The rate of mutations in certain types of DNA sequences are the same no matter the species, according to a paper in this week's PNAS, and suggest the presence of a molecular clock that operates in DNA, say the authors.

The molecular clock hypothesis resulted from studying how beta-globin proteins from different organisms appeared to be changing at a fairly constant rate, no matter which lineage was studied, Philip Green, one of the authors, told The Scientist. But prior to the findings described in the paper, scientists had believed that such a clock—due to changes that occur as the result of errors during replication—could not hold at the DNA level.

Green and Dick G. Hwang, both of the Department of Genome Sciences at the University of Washington, applied a Bayesian Markov chain Monte Carlo sequence analysis—"a statistical model for a complicated situation where you can write down...

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