Patterns of genetic variation in rare and widespread plant congeners

Rare species are typically considered to maintain low levels of genetic variation, and this view has been supported by several reviews of large numbers of isozyme studies. Although these reviews have provided valuable data on levels of variability in plant species in general, and rare species in particular, these broad overviews involve comparisons that may confound the effects of rarity with a multitude of other factors that affect genetic variability. Additionally, the statistical analyses employed assume the data to be independent, which is not the case for organisms that share a common phylogenetic history. As the role of evolutionary history and historical constraints has become better understood, more researchers have studied widespread congeners when investigating the genetic diversity of rare species in an effort to control for these effects. We summarize the available data from such studies, comparing for rare and widespread congeners (1) the levels of genetic variability at the population and species levels and (2) measures of population substructuring. At the population level, we summarized data for percentage polymorphic loci (%P(pop)), mean number of alleles per locus (A(pop)), and observed heterozygosity (H(o)). Species-level measures used were percentage polymorphic loci (%P(spp)), mean number of alleles per locus (A(spp)), and total genetic diversity (H(T)). Indices of population subdivision (either F(ST) or G(ST)) were also examined. Using Wilcoxon signed rank tests, we found significant, but small, differences between rare and widespread species for all diversity measures except H(T). However, there does not appear to be a difference between rare and widespread congeners in terms of how genetic variation is partitioned within and among populations. Levels of diversity, for all measures examined, between rare and widespread congeners are highly correlated.