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Abstract.-Attempts to relate estimates of regional F ST to gene flow and drift via Wright's (1931) equation F ST = 11(4Nm + 1) are often inappropriate because most natural sets of populations probably are not at equilibrium (McCauley 1993), as assumed by the island model upon which the equation is based, or ineffective because the influences of gene flow and drift are confounded in the product Nm. Evaluations of the association between genetic (F ST ) and geographic distances separating all pairwise populations combinations in a region allows one to test for regional equilibrium, to evaluate the relative influences of gene flow and drift on population structure both within and between regions, and to visualize the behavior of the association across all degrees of geographic separation. Tests of the model using micro satellite data from 51 populations of eastern collared lizards (Crotaphytus collaris collaris) collected from four distinct geographical regions gave results highly consistent with predicted patterns of association based on regional differences in various historical and ecological factors that affect the amount of drift and gene flow. The model provides a prerequisite for and an alternative to regional F ST analyses, which often simply assume regional equilibrium, thus potentially leading to erroneous and misleading inferences regarding regional population structure.Key words.-Correlations, Crotaphytus collaris, F-statistics, gene flow, genetic distance, Mantel's test, population structure, random genetic drift.Received March 24, 1998. Accepted June I, 1999.It is notoriously difficult to assess the individual effects of gene flow and drift on regional population structure. Traditional attempts to relate estimates of regional F ST to gene flow and drift via Wright's (1931) equation F ST = 1I(4Nm + 1) are often inappropriate because most natural sets of populations probably are not at equilibrium (McCauley 1993), as is assumed by the island model upon which the equation is based. Furthermore, the influences of gene flow and drift on regional population structure are confounded in the product Nm.In this paper we discuss a method that allows one to test for regional equilibrium before applying Wright's equation or other models that assume such a pattern. Although Nand m remain a product under this approach, it does permit evaluation of the relative historical influences of gene flow and drift on regional population structure by comparing the observed relationship between the genetic (F ST ; Reynolds et al. 1983) and geographic distances separating pairs of populations in a region (Nei 1972;Hartl and Clark 1989;Tilley et al. 1990) with those expected under a stepping-stone model of population structure (Kimura 1953;Malecot 1955;Kimura and Weiss 1964). Further refinement of the analysis is possible by taking into consideration various historical and ecological factors characteristic of a region as they pertain to the amount of drift and gene flow affecting the organisms under study. Finally, it is also p...

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