text: what is neutral mutation?   the definition that the other two gave is correct, a mutation that has no effect on reproductive fitness, also, on a more concrete level, a neutral mutation also refers to a mutation that causes a change in one amino acid in a polypeptide chain that doesn't have an effect on the function of the protein.  often this occurs when one amino acid is replaced by a chemically similar one, which could be caused by a single base pair mutation.