This book represents my attempt to convince the scientific world that the main cause of evolutionary change at the molecular level — changes in the genetic material itself — is random fixation of selectively neutral or nearly neutral mutants rather than positive Darwinian selection. This thesis, which I here call the neutral theory of molecular evolution, has caused a great deal of controversy since I proposed it in 1968 to explain some then new findings in evolution and variation at the molecular level. The controversy is not surprising, since evolutionary biology has been dominated for more than half a century by the Darwinian view that organisms become progressively adapted to their environments by accumulating beneficial mutants, and evolutionists naturally expected this principle to extend to the molecular level. The neutral theory is not antagonistic to the cherished view that evolution of form and function is guided by Darwinian selection, but it brings out another facet of the evolutionary process by emphasizing the much greater role of mutation pressure and random drift at the molecular level.

The neutral theory has two roots. One is the stochastic theory of population genetics whose foundation traces back to the pathbreaking work of R.A. Fisher, J.B.S. Haldane and Sewall Wright early in the 1930s, and is mathematical in nature.