Diversity and divergence in Cistus salvifolius (L.) populations from contrasting habitats

Cistus salvifolius L. is a widespread Mediterranean shrub, occurring over a wide range of environments. Given the degree of habitat differentiation, and geographic isolation of some populations, adaptation to local conditions and hence population divergence might be expected to have occurred. To test this hypothesis morphology and allozyme diversity was measured in 13 populations collected from contrasting habitats around the Mediterranean. Leaf morphology (length, width and petiole length) and internode length varied widely between populations. Leaf width and internode length were negatively correlated with longitude, and leaf length was negatively correlated with mean rainfall. All populations were polymorphic at all allozyme loci studied, and no populations showed significant difference between levels of expected and observed heterozygotes. Allelic diversity (H-s) within populations was high, and populations from the more extreme sites showed no decrease in diversity or predominance of rare genotypes, suggesting there is little selection for characters favouring survival in local conditions. Some populations from highly contrasting habitats, in terms of rainfall, appeared to be genetically similar. However, there were differences between some populations, in areas less than 1 km apart, which have similar geography and climate. Results suggest that the C. salvifolius populations examined may not be as adapted to local environmental conditions as expected. Periodic fires, gene flow, and environmental heterogeneity may all help maintain genetic diversity and hinder adaptation.