Evolution

Population Genetics of Frozen Bears

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Science  25 Feb 2000:
Vol. 287, Issue 5457, pp. 1365
DOI: 10.1126/science.287.5457.1365b

It is generally assumed that genetic differences between geographically isolated populations of a species are a direct consequence of their separation, which reduces the chance that genetic exchange will counteract divergence. Indeed, the degree of genetic divergence can be an indication of the endurance of isolation.

Leonard et al. have analyzed mitochondrial DNA sequences from samples of brown bears preserved for 15,000 years in Alaskan permafrost. The isolated populations of brown bears in present-day North America vary in their sequences of these regions of mitochondrial DNA; surprisingly, the sequences within a single population of Ice Age bears showed a similar range of variation. If this variation existed before the populations became geographically subdivided, then the current divergence is not the result of post-isolation genetic drift; rather, it must have arisen by a ‘sampling effect’, as the original population became fragmented and dispersed (see also Willis and Whittaker, Perspective, this issue, p. 1406).

The analysis of ancestral populations is a rare bonus for the population geneticist, and the exceptional preservation of the Ice Age brown bears is unlikely to be repeated in many organisms. But reconstructions of past population history by phylogenetic inference from extant populations now have an added twist.—AMS

Proc. Natl. Acad. Sci. U.S.A.97, 1651 (2000).

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