Molecular Clocks and Explosive Radiations

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Abstract
Molecular data are ideal for exploring evolutionary history because of its universality, stochasticity, and abundance. These features provide a means of exploring the evolutionary history of all organisms (including those that do not tend to leave fossils), potentially within a statistical framework that allows testing of evolutionary hypotheses. However, the discrepancy between molecular and paleontological dates for three key “explosive” radiations inferred from the fossil record—the Cambrian explosion of animal phyla and the post-KT radiations of modern orders of mammals and birds—have led to a reexamination of the assumptions on which molecular dates are based. Could variation in the rate of molecular evolution, perhaps associated with “explosive” radiations, cause overestimation of diversification dates? Here I examine four hypothetical causes of fast molecular rates in explosive radiations—body size, morphological rate, speciation rate, and ecological diversification—using available empirical evidence on patterns of variation in rate of molecular evolution.

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