 Kimura is responsible for the development of the neutral theory of molecular evolution, which discusses how most of the variation and evolution within species is caused by the random fluctuation of neutral allele frequencies, so by genetic drift, rather than natural selection.More recent studies have confirmed the works of these early evolutionary biologists, showing that rates of extinction decrease with increasing beneficial alleles, that extinction of deleterious alleles occurs faster than that of beneficial alleles, and that the process of adaptation can become very complex.To illustrate what a fixed allele, lets imagine a population of rabbits where there are three alleles for fur color brown, gray or white.