Histogram showing the frequency of f2 values of mouse:rat intertaxa ortholog pairs. (a). Observed data from all ortholog pairs (n>100), with the best fit Gaussian superimposed. μ = 0.89 (95% ci 0.886–0.893), σ = 0.029 (95% ci 0.028–0.031). (b). Theoretical histogram that assumes the null hypothesis that all sites diverge with equal rate constants, based on a simulation with the same distribution of characters. μ = 0.89 (95% ci 0.888–0.891), σ = 0.022 (95% ci 0.021–0.024). ci: confidence interval.