THE AGE OF THE ANGIOSPERMS: A MOLECULAR TIMESCALE WITHOUT A CLOCK

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Abstract
The age of the angiosperms has long been of interest to botanists and evolutionary biologists. Many early efforts to date the age of the angiosperms and evolutionary divergences within the angiosperm clade using a molecular clock have yielded age estimates that are grossly inconsistent with the fossil record. We investigated the age of angiosperms using Bayesian relaxed clock (BRC) and penalized likelihood (PL) approaches. Both of these methods allow the incorporation of multiple fossil constraints into the optimization procedure. The BRC method allows a range of values for among-lineage rate of substitution, from a nearly clocklike behavior to a condition in which each branch is allowed an optimal substitution rate, and also accounts for variation in molecular evolution across multiple genes. A topology derived from an analysis of genes from all three plant genomes for 71 taxa was used as a backbone. The effects on age estimates of different genes, single-gene versus concatenated datasets, and the inclusion and assumptions of fossils as age constraints were examined. In addition, the influence of prior distributions on estimates of divergence times was also explored. These results indicate that widely divergent age estimates can result from the different methods (198–139 million years ago), different sources of data (275–122 million years ago), and the inclusion of temporal constraints to topologies. Most dates, however, are between 180–140 million years ago, suggesting a Middle Jurassic-Early Cretaceous origin of flowering plants, predating the oldest unequivocal fossil angiosperms by about 45–5 million years. Nonetheless, these dates are consistent with other recent studies that have used methods that relax the assumption of a strict molecular clock and also agree with the hypothesis that the angiosperms may be somewhat older than the fossil record indicates.