Fit of Poisson and negative binomial distributions to the site-specific substitution counts in Figure? 1, estimates of α, the ratio of the P values for both distributions, the proportion of constant sites for five data sets and the substitution rate estimate for each data set.

aThis estimate of the shape parameter for gamma-distributed rates among sites was obtained by minimising χ2.
bSites having <0.5 mutations as inferred by stochastic mutational mapping in SIMMAP.