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Comparisons between insular and continental radiations have been hindered by a lack of reliable estimates of absolute diversification rates in island lineages. We took advantage of rate-constant rDNA sequence evolution and an ''external'' calibration using paleoclimatic and fossil data to determine the maximum age and minimum diversification rate of the Hawaiian silversword alliance (Compositae), a textbook example of insular adaptive radiation in plants.Our maximum-age estimate of 5.2 ؎ 0.8 million years ago for the most recent common ancestor of the silversword alliance is much younger than ages calculated by other means for the Hawaiian drosophilids, lobelioids, and honeycreepers and falls approximately within the history of the modern high islands (<5.1 ؎ 0.2 million years ago). By using a statistically efficient estimator that reduces error variance by incorporating clock-based estimates of divergence times, a minimum diversification rate for the silversword alliance was estimated to be 0.56 ؎ 0.17 species per million years. This exceeds average rates of more ancient continental radiations and is comparable to peak rates in taxa with sufficiently rich fossil records that changes in diversification rate can be reconstructed.The Hawaiian silversword alliance (Argyroxiphium, Dubautia, Wilkesia; Compositae) has been considered ''the best example of adaptive radiation in plants'' (1). Life-form diversity among the 28 Hawaiian-endemic species in the group encompasses trees, shrubs, subshrubs, mat-plants, monocarpic and polycarpic rosette plants, cushion plants, and vines that occur across a broad environmental spectrum, from rainforests to desert-like settings (2). Although monophyly of the group is well established (3), absolute ages of the silversword alliance and other Hawaiian lineages have been difficult to estimate. Ages are essential in the reconstruction of the absolute diversification rates-the rate of speciation minus extinction (S Ϫ E). Availability of such rate estimates would permit comparisons with other insular radiations and with continental radiations, which may exhibit qualitatively or quantitatively different patterns of diversification.Unfortunately, knowledge of island ages from potassiumargon dating is of minimal value for placing upper limits on ages of lineages in ancient hot-spot archipelagos such as the Hawaiian Islands. The age of terrestrial groups in the Hawaiian archipelago can conceivably date back to the formation of Kure [29 million years ago (Ma)], under the assumption of an unbroken succession of dispersals from older to younger islands since that time (4). An upper age limit of 29 Ma is too high to be of any value for dating island radiations in groups such as Asteraceae-the oldest unequivocal fossil evidence of the family worldwide is more recent, from the mid-Oligocene (5). Moreover, calibration of clock-like divergence of molecular sequences is complicated if individual island ages are used to estimate the age of an entire insular radiation (''internal'' calibration)...

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