Rates of protein evolution: a function of amino acid composition.

Conservation of secondary and tertiary structure in proteins suggests that rates of sequence variation reflect differences in the total number of amino acid replacements that are compatible with preservation of structure. Consequently, rates of sequence variation depend on whether the constituent amino acids of individual proteins are, over-all, more subject or less subject to evolutionary substitution than normal. Such rates correlate well with a mutability term based on amino acid composition.