Age and rate of diversification of the Hawaiian silversword alliance (Compositae)

Abstract

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.

Figure 1

Time-calibrated phylogeny of one of four most-parsimonious rDNA ITS trees of the Hawaiian silversword alliance (Argyroxiphium, Dubautia, and Wilkesia) and closest continental perennial relatives in Madia and Raillardiopsis (8, 9). Outgroup tarweed taxa, Adenothamnus validus and Raillardella pringlei, are not shown. Bootstrap values are shown along branches. Consistency index = 0.81. Retention index = 0.85. The tree shown is a clade that nests within the much larger tarweed radiation (ca. 114 species in 17 genera) (25).

Figure 2

Bootstrap (15) distribution of age estimates for the silversword alliance using a 15 Ma calibration, accounting for the stochastic sampling error in the sequence data used to infer ages. MRCA, most recent common ancestor.

Figure 3

Natural logarithm (ln) of species diversity through time based on the tree in Fig. 1, with scoring of additional species that are identical in ITS sequence with species included in the tree [At all ITS sites, A. grayanum (BGB 661) and A. kauense (BGB 773) are identical to Argyroxiphium sandwicense ssp. sandwicense (BGB 657); D. linearis (BGB 516) is identical to Dubautia arborea (BGB 527); D. platyphylla (BGB 524) is identical to Dubautia menziesii (BGB 522); W. hobdyi (GDC 1150) is identical to Wilkesia gymnoxiphium (Char 76.022) (see ref. 9)]. Ages are based on a 15-Ma calibration point for the most recent common ancestor of the Hawaiian silversword alliance and its Californian ITS sister group.