- Lab Exercise 1
Use and Construction of Dichotomous Keys

Dichotomous keys are so named because they are divided into a series of steps, called couplets, each of which consists of 2 mutually exclusive choices (dichotomy comes from Greek words meaning to cut into two parts). Keys of this sort are sometimes called diagnostic keys, because they are often used to identify a particular type of object from a large selection of types of objects. These keys may be used, for example, to identify a type of alloy by the pattern of sparks generated when it is touched to a grinding wheel, to identify a particular type of military installation from satellite imagery, or to allow a nonspecialist to diagnose a disease. In biology, we frequently use these keys in a special way.

While in other fields the diagnostic features used in keys may be simply based upon obvious physical characteristics and not designed to indicate relationships between objects, in biology these keys are often designed to identify and group organisms based on shared characteristics that indicate evolutionary relationships among the taxa (groups) represented in the key. We will explore how these relationships are determined in next week's lab.

Example:

Objects Available:

  • Frog
  • Kangaroo
  • Salamander
  • Bat
Key #1: Key #2:
1. Has long hind legs go to 2. 1. Has slimy skin go to 2.
1'. Has short hind legs go to 3. 1'. Has hairy skin go to 3.
       
2. Has slimy skin Frog 2. Has long hind legs Frog
2'. Has hairy skin Kangaroo 2'. Has short hind legs Salamander
       
3. Has wings Bat 3. Has wings Bat
3'. Has front legs Salamander 3'. Has front legs Kangaroo

Key 1 is not correctly prepared. While it is diagnostically correct - - it does allow you to correctly identify the animals -- it does not accurately reflect the evolutionary relationships between the objects. It groups frogs and kangaroos together because both have long hind legs. Key 2 correctly groups the mammals together based on a characteristic all mammals have (hair), and the same goes for amphibians (slimy skin). It then subdivides the two groups.

To use one of these keys:

  1. Select the object to be identified.
  2. Go to step 1 in the key, and select the proper choice.
  3. Go to the step indicated by your choice.
  4. Continue until you reach a step that identifies your object.
  5. Select another object and repeat steps 1-4.

A correctly prepared key will have one fewer steps than the number of different objects covered by the key. For example, if you have a key to 45 species of mammals, the key would have 44 steps or couplets.

If you are compiling a key to be used for strictly identification purposes and that does not need to indicate evolutionary relationships among organisms, follow these guidelines:

  • Choose characteristics that are readily observable.
  • Choose characteristics that do not change easily.
  • Use objective language. For example, "tall" is subjective, '183 cm.or taller' is objective.
  • Keep it simple. People that do not know much about the organisms may be using your key. For example, if creating a key to bats, you might want to use the term 'tail membrane' rather than the more formal 'uropatagium'.

Assignments:

1. Create a diagnostic key to students in this room - whole class participates.

2. Use a key to identify assigned organisms - individuals complete this in or out of class.

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