Estimates of the number of substitutions in unconstrained sites under a mixture of Poisson models (see Methods). (A) The proportion of unconstrained sites that have undergone 0, 1, 2, 3, 4, 5, or 6 substitutions when the average number of neutral substitutions over the tree relating the sequences is 0.8 (corresponding to human, mouse, and rat), 1.25 (corresponding to human, mouse, rat, cow, and dog), 1.5 (corresponding to all species of the CFTR region data set studied), and two larger arbitrary values, 3 and 5. The fraction of sites that has undergone 0 substitutions denotes those unconstrained sites that appear fully conserved. In the currently sequenced genomes (0.8 subs/site = H + M + R), this fraction is 49%. (B) the fraction of unconstrained sites that has undergone at least one, two, three, or four substitutions, as a function of the total length of the tree that relates the sequenced organisms.
