  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.
