The tendency of microsatellites to be highly polymorphic is a major factor responsible for their popularity as markers for ecological and evolutionary studies. Microsatellite polymorphism is generally attributed to slip-strand mispairing errors, causing the addition or deletion of repeat units during replication (Levinson and Gutman 1987). Valdes, Slatkin, and Freimer (1993) found that human microsatellite evolution appears to follow a stepwise mutation model (SMM) (Ohta and Kimura 1973). Following the SMM, single repeat units are added or deleted with equal and constant probability across all alleles. Several statistical methods to evaluate patterns of microsatellite variability and differentiation that assume variants of the SMM have since been developed (e,g., Goldstein et al. 1995a, 1995b; Slatkin 1995; Rousset 1996) and incorporated into widely used software programs such as GENEPOP (Raymond and Rousset 1995).
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