Community conservatism is widespread across microbial phyla and environments.

Phylogenetic signal describes the tendency of related organisms to resemble each other in morphology and function. Related organisms tend to also live in similar ecological niches, which is termed niche conservatism. The concepts of both phylogenetic signal and niche conservatism are widely used to understand crucial aspects of evolution and speciation, and they are well established in animals and plants. However, although assumed to be present, the extension of these concepts to microorganisms is challenging to assess. Here we hypothesize that two closely related microbial species should be found in samples with similar community compositions, reflecting their ecological similarity. We propose 'community conservatism' to refer to this phenomenon and leverage a database with millions of samples and hundreds of thousands of pairs of microorganisms to assess their relatedness and the similarity of the communities they occupy. Our findings reveal that community conservatism can be observed globally in all environments and phyla tested, over nearly all taxonomic ranks, but to varying extents. Analysing community conservatism shows promise to advance our understanding of evolution, speciation and the mechanisms governing community assembly in microorganisms. Furthermore, we propose that it can be used to reintegrate ecological parameters into operational taxonomic unit delimitation.