Evolution at the front*

Bacterial motility is intimately linked to growth through the allocation of cellular resources. This link creates physiological constraints which govern the spatiotemporal dynamics of growing and migrating bacterial populations. Here we present results which illuminate the evolutionary implications of these constraints. We select Escherichia coli for faster migration through a porous environment, a process which depends on both motility and growth. Using high-throughput single-cell tracking we find that a trade-off between swimming speed and growth rate constrains the evolution of faster migration. Evolving faster migration in rich medium results in slow growth and fast swimming, while evolution in minimal medium results in fast growth and slow swimming. Whole genome sequencing shows that in each condition parallel genomic evolution drives adaptation through different mutations. Through precise genetic engineering we show that the trade-off is mediated by antagonistic pleiotropy through mutations that likely disrupt negative regulation. A geometric model of the evolutionary process shows that the genetic capacity of an organism to vary traits can qualitatively depend on its environment, which in turn alters its evolutionary trajectory. Our results suggest that understanding how genetic architecture interacts with constraints on phenotypic variation may provide a route to predicting evolutionary dynamics more broadly. We present recent work which explores this possibility experimentally.