  It is widely accepted that population genetics theory is the cornerstone of
evolutionary analyses. Empirical tests of the theory, however, are challenging
because of the complex relationships between space, dispersal, and evolution.
Critically, we lack quantitative validation of the spatial models of population
genetics. Here we combine analytics, on and off-lattice simulations, and
experiments with bacteria to perform quantitative tests of the theory. We study
two bacterial species, the gut microbe Escherichia coli and the opportunistic
pathogen Pseudomonas aeruginosa, and show that spatio-genetic patterns in
colony biofilms of both species are accurately described by an extension of the
one-dimensional stepping-stone model. We use one empirical measure, genetic
diversity at the colony periphery, to parameterize our models and show that we
can then accurately predict another key variable: the degree of short-range
cell migration along an edge. Moreover, the model allows us to estimate other
key parameters including effective population size (density) at the expansion
frontier. While our experimental system is a simplification of natural
microbial community, we argue it is a proof of principle that the spatial
models of population genetics can quantitatively capture organismal evolution.
