Dynamics of the model. Dynamics of the model. (A) Snapshot of a simulation. At this time, the population has adapted to the drug concentrations in compartments ≤ 6. The next adaptation step most likely unfolds in one of two ways, indicated with blue and red arrows. The adaptation rate critically depends on the rates of these two “paths”. (B) The spread of the bacteria for the simulation in Fig. 1. For each tile, an arrow indicates the origin of its first inhabitant. Colors indicate for each adaptation step whether the red or the blue path was taken. The blue path is dominant. (C) The red path becomes more likely if the mutation rate and the carrying capacity are high. Here, μfK = 102; now the red and blue paths occur with equal probability. (D) If drug resistance comes at a large fitness cost s, the blue path is repressed. Here, s = 0.2; both the population density at the end of the simulation and the spread of the population are shown. No comet tail is found. (Parameters: as in Fig. 1). Rutger Hermsen et al. PNAS 2012;109:27:10775-10780 ©2012 by National Academy of Sciences