| Islands have historically played an important role in the development of evolutionary theory. The rapid rate at which divergence occurs and the ability to date groups based on island age make island lineages important subjects of evolutionary study. Despite this, there have been relatively few studies of adaptive radiations in plants on islands. The endemic Hawaiian species in the plant genus Plantago lend themselves well to studies of adaptive radiation as they occur in habitats as varied as bogs, subalpine shrub land, mesic forest, rainforest, and waterfalls, and they have diverged in growth form, leaf morphology, and reproductive biology, likely in response to their environment. Nevertheless, the group is understudied and relatively poorly known. We assessed relationships among the Hawaiian Plantago species as well as gene flow among populations using sequence data and microsatellite analysis. Overall findings suggest that Hawaiian Plantago is likely comprised of many more taxa than currently recognized, and that speciation has largely occurred allopatrically, with divergence a result of interisland dispersal and adaptive shifts from bog to understory habitats. Results support initial colonization of the oldest island, Kaua'i, followed by dispersal to the younger islands of O'ahu, Maui Nui, and Hawai'i, indicating that Hawaiian Plantago is likely not older than 5.1 My. Additionally, ecophysiological analysis focusing on leaf structure and function in a subset of taxa in the group suggests that leaf diversification in Hawaiian Plantago has involved coordinated trait shifts, generating strong and apparently adaptive trait linkages. |
