Evolutionary Rates in Proteins: Neutral Mutations and the Molecular Clock
There is an interesting relationship between neutral mutations and the molecular clock. The theory of neutral mutations requires that the rate of fixation of a mutation be equal to the neutral mutation rate. Thus the fixation of neutral mutations should be clocklike, with each “tick” of the clock representing another fixation. Naturally the clock will not be metronomic but, like a radioactive clock, stochastic, with fixation events in the unit time interval showing a Poisson distribution. Although a test of the clock hypothesis is a test of the neutral hypothesis, the existence of a clock does not depend on the correctness of the neutral hypothesis (see also Sarich and Cronin, this volume). This has resulted in past confusion. In a similar fashion, the covarion (concomitantly variable codons) concept (Fitch and Markowitz, 1970) is also independent of the correctness of the neutral hypothesis. We shall consider in turn a statistical model to test evolutionary rates, the results of that test, a comparison of our estimated rates with those from other sources, and some problems in testing evolutionary clocks.
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