Evolution in Helianthemum oelandicum (Cistaceae)-evidence of Holocene differentiation in morphology on the Baltic island of Öland, south-eastern Sweden

We use Helianthemum oelandicum subsp. oelandicum as a model for studies of mechanisms shaping the diversity in a rapidly radiating lineage of the H. oelandicum aggregate. Locally, correlations between drainage conditions (using GIS to measure variation in the micro-Topography on the more or less horizontal bedrock) and frequency of alleles of Mendelian genes for pubescence indicated that hairs are markers of adaptation to the unique environmental conditions (drought and periodic flooding) on the alvar habitats of Öland. An allozyme study showed that 1 and 4% of the genetic diversity is partitioned among two varieties and natural populations, respectively. FST for hair alleles was up to ten time greater than FST for allozymes. Öland rose above sea level after the LGM, which makes it possible to date the onset of the post-glacial diversification on the island. Only two, mainly allopatric, plastid DNA haplotypes were detected, H1 and H2. The southern Helianthemum oelandicum subsp. oelandicum var. canescens has only H1, whereas the more widespread var. oelandicum possesses both haplotypes. A restricted occurrence of haplotype H1 in the core area of var. oelandicum in the central Öland, coincided with the highest elevation on the island, probably representing the post-glacial arrival of the species on the island.