  We study an individual-based model in which two spatially-distributed
species, characterized by different diffusivities, compete for resources. We
consider three different ecological settings. In the first, diffusing faster
has a cost in terms of reproduction rate. In the second case, resources are not
uniformly distributed in space. In the third case, the two species are
transported by a fluid flow. In all these cases, at varying the parameters, we
observe a transition from a regime in which diffusing faster confers an
effective selective advantage to one in which it constitutes a disadvantage. We
analytically estimate the magnitude of this advantage (or disadvantage) and
test it by measuring fixation probabilities in simulations of the
individual-based model. Our results provide a framework to quantify
evolutionary pressure for increased or decreased dispersal in a given
environment.
