The Ontological Status of Species: Scientific Progress and Philosophical Terminology

(1) Biological species are not classes, as traditionally defined, because they have no essence and because they have various properties (see 2 and 3) that are incompatible with the class concept. (2) Among non-class properties of species are their spatiotemporal localization, their boundedness, their internal cohesiveness, and their capacity to change (evolve). (3) Other non-class properties of species are their propensity for splitting (speciating). for fusing (by hybridization), and for becoming extinct, none applicable to classes. (4) To call a species a set, and defining as set any aggregate of more than a single entity, would completely destroy the usefulness and unique characterization of the species in biological science. (5) Properties in common of taxa of all ranks are not essences, since they are variable and have the potential for evolution. (6) Even though ignored by philosophers, the non-class nature of species was recognized by naturalists from John Ray (1686) and Buffon (1753) to the 1960s. By that time it was virtually unanimously recognized by biologists. (7) In order to make the non-class nature of biological species more visible, Ghiselin (1974) and Hull (1976) have proposed to consider them individuals. (8) Most biologists and some philosophers have however been unhappy about calling a species an individual, when it actually may consist of millions or billions of individual organisms and show much less cohesion than a single individual. (9) It is proposed that the term population, applied by naturalists to species for more than 100 years, be added to the vocabulary of the philosophers of science to designate a phenomenon of nature, biological species, for which neither the term class (set) nor the term individual is appropriate. (10) Only sexually reproducing organisms qualify as species. Some other terminology, for instance paraspecies, will have to be found for uniparentally reproducing forms. Higher taxa are neither classes nor individuals but may be designated, following Wiley, as 'historical groups.' Grades are classes.