Genetic diversity in eight populations of P. mexicana before (light gray) and after (dark gray) the flood (for populations code see legend to Fig. 1). Depicted are mean (± S.E.) values across loci for observed (Ho, above) and expected heterozygosity (HE, middle), and allelic richness (A, below). Paired t-tests on locus-wise values within each population; significant comparisons are bold.