Reconciling Molecular and Fossil Evidence on the Age of Angiosperms

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9726856 Sanderson The age of the angiosperms has long been a topic of controversy in plant evolution. Traditionally, the problem was addressed primarily from a paleobotanical point of view. However, molecular studies relying on the hypothesis of a constant rate of evolution ("molecular clock") have added a new dimension to the problem, often suggesting a much older age than does fossil evidence. Many of these analyses conflict with each other, and therefore they call into question not only paleobotanical assumptions but also the validity of the molecular clock, a major issue in molecular evolution. Drs. Sanderson and Doyle will undertake a critical evaluation of the implications of molecular evidence for the age of angiosperms, especially its agreement or disagreement with fossil evidence, using both new data and new methods of analysis. They propose to combine existing and new sequence data from rbcL, atpB, and 18S rDNA genes with new data to be obtained from four slowly evolving chloroplast genes. Sampling of taxa will emphasize angiosperms with a good Cretaceous fossil record or which represent early branches in the angiosperm tree in addition to other relevant seed plants. Using new methodologies that allow hypothetical rates of evolution to vary, we hope to obtain a better assessment of (1) the error in molecular estimates of the age of angiosperms and the sources of those errors, and (2) the level of disagreement between fossil evidence for the age of angiosperms and molecular estimates when the error in divergence times is taken into account. Methods and insights derived from this study may be useful in dating other groups where there are apparent conflicts between molecular and fossil data.