AGE AND RATE OF DIVERSIFICATION OF THE HAWAIIAN SILVERSWORD ALLIANCE (COMPOSITAE)

Comparisons between insular and continental radiations have been hinde red by a lack of reliable estimates of absolute diversification rates in island lineages. We took advantage of rate-constant rDNA sequence e volution and an ''external'' calibration using paleoclimatic and Fossi l data to determine the maximum age and minimum diversification rate o f the Hawaiian silversword alliance (Compositae), a textbook example o f 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 me ans for the Hawaiian drosophilids, lobelioids, and honeycreepers and f alls approximately within the history of the modern high islands (less than or equal to 5.1 +/- 0.2 million years ago), By using a statistic ally efficient estimator that reduces error variance by incorporating clock-based estimates of divergence times, a minimum diversification r ate for the silversword alliance was estimated to be 0.56 +/- 0.17 spe cies per million years. This exceeds average rates of more ancient con tinental radiations and is comparable to peak rates in taxa with suffi ciently rich fossil records that changes in diversification rate can b e reconstructed.