Genetic variation in rare and common plants

Isozyme variation in 247 plant species is summarized as 57 generic-level comparisons of rare and common species. All species-level measures of variation (P-s, A(s), AP(s), H-es) and mean population-level measures (P-p, A(p), AP(p), H-ep, and H-o) show reductions significant at the p < 0.001 level, but F-IS and F-ST did not differ significantly, reflecting the similarity of breeding system in congeneric species and disparate ranges often sampled for rare and common species. The reduction in gene flow (Nm) among populations of rare species was significant when estimated from F-ST, but not when estimated from private alleles. Species monomorphic for isozymes are predominantly endemic and self-fertile. Although census populations of virtually all rare species are higher than levels at which theory would predict genetic erosion, and higher than levels protected by the U.S. Endangered Species Act (ESA), rare plants evidently have more significant reductions in genetic variation and gene flow than have been recognized previously.