The sum of the genomes of an organism and all its symbiotic microorganisms (i.e. a holobiont)—for instance, a termite, the cellulose-digesting flagellate protists in the termite’s gut, and the protists’ own endosymbiotic bacteria. This concept underlies the hologenome theory of evolution, in which natural selection is viewed as acting on holobionts, rather than on individual organisms. It is argued that the microbial community of the host (i.e. its microbiome) can evolve far more rapidly than a large and complex host, and that the host can achieve selective advantage in just a few microbial generations simply through acquired changes in the makeup of its microbiome. Such a process invokes the Lamarckian concept of inheritance of acquired characteristics. The microbiome can also on occasion influence mating choices, for instance by altering the scent produced by an animal, which has led to speculation about its possible role in contributing to mating incompatibility and the splitting of species.