Strain-specific higher-order interactions break phylogenetic conservatism in microbial communities

Whether strains of the same species sharing the same ecological role are truly interchangeable has not been directly tested experimentally in communities, where higher-order interactions govern outcomes. Using 79 co-occurring Bacillaceae isolates from the Cuatrociénegas Basin, we combined whole-genome analyses with systematic strain substitutions in BARS, a three-species community in which a resistant strain protects a sensitive strain from an antagonist. Trait conservatism strongly predicted pairwise antagonistic outcomes. Yet across 34 assembled triads, 13 showed rescue beyond pairwise expectations and five showed suppression; single-strain substitutions rarely restored protection, although protection was recovered in one fully reconfigured triad. These outcomes could not be inferred from taxonomic relatedness, pangenomic content or pairwise assays; phylogenetic conservatism thus reaches its limit at the transition from pairwise to higher-order interactions. To explain this limit, we propose that strains behave as non-equivalent biological tiles whose interaction edges must align, a dynamic biological tessellation .