Illustration of the stepwise buildup of antibiotic resistance in a population of bacteria in the presence of a gradient of antibiotic. Initially, the bacteria are confined to regions of low drug concentration, where the nonmutant “wild-type” strain (WT) can survive and grow. After some generations, a slightly more resistant mutant (MT1) arises at the boundary of the region with high bacterial population density (dashed region), where it has a high chance to reproduce and become the dominant bacterial strain. This mutant strain is able to invade the previously unoccupied region of higher antibiotics concentration until a new steady state is reached. The process repeats: A new mutation occurs in the “selective window” close to the wave front of the expansion of the MT1 population and establishes the next clone of more resistant bacteria (MT2). This process can quickly and efficiently generate drug resistance in situations where a homogeneous drug concentration would wipe out the bacterial infection.