Most discussions of the rates of DNA evolution have been with respect to the molecular clock hypothesis which states that there is a positive linear relationship between time since two species diverged and amount of genetic divergence (e.g., DNA sequence difference) between those species. These observations stated above indicate that there is not one molecular clock but probably many molecular clocks that "tick" at different rates. Lets say we identify a reliable molecular clock (e.g., number of amino acid substitutions in the cytochrome C gene), we can use this to date, or corroborate, evolutionary events of interest (e.g., the divergence times for species that do not have good fossil data). For example: we know that there are K XY substitutions between species X and Y and we know that they diverged T years ago (from fossil data). Thus the rate of molecular evolution is r = K XY /2T . The denominator has a 2 in it because there are two paths of evolution on which the divergence can accumulate (ancestor to
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