Tips and nodes are complementary not competing approaches to the calibration of molecular clocks.

Joseph E O'Reilly,Philip C J Donoghue

doi:10.1098/rsbl.2015.0975

Abstract

Molecular clock methodology provides the best means of establishing evolutionary timescales, the accuracy and precision of which remain reliant on calibration, traditionally based on fossil constraints on clade (node) ages. Tip calibration has been developed to obviate undesirable aspects of node calibration, including the need for maximum age constraints that are invariably very difficult to justify. Instead, tip calibration incorporates fossil species as dated tips alongside living relatives, potentially improving the accuracy and precision of divergence time estimates. We demonstrate that tip calibration yields node calibrations that violate fossil evidence, contributing to unjustifiably young and ancient age estimates, less precise and (presumably) accurate than conventional node calibration. However, we go on to show that node and tip calibrations are complementary, producing meaningful age estimates, with node minima enforcing realistic ages and fossil tips interacting with node calibrations to objectively define maximum age constraints on clade ages. Together, tip and node calibrations may yield evolutionary timescales that are better justified, more precise and accurate than either calibration strategy can achieve alone.

Highlights

The molecular clock has displaced the fossil record as the primary means of establishing an evolutionary timescale; the accuracy and precision of divergence time estimates and their fossil calibrations remain inextricably linked [1]
There has been little effort to demonstrate the effect of different approaches to calibration and, to determine whether the effective prior on node ages resulting from tip calibration is compatible with the fossil evidence usually employed in node calibration
We show that effective node age priors derived from tip calibration are often incompatible with fossil evidence, violating either minimum or maximum node age constraints

Summary

Introduction

The molecular clock has displaced the fossil record as the primary means of establishing an evolutionary timescale; the accuracy and precision of divergence time estimates and their fossil calibrations remain inextricably linked [1]. Divergence time estimation has achieved calibration based on geological (usually palaeontological) constraints on clade (node) ages This approach has been developed to the extent that further improvements in accuracy and precision are limited by the inherent uncertainty in fossil evidence. There has been little effort to demonstrate the effect of different approaches to calibration and, to determine whether the effective prior on node ages resulting from tip calibration is compatible with the fossil evidence usually employed in node calibration.

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Discussion

Conclusion