Because of their low effective population sizes, natural selection is expected to have reduced effectiveness in organisms such as mammals. By comparing the amino acid substitution rates between mammalian protein surfaces and interiors, it was found that almost a third of the proteins surveyed failed to reject the null hypothesis of neutral substitutions among surface residues. Proteins with such partly neutral evolution nonetheless have no fewer protein interactions than do other proteins. I suggest that natural selection can function to preserve protein interactions without requiring strict conservation of the individual residue contacts that impart those interactions.