The identification of living or non-living things can be greatly simplified by using a tool known as a dichotomous key. The word dichotomous literally means “divided in two parts”. It therefore stands to reason that a dichotomous key divides a set of objects into two subsets based on the presence or absence of a particular character. Each question within the set leads to another set of more specific questions and so on, until the user reaches a conclusion as to the identity of the object. This form of classification is the one used most often by taxonomists because it offers the user a quick and easy way of identifying unknown organisms.
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 another 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.
DID YOU KNOW?
- Dichotomous keys are valuable tools for taxonomists and naturalists. Many related organisms closely resemble one another, and it can be difficult to differentiate between similar species. A properly designed dichotomous key can provide researchers with an easy to use method for identifying species.
- Aristotle (4th century B.C.) was one of the first men to attempt to group organisms based on their form and structure. He grouped animals according to whether they inhabited water, land, or air, and plants by differences in their stems. Linnaeus (18th century) took this classification system further by developing the binomial naming system we use today.
- It is often impossible to determine the species or sub-species of organisms such as plants and small animals or fishes before they reach maturity. Recent biotechnological advances allow scientists to identify organisms early in their development using species-specific molecular markers.
Related Products
