Population genetics in microchannels*

Spatial constraints such as rigid barriers affect the dynamics of cell populations, potentially altering the course of natural evolution. We investigate the population genetics of bacteria Escherichia coli proliferating in microchannels with open ends. We base our analysis on a population model in which reproducing cells push entire lanes of cells towards the open ends of the channel. The model predicts that diversity loss is exponentially fast along the axial direction of the microchannel and at a much slower pace in the transverse direction. These predictions are in quantitative agreement with our experiments. Our theory predicts that two neutral, fluorescently labeled E. coli strains should organize into a regular stripe pattern in the course of a few generations, as we experimentally confirm.  We also demonstrate that random mutations appearing in the middle of the channel and close to its walls are much more likely to reach fixation than those occurring elsewhere.