"Phylogenetics" by Wiley (1981), p. 6
- A species or a group of species (= supraspecific taxon) that exists in nature as a result of a unique history of descent with modification (i.e., evolution). Angiospermae is hypothesized a natural taxon because all of its member species share a unique common history not shared by any species that is not an angiosperm. Invertebrata is a taxon, but it is not a natural taxon because some of its members (echinoderms for example) share a unique history of descent with noninvertebrates (specifically with Vertebrata) that is not shared by other invertebrates.
- Wiley, E. O. 1981. Phylogenetics: the Theory and Practice of Phylogenetic Systematics. -xv + 439 pp. John Wiley & Sons, Inc., New York.