[Paper Review] Counting chickens before they hatch: reciprocal consistency of calibration points for estimating divergence dates

David A. Morrison|arXiv (Cornell University)|Jan 20, 2010

Genetic diversity and population structure21 references3 citations

TL;DR

This paper re-evaluates the reliability of secondary calibration points in molecular clock dating by testing their reciprocal consistency with primary calibration points. Using improved statistical methods including confidence intervals, the author demonstrates that no inconsistencies exist between primary and secondary timepoints, though large confidence intervals often limit the utility of molecular data for precise hypothesis testing.

ABSTRACT

There has been concern in the literature about the methodology of using secondary calibration timepoints when estimating evolutionary divergence dates. Such timepoints are divergence time estimates that have been derived from one molecular data set on the basis of a primary external calibration timepoint, and which are then used independently on a second data set. Logically, the primary and secondary calibration points must be mutually consistent, in the sense that it must be possible to predict each time point from the other. However, the attempt by Shaul and Graur (2002, Gene 300: 59-61) to assess the reliability of secondary timepoints is flawed because they presented time estimates without presenting confidence intervals on those estimates, and so it was not possible to make any explicit hypothesis tests of divergence times. Also, they inappropriately excluded some of the data, which leads to a very biased estimate of one of the divergence times. Here, I present a re-analysis of the same data set, with more appropriate methodology, and come to the conclusion that no inconsistencies are involved. However, it is clear from the analysis that molecular data often have such large confidence intervals that they are uninformative, and thus cannot be used for reliable hypothesis tests.

Motivation & Objective

• To assess the validity of secondary calibration points used in molecular clock dating.
• To address flaws in prior methodology, particularly the lack of confidence intervals in time estimates.
• To test whether primary and secondary calibration points are reciprocally consistent.
• To evaluate the reliability of molecular data for hypothesis testing in divergence time estimation.
• To correct biased data exclusions in previous studies, such as those by Shaul and Graur (2002).

Proposed method

• Re-analyzed the same dataset used by Shaul and Graur (2002) using proper statistical methods.
• Applied confidence intervals to divergence time estimates to enable formal hypothesis testing.
• Assessed reciprocal consistency by testing whether each calibration point could predict the other within its uncertainty range.
• Avoided arbitrary data exclusions that previously led to biased estimates.
• Used standard molecular clock models with proper error propagation to evaluate uncertainty.
• Performed explicit statistical tests of divergence time consistency between primary and secondary calibration points.

Experimental results

Research questions

• RQ1 Are secondary calibration points logically consistent with primary calibration points in molecular dating?
• RQ2 Does the absence of confidence intervals in prior studies invalidate their conclusions about calibration reliability?
• RQ3 Can molecular data with large confidence intervals support reliable hypothesis testing of divergence times?
• RQ4 How do data exclusions affect the accuracy of divergence time estimates?
• RQ5 Is the reciprocal consistency of calibration points sufficient to validate secondary timepoints?

Key findings

• No inconsistencies were found between primary and secondary calibration points when proper confidence intervals were applied.
• The prior study by Shaul and Graur (2002) was flawed due to omission of confidence intervals, preventing valid hypothesis testing.
• Excluding certain data points led to a biased estimate of one divergence time, undermining the reliability of their conclusion.
• Molecular data often yield such wide confidence intervals that they are uninformative for precise divergence time inference.
• Reciprocal consistency is a necessary but not sufficient condition for reliable calibration, given the high uncertainty in many estimates.
• The re-analysis confirms that secondary calibration points can be reliable when statistical rigor is applied.