A HIERARCHICAL VIEW OF GENETIC-STRUCTURE IN THE RARE ANNUAL PLANT CLARKIA-SPRINGVILLENSIS

Genetic structure at several spatial scales was examined in the rare C alifornia annual, Clarkia springvillensis. Using seven isozyme-encodin g loci as genetic markers, we assessed the amount and distribution of genetic variation among three populations and eight subpopulations. To tal genetic variation was lower than in species with similar life hist ory traits but equivalent to that of other endemic plants. Spatial aut ocorrelation showed some evidence for very limited differentiation wit hin subpopulations at a scale of 1-2 m. The subpopulations, separated by tens of meters, were found to be more differentiated from each othe r (F-sp = 0.084) on average than were populations (F-pt = 0.017). This local genetic differentiation was not correlated with physical distan ce between subpopulations. The low F-pt estimates suggest that substan tial gene flow is occurring among populations. However, the lack of co rrelation between genetic and geographic distances and the significant differentiation of subpopulations suggest that genetic drift is occur ring within populations. Therefore, we believe the apparent homogeneit y of populations is due to each population's gene frequencies' being a n average of several divergent subpopulations. If drift is causing dif ferentiation within populations, it may eventually cause differentiati on between populations. The importance of using a hierarchical approac h to evaluating genetic structure is clear. Patterns occurring at one spatial scale may not be evident at others. One should not necessarily conclude that gene flow is substantial and that the risk of genetic e rosion via drift is negligible just because differentiation between po pulations is small; the system may not be at equilibrium. This lesson is particularly important when recent changes in climate or land use a re apparent.