Although the species concept is frequently used, there is no clear definition of what a species actually is. The prevalent biological species concept, introduced by Ernst Mayr, defines a species as a group of interbreeding natural populations that are reproductively isolated from other such groups. However, it is often difficult to assess whether groups of organisms definitely cannot interbreed, especially in the case of hybrids, clones and fossils. In such cases, molecular biology analysis and the morphological species concept are applied in order to obtain further clarification. The morphological species concept defines species as the smallest groups that are consistently and persistently distinct, and are physically distinguishable by certain characters or combinations of characters.
Mayr, Ernst (1942): Systematics and the Origin of Species. New York: Columbia University Press.
Mahner, Martin: (2005): Biologische Klassifikation und Artbegriff. (Biological classification and species concept.) In: Krohs, Ulrich / Toepfer, Georg (ed.): Philosophie der Biologie: eine Einführung. Frankfurt am Main: Suhrkamp, 231-248.