RNA viruses, retroviruses and pararetroviruses, are known for their rapid evolution, because of high mutation rates and short generation times. Replication errors (mutations and imperfect recombinations) generate a great variability. However, selection pressures shape viral populations diversity, by acting at different steps of virus infectious cycle : strategies of genome expression, interactions with host(s) and vector(s) are the targets of selection. Such a selection sorts out genotypes according to their fitness. The second most important parameter acting on viral population diversity, genetic drift, occurs when population effective size is low, which seems to be frequent during host or tissue infection initiation. By contrast to selection, drift is a random, fitness‐independent process, which may lead to the fixation of deleterious mutations. Despite their exceptionally high mutation rates, RNA viruses seem to follow classical population genetics principles, as showed during the last two decades.http://www.john-libbey-eurotext.fr/en/revues/bio_rech/vir/e-docs/00/04/04/91/resume.md?type=text.htm
