Evolutionary coexistence in a fluctuating environment by specialization on resource level

Microbial communities in fluctuating environments, such as the human gut or repeated dilutions in the laboratory, contain a wealth of diversity. Diversity contributes to the stability and function of communities and is maintained by underlying mechanisms. When nutrient levels fluctuate over time, one possibly relevant mechanism is that types specialize on low and high nutrient levels. The relevance of this process is supported by observations of coexistence in the laboratory, and by simple models, that show that negative frequency dependence of two such types can stabilize coexistence. However, as microbial populations are often large and numerous, they evolve. Our aim is to determine what happens when species can evolve; whether evolutionary branching can create diversity or whether evolution will destabilize coexistence. We derive the selection coefficient in fluctuating environments and use adaptive dynamics to find that evolutionary stable coexistence requires a special type of trade-off between growth at low and high nutrients. We do not find support for the necessary trade-off in data available for the bacterium Escherichia coli and the yeast Saccharomyces cerevisiae on glucose. However, this type data is scarce, and might exist for other species or in different conditions. Moreover, we do find evidence for the right trade-off and evolutionarily stable coexistence of the two species together. Since we find this coexistence in the scarce data that is available, we predict that specialization on resource level is a relevant mechanism for species diversity in microbial communities in fluctuating environments in natural settings.