Kimura’s neutral theory of molecular evolution. By postulating the revolutionary new concept of neutral variants, Kimura’s neutral theory summarizes molecular evolution in one the most elegant mathematical expressions in science. The expression $K={\mu }_0$ (the rate of molecular evolution equals the neutral mutation rate) unifies the three levels of genetic variation from its origin to its substitution in the population: mutation (individual level), polymorphism (population level), and divergence (species level). According to the neutral theory, intrapopulation polymorphism is just a random walk of variants in their process to fixation or loss (represented for species A: gray, neutral mutations; maroon, strongly deleterious mutations; see also Figure 3B). Orange arrows represent the average lifetime of a neutral mutation from its appearance to its fixation in the population (1/μ₀).