BIOLOGY Volume 1 - A Guide to General Biology - 2004

2. THE DIVERSITY OF LIFE ON EARTH

2.1. Classification

2.1.6. Identification and Keys

Identification keys greatly assist biologists in identifying organisms. To achieve this, a list of characteristics for a given Organism is first compiled and then compared against the diagnostic features of individual taxonomic groups. Identifications typically rely on readily distinguishable traits, such as shape, color, number of appendages, segments, and so forth. Consequently, identification is artificial or phenotypic, as it relies exclusively on the external appearance (phenotype) of the organism. Despite this, almost all diagnostic keys enable the determination of an organism's placement within a specific taxon that forms part of a natural phylogenetic hierarchical Classification.

There are several types of diagnostic keys, the simplest of which are dichotomous keys. These keys consist of numbered (1, 2, 3, etc.) pairs of contrasting statements that form a couplet. Each couplet represents a specific character. The statements within a pair must be contrasting and mutually exclusive. To determine the taxonomic position of an organism, these pairs of statements are evaluated in sequence. As one moves from one couplet to the next, a large group of organisms is successively broken down into progressively smaller groups until the taxonomic group of the organism in question is established.

The characters used in identification keys must be morphological and easily discernible. They may be qualitative, such as abdomen shape and coloration, or quantitative, such as the number of hairs and stem height. Any features may be used for identification provided they remain constant for a given species and are not subject to environmental modification. In this regard, The Use of size and color is often unsatisfactory, as both traits can vary under the Influence of Environmental factors, seasonal changes, age, or the physiological state of the organism at the time of identification. Character states chosen for identification should ideally occur in two or more discrete variants. For instance, a character such as "stem shape" may occur in one of two variants: either "round in cross-section" or "rectangular in cross-section".

Each statement is followed by a number that directs the user to the corresponding couplet; if the statement in the current couplet matches the appearance of the organism, the number following it indicates the next couplet to be examined. For example, if when identifying cultivated legumes (Leguminosae), which include peas and beans (Table 2.1), you reach couplet 5 and observe that the plant stems lack thorns or leaf-derived spines, you must then skip couplet 6 and proceed directly to couplet 7, and so on (Table 2.1).

Class="center">Table 2.1. Excerpt from an Identification key for cultivated legumes (Leguminosae)