  The gut microbiota features important genetic diversity, and the specific
spatial features of the gut may shape evolution within this environment. We
investigate the fixation probability of neutral bacterial mutants within a
minimal model of the gut that includes hydrodynamic flow and resulting
gradients of food and bacterial concentrations. We find that this fixation
probability is substantially increased compared to an equivalent well-mixed
system, in the regime where the profiles of food and bacterial concentration
are strongly spatially-dependent. Fixation probability then becomes independent
of total population size. We show that our results can be rationalized by
introducing an active population, which consists of those bacteria that are
actively consuming food and dividing. The active population size yields an
effective population size for neutral mutant fixation probability in the gut.
