Abstract

Spatially growing populations are ubiquitous across scales, ranging from microbial biofilms in the soil to expanding tissues in developing organs and the spreading of diseases. In spatial settings, individuals experience inhomogeneities in the surrounding environment that result in different growth rates across the population. Since replication is necessary to transmit the genetic information from mother to daughter, the growth dynamics determined by the spatial constraints can deeply affect the spreading of mutations in a population and thus its evolution.

Here, I will present two examples in which the simple spatial constraints associated with two-dimensional growth affect both the genetic diversity and the adaptation of microbial and viral populations, respectively. Population sequencing and fluorescence imaging show that microbial colonies exhibit an excess of mutational jackpot events compared to a well-mixed population of the same size. At the same time, they also carry a large number of rare mutations that are homogeneously distributed across the population and continuously generated during the growth process. A very different growth and evolutionary dynamics is instead observed in ecoliphage T7 two-dimensional plaques. Here the phage has to find an optimal incubation time, which provides a sufficient offspring number without excessively delaying its diffusion.