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Dichotomous Key

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Author: Sarah Benton

Classification is very important to the field of biology. As we continue to discover new species, learn better techniques for analyzing relationships between species (i.e. DNA analysis) and share information internationally it is important to have systems in place to identify and classify organisms. A dichotomous key is a tool that helps to identify an unknown organism. A dichotomous key is a series statements consisting of 2 choices that describe characteristics of the unidentified organism. The user has to make a choice of which of the two statements best describes the unknown organism, then based on that choice moves to the next set of statements, ultimately ending in the identity of the unknown. Dichotomous keys are often used in field guides to help users accurately identify a plant or animal, but can be developed for virtually any object. They are particularly helpful when two species are very similar to one another.

This project is about learning how to use a dichotomous key to identify plants or animals. Through the process of creating their own dichotomous key and field guide the student will sharpen their observation and classification skills, both of which are necessary for success in upper level science courses. The student also builds an appreciation for nature through extended periods of observation.

Problem:

Learn how to make a dichotomous key. How can we use a dichotomous key to identify plants or animals?

Materials:

  • Materials are available at the library, office supply store or from home
  • Posterboard
  • Tree, bird, fungus, amphibian, wildflower, etc. field guides
  • Notebook
  • Camera

Procedure:

  • Study examples of dichotomous keys, like the ones below or others you might find in a field guide. There are a variety of ways you can design a dichotomous key, and they can be used to identify pretty much anything. Figure 1 below is a dichotomous key for types of potato chips, and Figure 2 is one for identifying organisms.
  • Practice making a dichotomous key with everyday items or people. Start with the most obvious features of the item and move to more specific statements. Remember, each statement must have 2 choices. For example you might start by creating a dichotomous key to identify students in your class. Begin with very general statements: Is the student male or female? Does the student have blue eyes or brown eyes? Does the student wear glasses? Etc. You can set up your key as a flow chart, or as a grid.

Here is an example of a partial dichotomous key for identifying classmates:

 
  • When you feel comfortable reading and creating a dichotomous key try to identify something in nature with a key. With an adult’s permission go outside and find a leaf from a tree you do not know. Use the dichotomous key in the tree identification guide you have to identify it.
  • You will create your own field guide that includes a dichotomous key for your project. Choose which group of organisms you will focus on (birds, trees, wildflowers, insects)
  • Pick 5 species in your group that you can see in the wild in your area.
  • Identify some of the features of the species. For example: if you choose 5 types of tree what are some of the shapes of the leaves? What color is the bark? etc.
  • It may be helpful to take photographs or make careful drawings of the features of your chosen species.
  • Work out the details on scrap paper before you try to draw out your dichotomous key on the poster board.
  • When you think you are finished with your dichotomous key try it out by using it to identify each organism. Does it work?
  • In addition to your key create a page of information about each species you have studied. Use pictures and text just like one of your field guides.
  • When you have finished you will have created your own field guide and dichotomous key for local organisms!
Figure 1
 
 
Figure 2

From: http://ashscience7.wikispaces.com/How+to+Use+a+Dichotomous+Key 

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