Polar Bears' Irish Ancestors
Polar bears have Irish roots, a new study suggests. Genetic evidence reveals that modern polar bears are the descendents of Irish brown bears that lived during the last ice age about 45,000 years ago.
The study was conducted by Beth Shapiro of Penn State University and Daniel Bradley of Trinity College Dublin. The results are published in the 7 July 2011 issue of the journal Current Biology.
Shapiro and Bradley report that the now-extinct Irish brown bears interbred with ancestral polar bears, leaving an unmistakable DNA fingerprint still carried by modern polar bears. That fingerprint, missing from the DNA of modern brown bears, confirms that interbreeding between polar bears and brown bears took place after the two lineages were already separate species.
The DNA fingerprint exists within the mitochondrial DNA of polar bears. Mitochondrial DNA is passed down the female lineage, from mother to offspring. This means that the ancient hybridization involved the mating of female Irish brown bears and prehistoric male polar bears.
Polar bears today differ markedly from their brown bear cousins. They two species differ in fur color, size, tooth structure, body shape, habitat and range. Yet the species can still interbreed. Polar-brown bear hybrids have been observed in the wild during recent years. Scientists think that many such crosses have occurred during the evolutionary history of the two species.
Study author Beth Shapiro suggests that climate fluctuations were, and are, an important factor in the interbreeding of polar bears and brown bears. As climate changes, so do the ranges of polar bears and brown bears. This cyclical expansion and contraction of the species’ ranges brings them into contact periodically. During times when their ranges overlap, brown bears and polar bears can interbreed.
References
Edwards, C., Suchard, M., Lemey, P., Welch, J., Barnes, I., Fulton, T., Barnett, R., O’Connell, T., Coxon, P., Monaghan, N., Valdiosera, C., Lorenzen, E., Willerslev, E., Baryshnikov, G., Rambaut, A., Thomas, M., Bradley, D., & Shapiro, B. (2011). Ancient Hybridization and an Irish Origin for the Modern Polar Bear Matriline Current Biology DOI: 10.1016/j.cub.2011.05.058
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