  Spatial constraints such as rigid barriers affect the dynamics of cell
populations, potentially altering the course of natural evolution. In this
paper, we investigate the population genetics of Escherichia coli proliferating
in microchannels with open ends. Our analysis is based on a population model in
which reproducing cells shift entire lanes of cells towards the open ends of
the channel. The model predicts that diversity is lost very rapidly within
lanes, but at a much slower pace among lanes. As a consequence, two mixed,
neutral E. coli strains competing in a microchannel must organize into an
ordered regular stripe pattern in the course of a few generations. These
predictions are in quantitative agreement with our experiments. We also
demonstrate that random mutations appearing in the middle of the channel are
much more likely to reach fixation than those occurring elsewhere. Our results
illustrate fundamental mechanisms of microbial evolution in spatially confined
space.
