Darwin's view of change being mostly driven by traits that provide advantage was widely accepted until the 1960s. While researching mutations that produce nucleotide substitutions in 1968, Motoo Kimura found that the rate of substitution was so high that if each mutation improved fitness, the gap between the most fit and typical genotype would be implausibly large. However, Kimura explained this rapid rate of mutation by suggesting that the majority of mutations were neutral, i.e. had little or no effect on the fitness of the organism. Kimura developed mathematical models of the behavior of neutral mutations subject to random genetic drift in biological populations. This theory has become known as the neutral theory of molecular evolution.