Bacterial systematics has not yet reached a consensus for defining thefundamental unit of biological diversity, the species. The past half-century ofbacterial systematics has been characterized by improvements in methods fordemarcating species as phenotypic and genetic clusters, but species demarcationhas not been guided by a theory-based concept of species. Eukaryotesystematists have developed a universal concept of species: A species is agroup of organisms whose divergence is capped by a force of cohesion;divergence between different species is irreversible; and different species areecologically distinct. In the case of bacteria, these universal properties areheld not by the named species of systematics but by ecotypes. These arepopulations of organisms occupying the same ecological niche, whose divergenceis purged recurrently by natural selection. These ecotypes can be discovered byseveral universal sequence-based approaches. These molecular methods suggestthat a typical named species contains many ecotypes, each with the universalattributes of species. A named bacterial species is thus more like a genus thana species.