G ENE flow is a powerful homogenizing force that may act to preventg enetic divergence arising amongp opulations by eitherd rift or selection (re- viewed by FELSENSTEIN 1976; SLATKIN 1985a). Evidence for the importance of gene flow in affecting the distri- bution of genetic variation among natural populations is often inferred from a species' potential for dispersal. For example, species with high dispersal capabilities often exhibit low levels of population differentiation (e.g., WAPLES 1987), whereas those with limited means of dispersal, or insurmountable barriers to movement, often display a significant degree of population struc- ture (e.g., LARSON et al. 1984). In these instances, indi- rect estimates of gene flow agree with migration rates expected to occur among populations. In other studies, however, indirect and direct estimates of gene flow have produced conflicting results (.g., EHIUICH et al. 1975; BAKER 1981) that have been interpreted as reflecting the highly stochastic nature of gene flow or the possible action of selection (see discussion in SLATKIN 1985a). The spatial patterns of variation observed at electro
G ENE flow is a powerful homogenizing force that may act to preventg enetic divergence arising amongp opulations by eitherd rift or selection (re- viewed by FELSENSTEIN 1976; SLATKIN 1985a). Evidence for the importance of gene flow in affecting the distri- bution of genetic variation among natural populations is often inferred from a species' potential for dispersal. For example, species with high dispersal capabilities often exhibit low levels of population differentiation (e.g., WAPLES 1987), whereas those with limited means of dispersal, or insurmountable barriers to movement, often display a significant degree of population struc- ture (e.g., LARSON et al. 1984). In these instances, indi- rect estimates of gene flow agree with migration rates expected to occur among populations. In other studies, however, indirect and direct estimates of gene flow have produced conflicting results (.g., EHIUICH et al. 1975; BAKER 1981) that have been interpreted as reflecting the highly stochastic nature of gene flow or the possible action of selection (see discussion in SLATKIN 1985a). The spatial patterns of variation observed at electro
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