Fig. 10

Changing the number of arcs by free (black curve) and quasineutral (blue curve) mutations. Top: Probabilities ρ and ρ_∗ are computed for 100 randomly selected MFE structures and the average of ρ(S,S^′) (respectively ρ_∗(S,S^′)) is obtained restricted to the condition that S,S^′ differ by a given number of arcs. We observe that even though neutral mutations are excluded from the computation of both ρ and ρ^′, quasineutral mutations have a much higher likelihood of preserving the number of arcs. The probability of a positive (resp. negative) change in the number of arcs for free mutations is 0.5 (resp. 0.35) and for quasineutral mutations it is 0.46 (resp. 0.28). This means that quasineutral mutations are slightly less likely to decrease the number of arcs. Bottom: The same probabilities for structures S with 4 or 5 arcs. We see that when the number of arcs is low, quasineutral simulations are less likely to decrease the number of arcs and so to result in the structure to mutate to an open chain