In biology, an identification key, taxonomic key, or frequently just key, is a printed or computer-aided device that aids in the identification of biological organisms.
Historically, the most common type of identification key is the dichotomous key, a type of single-access key which offers a fixed sequence of identification steps, each with two alternatives. The earliest examples of identification keys originate in the seventeenth century, but their conceptual history can be traced back to antiquity. Modern multi-access keys allow the user to freely choose the identification steps and any order. They were traditionally performed using punched cards but now almost exclusively take the form of computer programs.
History
thumb|Identification key published in Lamarck's Flore française, Volume 1.
The conceptual origins of the modern identification key can be traced back to antiquity. Theophrastus categorized organisms into "subdivisions" based on dichotomous characteristics. The seventeenth-century Chinese herbalist, Pao Shan, in his treatise Yeh-ts'ai Po-Iu, included a systematic categorization of plants based on their apparent characteristics specifically for the purposes of identification.
Within the biological literature, identification keys are referred to simply as keys. They are also commonly referred to in general as dichotomous keys, though this term strictly refers to a specific type of identification key (see Types of keys).
Use
Identification keys are used in systematic biology and taxonomy to identify the genus or species of a specimen organism from a set of known taxa. They are commonly used in the fields of microbiology, plant taxonomy, and entomology, as groups of related taxa in these fields tend to be very large. However, they have also been used to classify non-organisms, such as birds nests, and in non-biological sciences such as geology. Similar methods have also been used in computer science.
A user of a key selects from a series of choices, representing mutually exclusive features of the specimen, with the aim to arrive at the sole remaining identity from the group of taxa. Each step in the key employs a character: a distinguishing feature of an organism that is conveniently observable.
Presentational variants
Dichotomous keys can be presented in two main styles: linked and nested. In the linked style (also referred to as open, parallel, linked, and juxtaposition Nested keys are more commonly known as indented, but unfortunately this refers to an accidental (albeit frequent) rather than essential quality. Nested keys may be printed without indentation to preserve space (relying solely on corresponding lead symbols) and linked keys may be indented to enhance the visibility of the couplet structure. More recently, Richard Pankhurst published a guidelines and practical tips for key construction in a section of his 1978 book, Biological Identification.
Computer-aided key construction
The first computer programs for constructing identification keys were created in the early 1970s. Since then, several popular programs have been developed, including DELTA, XPER, and LucID.