Symbiosis is the tight relationship between two or more species that are living together. In many cases the relationship is obligate, a situation in which we have to face some terminological problems. If the two symbionts are, in some way, forced to live together, what is the biological entity resulting from the relationship? A common way to think is to consider the biggest organism, usually defined as host, the individual or the species (according to the taxonomic rank taken into account). However, is this correct? Recent studies clearly underline that the unit of selection is not the host only, but the sum of all the organisms involved in the symbiosis. But if we evolve as a community of organisms, what is an individual or a species? To answer these questions it is necessary to incorporate Lamarckian mechanisms in Darwinian theory of evolution. Time is arrived for a real integration of symbiosis in the evolutionary studies, with all the consequences that are following. Parole chiave: Comunità di organismi, Evoluzione, Individuo, Specie, Simbiosi, Untà selettiva.
Keywords: Community of organisms, Evolution, Individual, Species, Symbiosis, Unit of selection.