The neutral theory of molecular evolution proposes that most molecular changes are selectively neutral.
Japanese geneticist Motoo Kimura introduced the theory in 1968. It does not claim that natural selection is unimportant. Instead, it argues that many changes at the DNA and protein level have little effect on an organism’s reproductive success and can spread or disappear mainly through genetic drift.
The theory helped explain why molecular evolution often proceeds at roughly regular rates and provided a foundation for using molecular differences to estimate divergence times. Later versions, including nearly neutral theory, recognize that slightly harmful or beneficial mutations can also matter, especially when population size changes.
Neutral theory applies primarily to molecular substitutions, not to every visible trait. Adaptations such as camouflage or antibiotic resistance can clearly be shaped by natural selection.