Phase transitions, hysteresis and memory in microbial ecologies

Studies of isolated populations have provided general principles by which organisms adapt to environmental change. Molecular mechanisms are subject to evolutionary pressures to operate in changing environments, but also to distinct pressures arising from competitions within larger communities and in more complex ecologies. Therefore, selection forces can occur at different scales spanning from single populations to large ecological structures. While the dynamics at the population level are well studied, little is known how the molecular details benefit from pressures at the ecological level.

Here, we present a layered ecological model where the evolutionary forces which hone the adaptation mechanisms at the molecular level are enacted through interactions among distinct populations that grow and compete across an ecology of localized patches. Migrations and invasions generate the pressure that allows different adaptation strategies to emerge and eventually stably coexist at the level of the ecology. As the molecular parameters are varied, the ecology undergoes a sharp or a continuous phase transition, which we analytically describe. The phase diagram can also exhibit a hysteresis loop, which points to the role of memory in the evolution of adaptation strategies.