Molecular evolution of human and rabbit beta-globin mRNAs.

The primary structures of human and rabbit beta-globin mRNAs are compared. Using as a standard the extent of nucleotide substitutions inferred from the hypervariable amino acid residues of fibrinopeptides A and B, which are thought to change largely by neutral evolution, we show that not all silent mutations in globin mRNA are neutral. The divergence of the sequences is limited in part by the selective usage of synonymous codons. The divergent nucleotides tend to be distributed nonrandomly: in the coding region silent substitutions are most rare in segments that are also deficient in substitutions leading to replacements.