The dynamics of the substitution process for mammalian mitochondrial DNA have been modeled. The temporal behavior of several quantities has been studied and the model's predictions have been compared with estimates obtained from recent mtDNA sequence data for an increasingly divergent series of primates, the mouse and the cow (Anderson et al. 1981, Bibb et al. 1981. The results are consistent with the hypothesis that the decrease in the proportion of transitions observed as divergence increases is a consequence of the highly biased substitution process. In addition, the results support the hypothesis that, although a portion of the mtDNA molecule evolves at an extremely rapid rate, a significant portion of the molecule is under strong selective constraints
