The species is the basic (fundamental) unit of biological taxonomy. It is the most specific level of classification, defined by Ernst Mayr's Biological Species Concept as a group of actually or potentially interbreeding natural populations that are reproductively isolated from other such groups. The taxonomic hierarchy from broadest to most specific is: Domain → Kingdom → Phylum → Class → Order → Family → Genus → Species. Binomial nomenclature (Linnaeus) uses Genus species (e.g. Homo sapiens).