left still searching for that elusive common ancestor between brown and polar bears.
Read the original study in the journal PLOS Genetics.
Learn more at livescience.com.
The Paleontological Research Institution, Ithaca, New York, is pleased to sponsor Paleontology content for This View of Life. Founded in 1932, PRI has outstanding programs in research, collections, and publications, and is a national leader in development of informal Earth science education resources for educators and the general public.