Spatial constraints such as rigid barriers affect the dynamics of cell populations, potentially altering the course of natural evolution. In this paper, we study the population genetics of Escherichia coli proliferating in microchannels with open ends. Our experiments reveal that competition among two fluorescently labeled E. coli strains growing in a microchannel generates a stripe pattern aligned with the axial direction of the channel. To account for this observation, we study a lattice population model in which reproducing cells push entire lanes of cells towards the open ends of the channel. By combining mathematical theory, numerical simulations, and experiments, we find that the fixation dynamics is extremely fast along the axial direction, with a logarithmic dependence on the number of cells per lane. In contrast, competition among lanes is a much slower process. We also demonstrate that random mutations that appear in the middle and at the boundaries of the channel are highly likely to reach fixation. By theoretically studying competition between strains of different fitness, we find that the population structure in such a spatially confined system strongly suppresses selection.
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