Figure 2

Distribution of the proposed statistic under the null hypothesis. In a simulation of molecular evolution under the null hypothesis (see text for details) the statistic proposed shows good agreement with the expected standard normal behavior (dotted trace) either using the mean and variance of ΔS computed exactly (unfilled bars) or in the long-time limit (filled bars). Inset is a comparison of the P-value as computed under the standard normal assumption and the number of times that value of statistic or greater was observed in 1000 simulations, using either the exact (Xs) or long-time limiting (squares) values for the mean and variance of ΔS.