Modeling the rate of nucleotide substitutions in DNA as a dichotomous
stochastic process with an inverse power-law correlation function desc
ribes evolution by a fractal stochastic process (FSP). This FSP model
agrees with recent findings on the relationship between the variance a
nd mean number of synonymous and nonsynonymous substitutions in 49 dif
ferent genes in mammals. that being a power-law increase in the ratio
of the variance to the mean, the index of dispersion, with the number
of substitutions in a protein. The probability of a given number of su
bstitutions occuring in a time t is determined by a fractional diffusi
on equation whose solution is a truncated Levy distribution implying t
hat evolution is a Levy process in time and yields the same functional
behavior for the variance in the number of substitutions as does the
FSP model. In addition to obtaining these relationships, the FSP model
implies lognormal statistics for the index of dispersion as a functio
n of the mean number of substitutions in a protein, which is confirmed
in the regression of the FSP model to data. Lognormal statistics sugg
est that molecular evolution can be viewed as a multiplicative stochas
tic process, rather than the linear additive processes of Darwinian se
lection and drift. (C) 1998 Elsevier Science B.V. All rights reserved.