An examination of the constancy of the rate of molecular evolution

Summary

The vertebrate evolution of four proteins (6 andβ hemoglobins, cytochromec, and fibrinopeptide A) is examined via a maximum likelihood procedure. The fundamental hypothesis is that the process of nucleotide substitution as revealed by the minimum phyletic distance procedure (Fitch, 1971) is Poisson with a constant time average for each protein. The method allows the simultaneous estimation of the relative times of divergence of all common ancestors while utilizing the information from all four proteins. It also affords the possibility of statistically testing several biologically meaningful hypotheses. The results are the following:

• 1. The total rate (sum of all proteins) of nucleotide substitution is not constant in time throughout the evolution of vertebrates.

• 2. The relative rates (among proteins) of nucleotide substitution are not constant throughout vertebrate evolution.

• 3. Despite the variation in the rates of nucleotide substitution the procedure employed provides estimates of the relative time of divergence which correlate well with paleontological dates.

• 4. The overall rate of nucleotide substitution within the primates is again found to be less than the rest of the mammals.