Introduction and Application
Classification systems are used constantly in everyday life. In a grocery store, for example, items are classified into food or non-food items; food items into perishable or non-perishable; perishable into meats, produce, dairy or frozen; meats into chicken, beef, pork, lamb or fish; produce into fruits or vegetables; etc. When you go for a walk outside you automatically classify things you see (examples: trees, flowers, birds, insects, etc.). Classification by using a dichotomous key is a way of identifying distinct traits specific to one individual object or organism.
In science, as well as in everyday life, people attempt to organize objects, ideas or information. Placing objects, ideas or information into groups on the basis of similarity is called classification. We classify things in order to locate them more easily, see how they are similar or different, find order in the world and so we can communicate with each other by knowing we are talking about the same thing.
A classification key is a tool which organizes the traits of a particular group of organisms so that these traits may be used to identify a particular organism in the group. One type of classification device is a dichotomous key. Dichotomous keys have a series of numbered steps which take the user from very general to more specific traits. Each step usually consists of two statements (there are exceptional times when ther may be more than two choices) about a single trait. Only one of these statements can be true about a single organism or object. The key helps the user narrow his choices until he arrives at the step which identifies the organism or object. Each key is specific for a group of organisms or objects. For example, you may have a key for wildflowers in a particular
region, trees in a particular region, freshwater fish, insects, etc. A key can only be used to identify organisms or objects that are present in that key.
The process of using a dichotomous key is the same from key to key even though they identify different organisms or objects. Begin by observing the organism or object in question and noting the general characteristics.
Using the key, read both statements in the first step. Decide which of the statements is true. This will lead you to another step in the key. Continue to read the statements in the steps to which your choices lead you until you arrive at the name of the organism.
In this particular exercise you are going to create your own classification key for the objects provided.
Materials and Methods:
1. Students will be arranged into small groups of 2-4 students per group.
2. Each group will receive the following materials: different pieces of
a. Remove all the hardware pieces from the plastic bag and observe
some general traits.
b. Divide the objects into at least two groups based on the presence
or absence of one trait. (Example: Objects with threads and
objects without threads.)
c. Place each object into the proper group.
d. Write the statements at the bottom of a piece of paper, one to the
left side and one to the right. (Keep in mind it is possible to have
more than two statements).
e. Draw 2 diverging lines above each statement on the paper. (See
f. Observe the objects in one group at a time. Further divide the
objects based on a single trait for each group. Write the
statements above each group and place the objects in the proper
group. (Example: In the group of objects without threads, you may
observe that some have heads and some do not.)
g. Continue to divide objects in each group into two subgroups until
you arrive at the level where there is only one object in each
h. At the final level, draw or trace each object next to the
i. Place all hardware back into the bags and write the names of all
the group members on the back of your papers.
objects with objects without
The following diagrams and terms are designed to provide some clues to your investigation.
Binomial Nomenclature: A two-name system of naming organisms.
Classify: To put objects, ideas or organisms into groups based on
Dichotomous: Divided into two parts; branched or forked into two more or
less equal divisions.
Genus: A group of similar species.
Organism: Any living thing.
Scientific Name: The two-part name of an organism consisting of the
genus and species.
Species: A group of closely related organisms capable of mating and
producing viable offspring.
Trait: An inherited characteristic.