Confidence Intervals for the Divergence Time of Two Clades

We describe a simple method for generating tighter confidence intervals for the date of divergence of two monophyletic groups of taxa. This technique exploits the variation that exists within each of two groups that have evolved separately from a common ancestor. We illustrate the method (plus a technique to test the molecular clock hypothesis) using sequence dissimilarity within the orders of ratites and of tinamous (small birds from South America commonly regarded as the closest relative to ratites).