Telling the Evolutionary Time: Molecular Clocks and the Fossil Record

Telling the Evolutionary Time: Molecular Clocks and the Fossil Record

Telling the Evolutionary Time: Molecular Clocks and the Fossil Record

Telling the Evolutionary Time: Molecular Clocks and the Fossil Record

Synopsis

This book presents a discussion between molecular biologists and paleontologists, in which they investigate the significance of competing sources of data, explain the nature of molecular clocks and the fossil record, and attempt to develop compromise models that incorporate conflicting opinions. These are presented as a series of case studies dealing with many of the most important groups of complex organisms. Research scientists and advanced undergraduates across the field of evolutionary biology will find Telling Evolutionary Time: Molecular Clocks and Fossil Record a stimulating volume, bringing new insight and perspective to a thorny debate.

Excerpt

Molecular clocks and the fossil record-towards consilience?

Molecular clocks or the fossil record: which approach tells evolutionary time? the papers presented in this volume stem from a joint Palaeontological Association/ Systematics Association symposium held at the Third Biennial Meeting of the Systematics Association, at Imperial College, London on 5 September 2001, which brought together palaeontologists and molecular biologists with the aim of addressing the disparity between molecular and palaeontological perspectives on evolutionary time.

Resolving the timing and tempo of major evolutionary radiations represents one of the most prominent and controversial debates currently underway in evolutionary biology. Although this temporal dimension has traditionally been the preserve of palaeontology, with a burgeoning molecular database that is progressively more representative of the tree of life, attempts to establish a molecular timescale for evolutionary history have become ever more sophisticated. the so-called 'molecular clock' works, at least at a simplistic level, by calibrating molecular distance between a pair of organisms to time using a reliable fossil estimate. By extrapolating this scaling it is possible to estimate the time of divergence between other taxa, for which the record is perhaps unreliable, or simply as a test of fossil-based estimates. Because of the unlikelihood of the fossilization of the earliest representatives of taxa, even the apparently reliable fossil records of taxa used in calibrating the clock can provide only a conservative estimate of the true divergence date, and so it is to be expected that molecular clock estimates should also be conservative.

However, this is not the case; molecular clocks almost always provide divergence estimates that are not just older, but considerably so (e.g. Hedges, Chapter 2), and sometimes as much as double the age of fossil-based estimates. By implication the fossil record is not just incomplete, but represents only half of evolutionary history, and the latter half at that.

No one would defend total completeness of the fossil record, but it is the degree to which evolutionary history is unrepresented by the fossil record that is in question (Fortey et al., Chapter 3; Benton, Chapter 4). Indeed, the gross disparity between molecular and fossil-based estimates has led to a period of introspection amongst the palaeontological community, and the development of a variety of methods for the evaluation of the quality of the fossil record. These include attempts to assess confidence in the fossil record as a reflection of the true time of origin of a particular clade (e.g. Benton, Chapter 4; Donoghue et al., Chapter 10). These methods are becoming progressively more realistic and, inevitably, more complex in their application (e.g. Tavaré et al. 2002), but they do provide a means of testing the timing of evolutionary events within

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