Figure 4. Accelerated Evolution of Some Operons.

(A) New operon pairs are more likely to be adjacent to each other than expected by chance. The surplus of adjacent pairs of the same age is particularly striking. The error bars show 95% confidence intervals from a c² test of proportions. The model for random evolution is detailed in Materials and Methods.

(B) The frequency of different types of modifications to pre-existing operons. The excess of append over prepend pairs is not quite statistically significant (p = 0.06, binomial test).