Summary The hologenome theory of evolution emphasizes the role of microorganisms in the evolution of animals and plants. The theory posits that the holobiont (host plus all of its symbiont microbiota) is a unit of selection in evolution. Genetic variation in the holobiont that can occur either in the host and/or in the microbial symbiont genomes (together termed hologenome) can then be transmitted to offspring. In addition to the known modes of variation, i.e. sexual recombination, chromosomal rearrangement and mutation, variation in the holobiont can occur also via two mechanisms that are specific to the hologenome theory: Amplification of existing microorganisms and acquisition of novel strains from the environment. These mechanisms are Lamarckian in that (i) they are regulated by 'use and disuse' (of microbes) and (ii) the variations in the hologenome are transmitted to offspring, thus satisfying also the Lamarckian principle of 'inheritance of acquired characteristics'. Accordingly, the hologenome theory incorporates Lamarckian aspects within a Darwinian framework, accentuating both cooperation and competition within the holobiont and with other holobionts.