Classification is important in understanding the natural world. All of the information known about living species is used to organize them into a sort of family tree, which helps us to see the similarities and differences that exist in the living things all around us. A dichotomous key is a guide for classification and identification, somewhat like a map through a classification system that was developed previously. Dichotomous comes from the Greek root dich-, meaning "two" and temnein, meaning "to cut". By asking a series of questions to which there are only two possible answers with respect to the object to be identified, the key leads users toward the proper identification. Many parts of the natural world that have been classified, categorized and grouped can be identified using a dichotomous key. Dichotomous keys can be developed to identify anything in any sort of classification. Let’s develop our own dichotomous key to better understand them.
Collection of various seeds and leaves -OR- Variety of buttons
What To Do
Beginning with the collection in the center of the working area, instruct the students to decide upon a way to divide the collection into two groups, A and B. One characteristic must be defined and used to decide which pieces are placed in which group. For example, a collection of buttons might be divided into groups of buttons with four holes and buttons that have some number of holes other than four. Record what factor was used to make the division and note which members of the original collection belonged to each resulting group. After the collection has been divided into two groups, divide the first group (A) into two more groups, based on one criterion (C and D). For example, if group A contains four-holed buttons, group C might contain four-holed buttons made of metal and group D would have four-holed buttons made from material other than metal. Divide group B into two more groups (E and F) based on one decisive factor.
Record data on the new groupings and dividing criteria. Continue to divide the groups until each item is by itself, then name each of the individual objects. Keep careful records. Looking at your records and the divisions you made, create a dichotomous key that would lead someone else to make the same distinctions you did. Begin by recalling the first factor you used to divide into two groups. If your two groups depended upon a button having four holes or some other number of holes, then the first question in your dichotomous key would have to ask the user to make that same distinction. For example, Does your button have four holes? If so, continue to step 2. If not, skip to step 5. When your key is complete ask a classmate to use it to classify and identify the same collection of items. Does your key lead others to the same identifications you made?
1. Is it possible to create more than one dichotomous key for classifying and identifying the same group of objects?
2. When two people use the same dichotomous key to identify the same object, is it possible (should it be possible) for them to have different final answers?
3. Why are classification and identification important?
Classification is a specific way of organizing information so that it can be more useful. There are as many as 100 million species of living things on earth today. Classification of these living things helps us see similarities and differences among the living world. Scientists have classified many millions of living species based on their physical characteristics and they have given a unique name to each unique species. The scientists who classify living things record their classifications so that later, others who encounter a certain species will be able to identify it in the same way. Making sure that two scientists are referring to the same thing when using a certain name is important for clear communication. Dichotomous keys help guide scientists toward identification so classifications can be shared and used mutually. Dichotomous keys ask a series of questions about the object to be identified; each question has only two possible answers, and the answer in reference to the certain object will lead the user in one of the two possible directions laid out by the answer.
Leonard, Joan. Green House Director, Department of Plant Biology, The Ohio State University.
© S. Olesik, WOW Project, Ohio State University, 2003.