molecular clock The concept that during evolution the number of substitutions in the nucleotides of nucleic acids (DNA or RNA), and hence in the proteins encoded by the nucleic acids, is proportional to time. Hence, by comparing the DNA or proteins of species that diverged a known length of time ago (e.g. determined from fossil evidence), it is possible to calculate the average substitution rate, thereby calibrating the `molecular clock'. Comparative studies of different proteins in various groups of organisms tend to show that the average number of amino-acid substitutions per site per year is typically around 10^–9. These results indicate a fairly constant rate of molecular evolution in comparable sequences of macromolecules in different organisms.

molecular clock The idea that molecular evolution occurs at a constant rate, so that the degree of molecular difference between 2 species can be used as a measure of the time elapsed since they diverged. Its accuracy depends on the validity of the neutral mutation theory.

molecular clock The idea that molecular evolution occurs at a constant rate, so that the degree of molecular difference between two species can be used as a measure of the time elapsed since they diverged. Its accuracy depends on the validity of the neutrality theory of evolution.

molecular clock The idea that molecular evolution occurs at a constant rate, so that the degree of molecular difference between two species can be used as a measure of the time elapsed since they diverged. Its accuracy depends on the validity of the neutrality theory of evolution.

mo·lec·u·lar clock • n. Genetics the average rate at which a species' genome accumulates mutations, used to measure their evolutionary divergence and in other calculations.