Yan BOUCHER (MIT, Boston) Evolutionary Units: Breaking down Species Concepts An important goal of biologists is to classify organisms into natural groups. This aim, once considered virtually impossible by microbiologists as notable as Stanier and Van Niel, was given a new life by the discovery of DNA. Among the first molecular microbiologist, Carl Woese encouraged two decades of efforts to create a molecular taxonomy and uncover the nature of microbial species. The advent of genomics, making feasible the sequencing of entire genomes, brought forward an unexpected conclusion: prokaryotes are not clonal, they exchange considerable amounts of DNA. This process, termed lateral gene transfer (LGT), required a fundamental shift in how we described the evolution of bacteria and archaea. Moreover, population genetics studies are now suggesting that LGT could be responsible for the majority of genetic changes occurring in many prokaryotes. The fact that the genetic diversity required for the evolution of microbes through selection and drift could in majority originate through processes other than inheritance from a progenitor requires a revision of prokaryotic species concepts. Many new concepts have been put forward, but all are based on classifying organisms in a natural species category, and none agree on which criteria should be used. We propose that since organisms can be broken down in parts that are exchangeable and can move between hosts, so too should concepts trying to define natural groups. Inspired by the work of Splitter and other philosophers, we argue that any elements that can be replicated, and are held together by some biological mechanism, can form natural groups. It can be useful to directly study such evolutionary units, because they have a causal effect and a role in the natural world. Furthermore, since they are not required to describe groups of organisms, evolutionary units can be defined at any level of biological complexity, from genes to populations. We demonstrate that this concept is not only useful, but also essential to new scientific disciplines that are rapidly gaining in importance, such as metagenomics.