Definitions in taxonomy

There are four ways of defining a term or a datum (Hubbard, 1951). The most obvious might be by description, or listing its essential characteristics. Then something can also be defined by its differences from similar things with which it might be confused, or conversely by classifying or associating it with things that it resembles. But the most useful is an action definition that specifies the cause and function of the thing.

This is very relevant to biological taxonomy, which is concerned with defining taxa as relatively stable entities within the diversity of living organisms.

Traditional taxonomic literature uses descriptive definitions (full description with reference to a type specimen) to establish taxa. It also uses differentiative definitions (diagnoses and keys) as tools for identification. In practice, differentiation becomes definition by elimination, as in the dichotomous keys used in floras and monographs to narrow down the possible identity of a specimen to a single name by gradient steps.

Associative definitions may also be used in the protologue to discuss the relationships of a new taxon. Association was implicit in Aristotle’s concepts of genus and species, which Linnaeus adopted in the binomial nomenclature of plants and animals. Thus placing a new kind of cat into the genus Felis is a starting point for defining it. But an associative definition depends on the user’s prior knowledge of the thing being associated. It’s little use describing a lynx as a cat with certain characteristics to someone who has never seen any cats.

An action definition of a species would include its origin (the genus within which it evolved), its niche (functional role in the ecosystems where it lives), and its limits. e.g. “Watsonia hysterantha is a local derivative of W. meriana flowering in autumn from stored reserves, reduced in size and ground cover to exploit soil pockets in granite cliffs.”

An action definition of a higher taxon would similarly include its origin from a taxon of similar rank or the next higher taxon to which it belongs, the functional adaptations that caused this line to diverge, and its limits. e.g. “Cactaceae are caryophyllids adapted to seasonal drought by crassulacean acid metabolism and water-storing stems, with consequent slow growth rates making them vulnerable to competition”.

A species is a solution to the problems posed by its niche and environment. It is the lowest level taxon that can be called stable in that it is not liable to vanish within a few generations due to gene flow. There is no guarantee that a morphological species, based on museum specimens of distinctive appearance, has such stability. For practical purposes a taxonomic species is often taken as equivalent to a biological species: a reproductively isolated population or group of populations. The biological species concept was adopted by zoologists who considered only sexually reproducing organisms and ignored such things as clones and apomicts. But sterile or apomictic clones are morphologically distinct and function as species in ecology even though they do not fulfil the biological definition of a population as a unit within which genes are exchanged.

Like a key that fits a lock it wasn’t meant for, a species may fortuitously fit another environment quite different to the one where it evolved, as when an introduced crop species unexpectedly becomes a weed. But a population moved to a new habitat immediately becomes a new biological species because it is reproductively isolated from the original population and subjected to different selection pressures. Morphological changes may occur later, but it has already started on a new evolutionary trajectory. Bulbil watsonia is an example, a vegetatively reproducing biotype of Watsonia meriana that was reproductively isolated by becoming triploid and found a niche in habitats disturbed by human activity, first in its native South Africa and later overseas.


Hubbard, L. Ron (1951) Advanced Procedure and Axioms.


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