Mehlferber, E. C.; Irby, I.; Yarter, M.; Lowery, N.; Lowhorn, R.; Appaji, Y.; Eum, J.; Song, H.; Stone, B.; Brown, S. P.

Microbes that span environmental reservoirs and diverse human infections are often described as generalists or as ubiquitous, yet ecological theory predicts generalism should be unstable when specialists outperform within any one niche. Here we show that Pseudomonas aeruginosa's apparent ecological breadth reflects convergent, cryptic specialism (CCS) - repeatable, environment-linked genomic differentiation that is not confined to deep lineages - rather than strict specialism or generalism. An analysis of 6,627 genomes with source-environment metadata reveals that environments are broadly dispersed across the phylogeny, inconsistent with lineage-locked specialization. Despite this shallow phylogenetic structure, genome content predicted environment-of-isolation across nine genotype-discernible environments, including under cross-validation that blocks phylogenetic relatedness. Interpretable feature profiles recovered both shared and environment-specific signals, including genomic signatures distinguishing distinct chronic and acute human infections. Finally, phenotypes from 47 diverse strains clustered more strongly by environmental source than by phylogenetic relatedness. Together, these results indicate that widely distributed bacterial "generalists" can comprise mixtures of cryptic, convergent specialists. By mapping genotype-defined ecological structure, our approach can identify when apparent generalists harbor hidden structure relevant to infection risk.