Abstract
In 1968, Motoo Kimura submitted a note to Nature entitled “Evolutionary Rate at
the Molecular Level,” in which he proposed what has since become known as the neutral
theory of molecular evolution. This is the view that the majority of evolutionary changes
at the molecular level are caused by random drift of selectively neutral or nearly neutral
alleles. Kimura was not proposing that random drift explains all evolutionary change.
He does not challenge the view that natural selection explains adaptive evolution, or, that
the vertebrate eye or the tetrapod limb are products of natural selection. Rather, his
objection is to “panselectionism’s intrusion into the realm of molecular evolutionary
studies”. According to Kimura, most changes at the molecular level from one generation
to the next do not affect the fitness of organisms possessing them. King and Jukes (1969)
published an article defending the same view in Science, with the radical title, “Non-
Darwinian Evolution,” at which point, “the fat was in the fire” (Crow, 1985b).
The neutral theory was one of the most controversial theories in biology in the
late twentieth century. This chapter will review the debate over the netural theory subsequent to Kimura.