near neutrality

Hartl DL & Taubes CH 1996 Compensatory nearly neutral mutations: selection without adaptation. J Theor Biol 182:303-309.

• a significant fraction of nucleotide substitutions should be slightly detrimental

• most genes undergo selectively driven nucleotide substitutions
• though not owing to adaptation to external conditions but rather to compensation for deleterious mutations previously incorporated into the gene
• this process makes it difficult to interpret statistical evidence for natural selection s being in any way related to the Darwinian notion of adaptive evolution

• a gene whose function remains unchanged may nevertheless undergo selectively driven nucleotide substitutions
• genes that carry out the same function in different species are not functionally fully equivalent in the sense of being able to carry out complete genetic complementation in interspecific transformation experiments