Empirical support for a stochastic model of evolution

Summary The stochastic model of molecular evolution was used to makea priori predictions for the total number of one-step nucleotide changes required to account for a given number of amino acid substitutions between two homologous proteins. These predictions are now found to be concordant with empirical data summarized by Dayhoff, Eck and Park (1969). Correction factors are derived for adjusting the leg lengths of phylogenetic trees. It is shown that the operations of constructing the phylogenetic tree and applying the correction algorithm are not commutative with respect to obtaining the leg lengths. The effect of this on certain published phylogenies is discussed. It is suggested that, as a first approximation, at any given point in evolutionary time, enthalpic (selective) forces determine the number and position of those codon sites which are free to vary, whereas within these variable sites, entropic (random) processes determine the course of evolution at the molecular level.