Composite stepwise mutation model under the neutral mutation hypothesis.

As an extension of the conventional ("Ohta-Kimura") stepwise mutation model, a new model is proposed. In this model, it is assumed that each charge state ("electromorph") is represented by K alleles and that a mutation changes an allele either by one step in the charge space or to one of the other members of the identical electromorph. It is shown that the net genetic variability within a population is similar to that predicted by the infinite-allele model ("Kimura-Crow" model) rather than to that predicted by the stepwise mutation model, and the K-dependence of genetic variability is rather weak when K greater than or equal to 2 and the effective population size is not much greater than the reciprocal of mutation rate. The results are compared with the recent observations at the xanthine dehydrogenase locus in Drosophila pseudoobscura.