near neutrality

Nei M & Kumar S 2000 Molecular evolution and phylogenetics. Oxford UP. ISBN:9780195135855

• the problem is the fate of slightly deleterious mutations
• the continuous accumulation of slightly deleterious mutations in a protein will eventually deteriorate its function
• if slightly deleterious mutations are fixed, there must be slightly advantageous mutations as well to maintain the function of the protein
• the average rate of amino acid substitution may become more or less constant when long-term evolution is considered
• (p. 31)