ISLAND COLONIZATION AND EVOLUTION OF THE INSULAR WOODY HABIT IN ECHIUM L (BORAGINACEAE)

Numerous island-inhabiting species of predominantly herbaceous angiosp erm genera are woody shrubs or trees. Such ''insular woodiness'' is st rongly manifested in the genus Echium, in which the continental specie s of circum-mediterranean distribution are herbaceous, whereas endemic species of islands along the Atlantic coast of north Africa are woody perennial shrubs. The history of 37 Echium species was traced with 70 kb of noncoding DNA determined from both chloroplast and nuclear geno mes. In all, 239 polymorphic positions with 137 informative sites, in addition to 27 informative indels, were found. Island-dwelling Echium species are Shown to descend from herbaceous continental ancestors via a single island colonization event that occurred < 20 million years a go. Founding colonization appears to have taken place on the Canary Is lands, from which the Madeira and Cape Verde archipelagos were invaded . Colonization of island habitats correlates with a recent origin of p erennial woodiness from herbaceous habit and was furthermore accompani ed by intense speciation, which brought forth remarkable diversity of forms among contemporary island endemics. We argue that the origin of insular woodiness involved response to counterselection of inbreeding depression in founding island colonies.