Dichotomous Keys
Classification is a way of separating a large group of closely related organisms into smaller subgroups. A dichotomous key is a listing of characteristics, such as structure and behavior, organized in such a way that an organism can be identified or classified.
A dichotomous key is a method for determining the identity of something (like the name of a butterfly, a plant, a lichen, or a rock) by going through a series of choices that leads the user to the correct name of the item. Dichotomous means "divided in two parts". At each step of the process of using the key, the user is given two choices; that are opposites of each other, so that only one choice can be correct. Each alternative leads to another question until the item is identified. (It's like playing 20 questions.)
To construct a dichotomous key, you must first identify and describe a property that some of the objects have but that none of the other objects have. This property may be presented to the user in the form of either a qualitative or quantitative description. Qualitative descriptions concern the physical attributes, or qualities, of the item being classified (e.g., “has blue hair”). Quantitative descriptions concern values that correspond with the item being classified (e.g., “has eight legs”). Often times separating the bigger group is the most difficult part of constructing a key. Obviously, the bigger the group, the greater the variation that is likely to be found in it, and the more exceptions there will be to any definite statement that is made about the group. The next step is to group all the objects displaying that property in one set (couplet) and all of the remaining objects in the other couplet. Some taxonomists use numbers to separate the couplets while others use letters. Also, some taxonomists prefer to place each couplet together, while others may separate couplets. One example of a simple dichotomous key for shapes is shown in Figure 1 below.
1a. The shape consists of four sides…..Go to 2
1b. The shape consists of three sides…..Go to 3
2a. Both pairs of opposite sides are parallel………..B (Parallelogram)
2b. Only one pair of opposite sides are parallel…..D (Trapezoid)
3a. One angle is equal to 90°………………………A (Right triangle)
3b. All angles are less than 90° and equal……….C (Equilateral triangle)
When constructing a dichotomous key, it is important to be as specific as possible so that the user does not become confused. The idea is to word each couplet in such a way that the user will arrive at the correct answer. If the wording leaves too much room for interpretation, the user could easily end up with an erroneous classification. When using a dichotomous key, it is important to consider both choices in the couplet. Jumping to conclusions may lead to the wrong classification of the item. When classifying a living (or once-living) specimen, it is best to study many specimens due to the variation between organisms. This will ensure that your results are representative of the majority.