Stepwise mutation model

The stepwise mutation model (SMM) is a mathematical theory, developed by Motoo Kimura and Tomoko Ohta, that allows for investigation of the equilibrium distribution of allelic frequencies in a finite population where neutral alleles are produced in step-wise fashion. The original model assumes that if an allele has a mutation that causes it to change in state, mutations that occur in repetitive regions of the genome will increase or decrease by a single repeat unit at a fixed rate (i.e. by the addition or subtraction of one repeat unit per generation) and these changes in allele states are expressed by an integer (. . . A-1, A, A1, .. .). The model also assumes random mating and that all alleles are selectively equivalent for each loci. The SMM is distinguished from the Kimura-Crow model, also known as the infinite alleles model (IAM), in that as the population size increases to infinity, while the product of the N_e (effective population size) and the mutation rate is fixed, the mean number of different alleles in the population rapidly reaches a peak and plateaus, at which time that value is almost the same as the effective number of alleles.