Figure 2.

Computing of the probability that the amount of information shared by two sequences, S, is lower than a threshold s. Given an initial sequence a, we can envisage different scenarios for its evolution into another sequence b. In a first step (Step 1), an elementary probability is computed by taking into account the evolution of just one residue (here a₁ into b₁). Considering one possible evolutionary scenario (Step 2), residues are considered as independent and the probability is the product of elementary probabilities for each positions aligned in this scenario, with approximations in the asymptotic limit of long sequences. The final probability (Step 3) is then estimated by taking into account all the possible evolutionary scenarios.