Jasper, R.; Kapopoulou, A.; Peischl, S.; Yilmaz, B.; Becker, J.; Bank, C.

Gut microbial communities are shaped by the physicochemical gradients and compartmentalization along the gastrointestinal tract. Whether spatially variable selection is strong enough to promote lasting genetic divergence within microbial species subject to migration and clonal expansion remains unclear. We performed a metagenomic population-genetic analysis of roe deer microbiota across metabolically and structurally distinct gut compartments. Remarkably, 10 of 20 prevalent bacterial species exhibited clear signatures of genetic local adaptation, despite the homogenizing effects of frequent migration. Nutrient availability and environmental stability emerged as primary drivers of selection, with horizontal gene transfer as a key mechanism enabling local adaptation. Our findings demonstrate that a comprehensive understanding of ecological and evolutionary dynamics in the gut necessitates considering fine-scale genetic variation within species.