Biosurfactants direct the evolutionary dynamics of surface-associated microbial systems.

Surface-associated microbial systems experience spatial constraints that can intensify stochastic lineage loss through genetic drift and alter how natural selection acts on competing lineages. A key feature of these systems is that steep resource gradients typically confine growth to a narrow band of cells located at the biomass periphery. This restricts the effective population size and constrains lineage interactions, thereby accelerating stochastic lineage exclusion. Here, we demonstrate that biosurfactants, which are secreted by many bacteria, can modulate the evolutionary dynamics of these systems by relaxing spatial constraints. Using Stutzerimonas stutzeri as a model bacterium, we used surface-associated growth experiments and individual-based simulations to demonstrate that rhamnolipid-induced reductions in surface friction widen the peripheral band of growing cells, consequently enhancing lineage intermixing. This relaxation of spatial constraints suppresses exclusion by genetic drift, mitigates competitive asymmetries, and increases the persistence of slower-growing variants. Our simulations reveal that reducing surface friction alone is sufficient to cause these effects, establishing a direct mechanistic link between spatial constraints and evolutionary outcomes in microbial systems. Our results demonstrate that biosurfactants not only promote ecological dispersal but also direct evolutionary dynamics by determining the spatial constraints acting on the system. Spatial constraints are therefore important determinants of microbial population dynamics and underappreciated regulators of evolutionary processes.