The relationships of 35 taxa endemic to the Chatham Islands and their close relatives were investigated using DNA sequence data. This study implies that the closest relatives of many of the endemic species are widespread and common species that occur on either of the main New Zealand islands. Hypotheses that the majority of the Chatham Island endemic plants have their closest relatives in northern North Island or southern South Island are not supported. The closest relative for the monotypic Myosotidium hortensium occurs in the Mediterranean, indicating successful long-distance transoceanic dispersal. Patterns of dispersal and estimates of divergence times were determined, and we relate these data to hypotheses of the islands' lateCenozoic geological history. The Chatham Island endemic taxa differ from their closest relatives by between 0.0 and 6.4% sequence divergence (mostly B1.5%). Divergence time estimates determined for 19 taxa using molecular clock analyses identified a large group of taxa whose divergence times wereB2.4 Myr, and four species with divergence times!3.54 Myr. Divergence time estimates for Chatham Island endemic plants are congruent with two late-Cenozoic geological events. First, the emergence of land 6.0 Myr following eruption of the Mangere Volcano, and second, uplift of the submerged Southern Volcanics, Mangere Volcano and Rangitihi Volcanics between 3.0 and 2.0 Myr.Keywords: Chatham Islands; dispersal; DNA sequence data; geology; molecular clock; New Zealand flora; speciation Introduction Phylogenetic studies of vascular plants endemic to isolated oceanic islands such as the Hawaian archipelago (Baldwin et al. 1991; Wagner & Funk 1995), Canary Islands (GJ Allan et al. 2004; Moore et al. 2002) and Juan Fernandez Islands (Crawford et al. 1998) have advanced understanding and facilitated the development of hypotheses on the evolutionary history of oceanic islands and their biotas (Chen & He 2009; Heaney 2007;Whittaker et al. 2008). New Zealand is also cited as an important biogeographic region and there has been much discussion about the flora having a Gondwanan ancestry and being vicariant (Craw 1989) or being more recent and having arrived primarily by dispersal (McGlone et al. 2001;Pole 1994). Within New Zealand, there has been debate about the effects of glaciation and glacial refugia on plant distributions (Burrows 1965;Wardle 1963Wardle , 1988 and the importance of the Alpine Fault (Heads 1998; McGlone 1985). Most of this debate has focused on the two main islands, North Island and South Island. Often overlooked biogeographic features of New Zealand are the more distant *Corresponding author. Email: heenanp@landcareresearch.co.nz Vol. 48, No. 2, June 2010, 83Á136 island groups of the Kermadec Islands, subantarctic islands and Chatham Islands.
New Zealand Journal of BotanyThe Chatham Island archipelago (1768W and 448S; Fig. 1) is located at the eastern end of the Chatham Rise, some 850 km to the east of the main islands of New Zealand, and is the easternmost extension of th...