neutral theory of molecular evolution
According to the neutral theory of molecular evolution, some classes of mutation have too small an effect on fitness to be affected by natural selection.
References
Chamary JV, Parmley JL, Hurst LD. Hearing silence: non-neutral evolution at synonymous sites in mammals. Nat Rev Genet. 2006 Feb;7(2):98-108. PMID: 16418745