Eco-evolutionary dynamics lead to functionally robust and redundant communities

Microbial communities are taxonomically diverse and variable: species presence and abundances widely fluctuate over time, space, and even across biological replicates under controlled experimental conditions. However, environmental conditions exert strong selection on the traits of community members and their functions. Similar environmental conditions are expected to produce functionally similar communities. This environmental selection, combined with taxonomic variability, leads to the influential concept of functional redundancy - the idea that many species can perform the same function, allowing communities with different species compositions to maintain identical functional profiles. Despite the centrality of functional redundancy in microbial ecology, we lack a theoretical understanding of its origin. Here we study the eco-evolutionary dynamics of communities interacting through competition and cross-feeding. We show that eco-evolutionary trajectories rapidly converge to a "functional attractor" - a functional composition uniquely determined by environmental conditions. Taxonomic composition follows non-reproducible dynamics while being constrained by the conservation of functional composition. Our framework provides a theoretical foundation for understanding functional robustness and redundancy in microbial communities.