Causes of transitions into mutator (M) and non-mutator (W) clones: an environmental change (Env), a mutation (Mut) and epigenetic stochastic switching (SS) due to the noise of protein production level.

We performed 100 independent runs for populations, which respectively experience environmental temperature jump (Tj), stationary phase (Sp) and starvation (St) as environmental stress (ES). Different runs achieve different levels of fitness after the initial adaptation. They can be broadly classified into three groups to which most of the runs belong - which are ∼0.33, ∼0.62, and ∼1, and the third column (Pathway) shows the number of runs which reached a corresponding fitness level. Columns 4–6 give numbers of runs where rise and fall of mutators was attributed to a specific molecular cause during the second adaptation events after t = 20000.