Rate constancy of globin gene evolution in placental mammals.

The molecular clock hypothesis is investigated by comparison of the rates of nucleotide substitution in globin genes of mice, cows and goats, humans, and rabbits, using the relative rate test. These comparisons are based on a branching order of genes and species established by cladistic analysis of nucleotide sequences. The species branching order is shown to be mouse, cow/goat, human, and rabbit. Relative rate tests involving paralogous and orthologous genes provide no evidence of heterogeneity, among species, in the rate of evolution of the genes. This result is discrepant with the conclusions of most other recent, similar studies. By comparison with previous studies, the present study is based on a sound phylogeny and involves a larger sample of species, genes, and genic regions. The result provides strong support for the neutral theory of molecular evolution and demonstrates that molecular evolutionary rate does not depend on generation time.