  How producers of public goods persist in microbial communities is a major
question in evolutionary biology. Cooperation is evolutionarily unstable, since
cheating strains can reproduce quicker and take over. Spatial structure has
been shown to be a robust mechanism for the evolution of cooperation. Here we
study how spatial assortment might emerge from native dynamics and show that
fluid flow shear promotes cooperative behavior. Social structures arise
naturally from our advection-diffusion-reaction model as self-reproducing
Turing patterns. We computationally study the effects of fluid advection on
these patterns as a mechanism to enable or enhance social behavior. Our central
finding is that flow shear enables and promotes social behavior in microbes by
increasing the group fragmentation rate and thereby limiting the spread of
cheating strains. Regions of the flow domain with higher shear admit high
cooperativity and large population density, whereas low shear regions are
devoid of life due to opportunistic mutations.

Populations and Evolution;Pattern Formation and Solitons;Biological Physics