| Abstract: | The purpose of this study was to assess the relative roles of population size and geographic isolation in determining population-genetic structure. Using electrophoretic techniques to quantify allozymic variation at 16 genetic loci, we measured genic variation within and among 16 natural populations of the California fan palm (Washingtonia filifera). Genotypes were determined for every individual in each population so that parametric values rather than sample estimates for measures of genic variability were obtained. Palm populations displayed low levels of within-population variability. The proportion of polymorphic loci and observed heterozygosity were 0.098 and 0.009 per population, respectively. Population size displayed a significant positive correlation with proportion of polymorphic loci, but not with observed heterozygosity. Low levels of genetic differentiation among populations were demonstrated by an F-statistic analysis and the computation of genetic similarity values. A hierarchical analysis of gene diversity revealed that only about 2% of the total gene diversity in W. filifera resides as among-population diversity. Climatic and geological changes since the Pliocene have eliminated widespread palm populations, and the species is presently restricted to isolated locations around the Colorado Desert. Existing populations in southern California are either relicts or recent recolonizations resulting from the dispersal of seeds from a refugium population in Baja California, Mexico. The observed patterns of low within- and low among-population genic diversity seem most consistent with a recent colonization by fan palms. It is hypothesized that stochastic processes reduced levels of genic variability in this refugium population during its formation. Dispersal of seeds from this refugium into suitable habitats in the Colorado Desert would produce populations with low variability and high genetic similarity because of their common ancestry. However, low intrapopulation variability and genetic homogeneity across populations could be the product of uniform selection pressures favoring a narrow array of specialized genotypes in either relict or colonizing populations. |
