Ecological interactions constrain the coexistence of generalists and specialists during coevolution in microbial communities

Natural microbial communities are complex ecosystems in which species with different metabolic strategies—both generalists and specialists—stably coexist. We do not understand how coevolution within a community can both lead to, and stabilize, such coexistence, and how generic we expect it to be. Here, we propose and study a minimal model of a co-evolving microbial community shaped by ecological interactions. Our model combines competition and cooperation for nutrients with evolutionary game theory. In doing so, it makes three broad and surprising predictions about co-evolved microbial communities. First, we find that generalists and specialists coexist only in a narrow range of ecological interactions; in all other regimes, specialists dominate. Second, we find that co-evolving with others makes it strikingly difficult for generalists to evolve a correlation between their nutrient preferences and growth rates; such a correlation is often observed in experiments. Finally, we find that communities with a greater fraction of generalists are also likely to have more stable states, which can be tested. This model provides a simple framework through which to quantify and make testable predictions about coevolution in microbial communities.