Evolution on a volcanic conveyor belt: using phylogeographic reconstructions and K-Ar-based ages of the Hawaiian Islands to estimate molecular evolutionary rates

The Hawaiian Islands form as the Pacific Plate moves over a 'hot spot' in the earth's mantle where magma extrudes through the crust to build huge shield volcanos. The islands subside and erode as the plate carries them to the north-west, eventually to become coral atolls and seamounts. Thus islands are ordered linearly by age, with the oldest islands in the north-west (e.g. Kauai at 5.1 Ma) and the youngest in the south-east (e.g. Hawaii at 0.43 Ma). K-Ar estimates of the date of an island's formation provide a maximum age for the taxa inhabiting the island. These ages can be used to calibrate rates of molecular change under the following assumptions: (i) K-Ar dates are accurate; (ii) tree topologies show that derivation of taxa parallels the timing of island formation; (iii) populations do not colonize long after island emergence; (iv) the coalescent point for sister taxa does not greatly predate the formation of the colonized younger island; (v) saturation effects and (vi) among-lineage rate variation are minimal or correctable; and (vii) unbiased standard errors of distances and regressions can be estimated from multiple pairwise comparisons. We use the approach to obtain overall corrected rate calibrations for: (i) part of the mitochondrial cytochrome b gene in Hawaiian drepanidines (0.016 sequence divergence/Myr); (ii) the Yp1 gene in Hawaiian Drosophila (0.019/Myr Kambysellis et al. 1995); and (iii) parts of the mitochondrial 12S and 16S rRNA and tRNA(val) in Laupala crickets (0.024-0.102/Myr, Shaw 1996). We discuss the reliability of the estimates given the assumptions (i-vii) above and contrast the results with previous calibrations of Adh in Hawaiian Drosophila and chloroplast DNA in lobeliods.