Fig. 4.

Starting from the quadruple mutant (1111), reverse evolution towards the ancestor (0000) is impeded across concentrations. These are illustrative examples of the most preferred pathways for evolution at each of the simulated pyrimethamine environments, starting with the 1111 quadruple mutant. Panels correspond to several simulation scenarios: a no drug, b low drug (~7.0 μM), c high drug (~3000 μM). In each case, one observes no accessible trajectories in the fitness landscape leading to the ancestral allele (0000), not even in the no-drug environment where the ancestor is the most fit