Snapshots of the population evolving antibiotic resistance. Snapshots of the population evolving antibiotic resistance. Shades of gray indicate the population density at each tile. The population, initially non-resistant and hence confined to compartment 1, adapts and expands simultaneously, in a probabilistic, stepwise fashion. While the population front “climbs the staircase”, a “comet tail” trails behind. Parameters: K = 105, r = 1/h, μf = 10-7/h, μb = 10-4/h, δ = 10-1/h, ν = 10-3/h. Rutger Hermsen et al. PNAS 2012;109:27:10775-10780 ©2012 by National Academy of Sciences