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From the standpoint of the neutral theory of molecular evolution, it is expected that a universally valid and exact molecular evolutionary clock would exist if, for a given molecule, the mutation rate for neutral alleles per year were exactly equal among all organisms at all times.  \end{quotation}    However the extent to which data supported the clocklike progression of molecular evolution was examined by a series of empirical studies that showed that the variance from lineage to lineage of empirically measured molecular clocks tended to be greater than would be expected from Poisson error alone \cite{LangleyFitch1974}. \cite{LangleyFitch1974,Kimura1987}.     
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