CHLOROPLAST DNA EVIDENCE OF COLONIZATION, ADAPTIVE RADIATION, AND HYBRIDIZATION IN THE EVOLUTION OF THE MACARONESIAN FLORA

Most evolutionary studies of oceanic islands have focused on the Pacif ic Ocean. There are very few examples from the Atlantic archipelagos, especially Macaronesia, despite their unusual combination of features, including a close proximity to the continent, a broad range of geolog ical ages, and a biota linked to a source area that existed in the Med iterranean basin before the late Tertiary. A chloroplast DNA (cpDNA) r estriction site analysis of Argyranthemum (Asteraceae: Anthemideae), t he largest endemic genus of plants of any volcanic archipelago in the Atlantic Ocean, was performed to examine patterns of plant evolution i n Macaronesia. cpDNA data indicated that Argyranthemum is a monophylet ic group that has speciated recently. The cpDNA tree showed a weak cor relation with the current sectional classification and insular distrib ution. Two major cpDNA lineages were identified. One was restricted to northern archipelagos-e.g., Madeira, Desertas, and Selvagens-and the second comprised taxa endemic to the southern archipelago-e.g., the Ca nary islands. The two major radiations identified in the Canaries are correlated with distinct ecological habitats; one is restricted to eco logical zones under the influence of the northeastern trade winds and the other to regions that are not affected by these winds. The pattern s of phylogenetic relationships in Argyranthemum indicate that interis land colonization between similar ecological zones is the main mechani sm for establishing founder populations. This phenomenon, combined wit h rapid radiation into distinct ecological zones and interspecific hyb ridization, is the primary explanation for species diversification.