Community conservatism is widespread across microbial phyla and environments

Evolution leaves recognizable ecological signatures. In animals and plants, closely related species tend to share traits and ecological niches - a pattern known as phylogenetic signal and niche conservatism. Whether similar principles apply to microorganisms has long remained unclear. By analyzing over one million microbiome samples from diverse environments worldwide (www.microbeatlas.org), we take the community a microbe is found in as a proxy for their realized niche. We show that phylogenetically related microbes consistently occur in more similar communities than distantly related ones. We term this global pattern community conservatism, and show it occurs in all environments and phyla analyzed - and the differences can help us to infer evolutionary patterns such as habitat specialism and generalism in different phyla. Despite immense diversity, frequent horizontal gene transfer and broad dispersal, microbial lineages retain measurable ecological continuity over evolutionary time. Community conservatism extends fundamental evolutionary concepts to the microbial world and shows promise to improve Operational Taxonomic Unit clustering in the future.