  Despite the importance that fluid flow plays in transporting and organizing
populations, few laboratory systems exist to systematically investigate the
impact of advection on their spatial evolutionary dynamics. To address this
problem, we study the morphology and genetic spatial structure of microbial
colonies growing on the surface of a nutrient-laden fluid $10^4$ to $10^5$
times more viscous than water in Petri dishes, the extreme but finite viscosity
inhibits undesired thermal convection and allows populations to effectively
live at the air-liquid interface due to capillary forces. We discover that S.
cerevisiae (baker's yeast) growing on a viscous liquid behave like "active
matter": they metabolically generate fluid flows many times larger than their
unperturbed colony expansion speed, and that flow, in return, can dramatically
impact their colony morphology and spatial population genetics. We show that
yeast cells generate fluid flows by consuming surrounding nutrients and
decreasing the local substrate density, leading to misaligned fluid pressure
and density contours, which ultimately generates vorticity via a thresholdless
baroclinic instability. Numerical simulations demonstrate that an intense
vortex ring is produced below the colony's edge and quantitatively predict the
observed flow. As the viscosity of the substrate is lowered and the
self-induced flow intensifies, we observe three distinct morphologies: at the
highest viscosity,compact circular colonies similar to those grown on hard agar
plates except with a stretched regime of exponential expansion, intermediate
viscosities give rise to compact colonies with "fingers" that are ripped away
to break into smaller cell clusters, and at the lowest viscosity, the expanding
colony breaks up into many genetically-diverse, mutually repelling, island-like
fragments of yeast colonies.
