Taxonomy

Taxonomic identification is the recognition of the identity or essential character of an organism. Taxonomists often present organized written descriptions of the characteristics of similar species so that other biologists can identify unknown organisms. These organized descriptions are referred to as taxonomic keys. A taxonomic key is often published with pictures of the species it describes. However, written descriptions are usually preferred over pictures, since pictures cannot convey the natural variation in the morphology of a species, nor the small, yet characteristic, morphological features of a species. In addition, matching an unidentified organism to one picture in a book of hundreds or thousands of pictures can be very time-consuming.

The dichotomous key is the best and most-used format for taxonomic keys. A dichotomous key sequentially presents pairs of alternative morphological features (dichotomies) and requires the user to decide which alternative best describes the unknown organism. A very simple hypothetical key for identifying four species of flowering plants illustrates the dichotomous key:

a. Flowers white; stem woody

b. Sepals present; bark deep-furrowed—species A

bb. Sepals absent; bark not deep-furrowed—species B

aa. Flowers not white; stem herbaceous

c. Flowers blue; flowers with five petals—species C

cc. Flowers red; flowers with three petals—species D

The first dichotomy asks the user to choose between a and aa. If alternative a is chosen, the user must choose between b and bb. If alternative aa is chosen, the user must choose between c and cc.

Taxonomic keys often require knowledge of the morphology of the taxa in question and consequently rely upon technical terminology. In the above example, it is assumed the user knows that sepals constitute the leaf-like, outermost whorl of a flower.

Interestingly, taxonomic keys often use the sexual organs of animals (genitalia), plants (flowers, fruits, and cones), and fungi (spore-bearing structures), because these differ significantly among closely related species. This indicates that sexual organs of animals, plants, and fungi tend to evolve rapidly and divergently, presumably because they are subjected to great evolutionary selection pressures. Differences in reproductive structures also indicate that mating may not be successful, and that two organisms may be reproductively isolated—that is, different species.