What In The World Is A Dichotomous Key?
Written by Dianne Smithwick , North Heights Alternative School

Imagine that you are at a family reunion. Everyone there is in some way related to each other. In this large group of closely related people (organisms), there are smaller groups. Each smaller group can be divided (classified) into smaller groups until an individual is identified.

Now imagine that someone listed all the family characteristics and organized them in such a way that a person (organism) could be identified. That list of characteristics would be called a dichotomous key.

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 similar to a scavenger hunt. A scavenger hunt has a starting point. If someone were conducting a scavenger hunt at a party, the party would be the starting point. Each hunter would begin at the party and follow the clue to the next destination. Directions to the next clue or the item being "hunted" are given at each destination. A dichotomous key also has a starting point. 1A of a dichotomous key is always the starting point with each object to be identified. A description of characteristics and a new "destination" is given at each point.                                              

For example,  a student wanted to classify these shapes. Using the dichotomous key below the student began to classify the collection.  The first shape to be classified was curved.  1A on the dichotomous key stated:  Object has only straight lines, go to 2.  The student then went to 1B.  1B stated:  Object has curved lines go to 4. The student then skipped to 4.  4A stated: Object has one continuous curving line, go to 5.  The student then skipped to 5A.  5A stated: Object is red--Ovalado rufus.  5B stated:  Object is green--Orbis crudus.  The student determined that based on the key the object was Orbis crudus.  Beginning at 1A again, the process was repeated until all of the shapes were identified.

DICHOTOMOUS KEY EXAMPLE

On the page, you will find members of the Button family reunion. Follow the directions and try your scavenger hunt skills. Happy hunting.

Identify the Button family members below using the attached dichotomous key. Remember to start at number 1 with each Button and follow the directions given you until you find the name of each family member.

Button 1____________________________		Button 2_____________________________
Button 3____________________________		Button 4_____________________________
Button 5____________________________		Button 6_____________________________
Button 7____________________________		Button 8_____________________________
Button 9____________________________		Button 10____________________________

BUTTON FAMILY DICHOTOMOUS KEY

How did you do?  Find a friend to work through the key and compare answers.  Do your answers match?  If not, why not?

Now you are ready to tackle an actual dichotomous key. 

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