Adaptive Radiation of Aquatic MIcrobial Species (ARAMIS). The action aims to understand the causes, mechanisms and consequences of adaptive radiation (i.e. the process in which organisms diversify rapidly to occupy ecological niches) in freshwater microbial communities. This will help solve several of the outstanding questions in the field of microbial ecology, such as how shifts in environmental conditions impact the trajectory of microbial evolution, the relative importance of horizontal versus vertical gene transfer in microbial ecosystems, or the definition of microbial ‘species’. As diversity is linked with stability, elucidating the factors driving diversification in freshwater taxa will also help us predict how such ecosystems may respond to future change. ARAMIS proposes the novel hypothesis that, while every freshwater clade has a unique evolutionary history, the environmental forces driving such evolution are fundamentally the same, both in nature and in relative importance. If validated, this simple paradigm would push the state of the art and allow the development of powerful conceptual models integrating environmental constraints, microbial diversification, and community stability. The hypothesis will be tested by combining metagenomics, metatranscriptomics and enrichment cultures over a set of more than 200 lake samples. This will contribute to a conceptual synthesis between community ecology and evolutionary biology by closing the gap between theoretical work and data-driven studies. ARAMIS has a high academic potential but, since it deals with the stability of freswater microbial populations, can also have social and public policy impact by bringing an ecoevolutionary perspective to the management of aquatic resources.