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Introduction to Biological Classification

Author(s): Deanne Erdmann, MS

Dichotomous Keys Identify Organisms

Identification is the process of finding the named group to which an organism belongs. Dichotomous keys are useful tools to help identify different organisms and usually are found in field guides. Identification in the field is based on features that are observable to the eye; therefore, it is important to remember that a key is an identification tool and is not synonymous with phylogenetic diagrams, which communicate hypothesized evolutionary history. 

Dichotomous keys are constructed of contrasting pairs of statements. To use a dichotomous key, begin with the first pair of statements and follow the directions at the end of each statement until you reach the name of the organism you are trying to identify. With each new organism, always start at the beginning of the key (1a and 1b). The ability to use dichotomous keys is an important skill and should be incorporated into instruction throughout the year. 

It is important to note that when constructing a dichotomous key, each pair of contrasting descriptions must deal with the same characteristic. For example the margin of the leaf might be used for the first pair of descriptors, and the shape of the leaf might be used for another pair. An incorrect pair of statements might be:

1a) Is the leaf heart shaped? 

1b) Are the edges lobed?