The species (from Latin: species "kind"), is in biology (including virology) the basic unit of biological systematics. Every biological species is a result of speciation. So far, no general definition of "species" has succeeded in meeting the theoretical and practical requirements of all biological subdisciplines equally. Rather, different species concepts exist in biology, leading to different classifications. Historically as well as currently, two approaches of species concepts play an important role:
- on the basis of an assumed reproductive community (biological species concept)
- on the basis of the assumption of a group of individuals with characteristic common species features, with the help of which the membership to a species should be clearly delimitable, and which are determined on the basis of external (morphological species concept) "or" hereditary features.
With the emergence of cladistics (from Greek: κλάδος kládos "branch", the chronologically defined phylogenetic species concept based on the biological species concept has been added since the 1950s, according to which a species begins with the species fission, i.e. the formation of two species from one original species, and ends with its renewed species fission or its extinction.