The study examines the mutational processes of simple sequence repeats (SSRs) in human populations, focusing on the coalescent process, repeat instability, and human diversity. The authors introduce a two-phase mutation model, which assumes that most mutations are single-step changes but also includes rare large jumps in repeat number. They use computer simulations to determine the expected values of statistics reflecting allele size frequency distributions for the two-phase model and compare these with observed values from 10 SSR loci in the Sardinian population. The two-phase model provides the best fit to the data for most loci. The study also genotyped the same loci in Egyptian and sub-Saharan African samples to assess the utility of SSRs for studying population divergence, finding that estimates of interpopulation distances from SSRs are similar to those derived from mitochondrial DNA (mtDNA) sequences. The results suggest that a two-phase mutation process, with frequent small changes and rare large changes, is responsible for the observed allelic variation at SSR loci.The study examines the mutational processes of simple sequence repeats (SSRs) in human populations, focusing on the coalescent process, repeat instability, and human diversity. The authors introduce a two-phase mutation model, which assumes that most mutations are single-step changes but also includes rare large jumps in repeat number. They use computer simulations to determine the expected values of statistics reflecting allele size frequency distributions for the two-phase model and compare these with observed values from 10 SSR loci in the Sardinian population. The two-phase model provides the best fit to the data for most loci. The study also genotyped the same loci in Egyptian and sub-Saharan African samples to assess the utility of SSRs for studying population divergence, finding that estimates of interpopulation distances from SSRs are similar to those derived from mitochondrial DNA (mtDNA) sequences. The results suggest that a two-phase mutation process, with frequent small changes and rare large changes, is responsible for the observed allelic variation at SSR loci.