It is hoped that this short series will fill a long felt want for a brief and reasonably simple introduction to taxonomy and to the technical terms used by zoologists in the classification of animals, both recent and fossil, and of the mollusca in particular.
The Species
The species is the most important taxonomy category, an understanding of the nature of species is indispensible for taxonomic work. The species concept of the biologist goes back to J. Ray, who in his 'Historia Plantarum' (l686) used the term species, much as it was used later by Linnaeus and the 19th. Century taxonomists.
A species definition is merely the verbalisation of a species concept. Species concepts are derived from a study of species in nature. A student of any local fauna finds that it is composed of well-defined 'Kinds' of animals and plants. In Great Britain for example there are 7 kinds of water snails of the genus Lymnaea. These are species. The individuals within a local population of such a species are freely interbreeding but are separated by a distinct gap from individuals of all other species. In spite of the morphological similarity of some of these 7 species, each one is separated from every other one by a definite gap. No intermediate has ever been found. They do not interbreed; they are reproductively isolated.
It is this discontinuity between natural populations that impressed the early naturalists, and which remains the corner-stone of the species concept of the modern systematist.
Species therefore may be defined as follows: Species are groups of actually (or potentially) interbreeding natural populations which are reproductively isolated from other such groups.
The Subspecies
The subspecies is the only infraspecific taxonomic category. It may be defined as follows: Subspecies are geographically defined aggregates of local populations which differ taxonomically from other subdivisions of a species.
When the species concept was first developed, the species was first thought to be something stable and uniform, composed of individuals that conform to type. Individuals that did not agree with the type were segregated as 'varieties1. Subsequently it was found that the ’variety1 was a composite concept, including both variant individuals and variant populations. The name variety for the latter category was eventually re¬placed by the term subspecies.
It must be borne in mind however that all taxonomic categories are somewhat heterogeneous and that completely uniform population groups do not exist in sexually reproducing species. Not only the species but the sub¬species also is an assemblage of populations, except in the rare case of exceedingly localised relict forms or insular populations, for example the land snail genus Chilinopsis on the island of St. Helena.
To qualify as a subspecies, such an assemblage of populations must be taxonomically different from other subspecies, but what is taxonomically different can be determined only by agr ement among taxonomists.
Further reading
Cain, A, J. , 1954 Animal Species and Their Evolution, Hutchinson's University Library.
Caiman, W. T., 1949 Classification of Animals. Methuen, London.
Huxley, J. S. (Editor), 1940 The New Systematics. Oxford University Press.
Schenk, E. T. and HcMasters, J. H., 1936, Procedure in Taxonomy, Oxford University Press.
T. Pain