Molecular Evolution Activities

The Molecular Clock
 

The Molecular Clock

The Neutralists' Mechanism

Motoo Kimura's original arguments in 1968 for the significance of neutral mutations and random drift were based in part on the hemoglobin data presented by Zuckerkandl and Pauling in 1965. In fact in 1971, Tomoko Ohta and Motoo Kimura asserted that the "remarkable constancy of the rate of amino acid substitutions in each protein over a vast period of geologic time constitutes so far the strongest evidence for the theory (Kimura 1968, King and Jukes 1969) that the major cause of molecular evolution is random fixation of selectively neutral or nearly neutral mutations" (Ohta and Kimura 1971, p. 18). The reason the constancy provided such strong support is that the neutral theory provided a mechanism for that constancy--the prediction of rate constancy followed easily from basic theoretical commitments of Kimura and the neutralists. According to the neutralists, the rate per generation of mutant substitutions in a population (k) is equal to the mutation rate per gamete (v): k = v.

This result is derived as follows. In a population of actual size N, there are 2Nv new mutations produced in the entire population per generation. Only a certain fraction of these new mutations will become established in the population--that is only a certain fraction will reach fixation. Let "u" represent he probability that a new mutation will reach fixation. Then, in Kimura's words, "in a steady state in which the process of substitution goes on for a very long time, the rate k of mutant substitution per unit time is given by the equation k = 2Nvu " (Kimura 1979, p. 108). For selectively neutral mutations, however, the probability of fixation (u) is equal to 1/(2N), because "any one of the 2N genes in the population is as likely as any other to be fixed, and so the probability that the new mutant will be the lucky one is 1/(2N)" (Kimura 1979, p. 108). Substituting 1/(2N) for u in equation 2 yields equation 1, k = v. It is important to note here that the rate of mutant substitution is independent of population size.

The rate of evolution for selectively advantageous mutants, in contrast to neutral mutants, is dependent on both population size and selection pressure. Kimura uses a lengthy derivation to show that for selectively advantageous mutants the equation for the rate of evolution is k = 4Nsv (Kimura 1979, p. 110). Thus, in order for a selectionist model to account for the constancy of the rates of evolution it must show how constancy is possible when the rates are strongly dependent on the environment, as represented by the selection coefficient s and the measure of population size N, which can be quite variable. Under the selectionist model, the rates of molecular evolution should show nearly the same variability as the rates of phenotypic evolution. Unfortunately for the neutralists, the rates of molecular evolution were found to be far from uniform.

 

The Molecular Clock

 

This page was last updated on 1 August 2002 by Michael Dietrich.