Sideritis L. subgenus Marrubiastrum (Moench.) Mend.-Heuer comprises 24 species of Macaronesian endemic perennials. The constituent taxa encompass a wide array of life forms and have exploited all ecological zones present in the Macaronesian archipelagos of Madeira and the Canary Islands. This group is especially intriguing because it exhibits the largest aneuploid series ever reported in oceanic island plants. The Macaronesian endemics are posited to be sister to continental species from the eastern Mediterranean region. Though recently revised, a phylogeny of the subgenus is still lacking. The present study uses a chloroplast DNA restriction site analysis to elucidate the pattern of evolution within and among the islands. By mapping chromosome number and geographical and ecological distribution onto the phylogenetic tree, we hope to shed light on the relative roles of the various factors that have contributed to diversification of the island endemics. Preliminary evidence suggests that adaptive radiation has been an important factor in the evolution of the subgenus within the islands. The insular taxa form a strongly supported monophyletic group, indicating a single colonization of Macaronesia. Monophyly of the subgeneric sections is not supported, suggesting that phylogenetic studies using additional molecular markers are needed to clarify sectional circumscription.

Key words: Island biology, Lamiaceae, Macaronesia, Sideritis