A biological key, which is sometimes called a dichotomous key, is a tool used to determine the identity of an organism by answering a series of questions about the organism’s characteristics. Dichotomous keys usually include a series of questions where each of which has two possible answers. By answering the questions in sequence, the operator of the key can determine the identity of the plant, animal, fungi or other organism.
An example of a question that may be found in a dichotomous key designed to identify trees is, “are the plant’s leaves broad or needle-like.” The answer to this question reduces the number of possible species that the operator must consider. The operator of the key then moves to the next question, depending on the answer to the first question. If the operator answered the first question with “needle-like leaves,” the second question may ask if the plant has cones or not, further reducing the list of possibilities.
The goal of any dichotomous key is to ask as few questions as possible to deduce the identity of the target organism. In most cases, it requires between five and 10 questions to deduce the species at hand, but dichotomous keys vary widely in their scope and thoroughness.