A Taxonomic key is a device, which when properly constructed and used, enables a user to identify an organism.In lifesciences, an identification key is a printed or computer-aided device that aids the identification of biological entities, such as plants, animals, fossils, microorganisms, and pollen grains. Identification keys are also used in many other scientific and technical fields to identify various kinds of entities, such as diseases, soil types, minerals, or archaeological and anthropological artifacts.

Traditionally identification keys have most commonly taken the form of singleaccess keys. These work by offering a fixed sequence of identification steps, each with multiple alternatives, the choice of which determines the next step. If each step has only two alternatives, the key is said to be dichotomous, else it is polytomous. Modern multi-access or interactive keys allow the user to freely choose the identification steps and their order.

At each step, the user must answer a question about one or more features (characters) of the entity to be identified. For example, a step in a zoological key for insect identification may ask about the number of bristles on the rear leg.

The need for taxonomic keys

We share the planet with at least 1.5 million other species. In order to communicate, retrieve, store, and accumulate information about our co-inhabitants, it has been necessary for biologists and others to:

  1. identify these organisms
  2. name them
  3. place the organisms into groups that reflect our current knowledge of their evolutionary relationships.

As we know, collectively these activities – identification, nomenclature and classification – make up the disciplineof taxonomy.

Types of taxonomic keys

A taxonomic key is a device, which when properly constructed and used, enables a user to identify an organism. There are two types of keys :-

  • Dichotomous
  • Polyclave

Dichotomous keys

di – two; chotomy – forked – These keys, which are the most common, were probably first published by Jean Baptiste-Lamarck in 1778. They consist of a series of paired statements, termed couplets, that describe some feature of the organism. The statements, or leads, are in direct contrast (i.e., mutually exclusive). To use the key, begin with the first couplet and select the statement that best fits our specimen. This will direct us to another couplet and ultimatel yprovide the identity of our specimen.

Writing of dichotomous key First collect our data by:

(a) laying out the animals to be keyed infront of us; and/or (b) recording data on note cards or in a computer spreadsheet or database; and/or (c) creating a table listing the species to key along one side and the characters to study along the other side. Once we have collected our data, start to group the objects. It is best to start with a feature that separates the things to be keyed into two groups of similar number and then subdivide these groups until individuals are distinguished.

Polyclave/Random Access/Synoptic Keys

Second type of key is termed multiple access or polyclave or synoptic key. The advantage of these keys is that they allow the user to enter the key at any point. These keys are a relatively new alternative to dichotomous keys and are becoming increasingly popular, especially because of the ease of computerizing them.

Identifying organisms with a polyclave is a process of elimination. In a written polyclave key there is a series of characters and character states. Each state is followed by a number or code for the species that possess that feature. The user selects any character and then copies down the list of species that possess the feature. Then the user selects another character and eliminates any species not common to both lists. This process continues until the specimen is identified.


The advantages of a polyclave (multiple-access) key are –

  • Easy to use;
  • Multi-entry – meaning the user can start anywhere. This is a significant advantage because the user can rely on characters that are most easy to observe, rather than having to deal with characters that may not be present in the specimen or are poorly developed
  • Order-free – meaning the user can work in any direction with character
  • Faster
  • Easily computerized. In fact, these keys are most commonly used in this form. Paper versions are typically large and unwieldy because each character needs to list all possible taxa.


A taxonomic key is a tool that is used to identify different types of organisms. An example of a key that could be used to identify different types of microorganisms is provided below. A taxonomic key contains a series of statements that describe the traits of the organisms, as shown in the sample key presented below.

The statements are grouped into 2 or 3 alternative descriptions (such as 1a and 1b) for each trait. Each group of statements represents only single trait.

To use the key, appropriate traits of the unknown organism are identified while following the steps of the key. When the description in the key and trait of the organism match, we are instructed to proceed to another set of descriptions or given an identification. Correctly following the steps of the key eventually leads to identification of the unknown organism.


Working in groups, obtain one specimen of each species, and construct a dichotomous key to the specimens. Do not use body size as a characteristic, since the key must be useful for small specimens of species that do become large.

Limit the use of color as a character. Color may fade in preserved specimens, and may vary among individuals. Use morphological characters such as body shape, position of fins, presence or absence of scales or spines in fin supports, etc.

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