Every mammalian species harbours a gut microbiota, and variation in the gut microbiota within mammalian species can have profound effects on host phenotypes. In this review, we summarize recent evidence that gut microbiotas have influenced the course of mammalian adaptation and diversification. Associations with gut microbiotas have: (i) promoted the diversification of mammalian species by enabling dietary transitions onto difficult-to-digest carbon sources and toxic food items; (ii) shaped the evolution of adaptive phenotypic plasticity in mammalian species through the amplification of signals from the external environment and from postnatal developmental processes; and (iii) generated selection for host mechanisms, including innate and adaptive immune mechanisms, to control the gut microbiota for the benefit of host fitness. The stability of specific gut microbiotas within host species lineages varies substantially across the mammalian phylogeny, and this variation may alter the ultimate evolutionary outcomes of relationships with gut microbiotas in different mammalian clades. In some mammalian species, including humans, relationships with host species-specific gut microbiotas appear to have led to the evolution of host dependence on the gut microbiota for certain functions. These studies implicate the gut microbiota as a significant environmental factor and selective agent shaping the adaptive evolution of mammalian diet, phenotypic plasticity, gastrointestinal morphology and immunity. This article is part of the theme issue 'The role of the microbiome in host evolution'.
Keywords: holobiont; host–microbe interactions; metagenome; microbiome; population genetics.