RESOURCE #1: What is a Dichotomous Key?
A dichotomous key is a sequence of steps that allows for the identification usually of a living thing. They key
consists of a series of choices that lead the user to the correct name of a given item. The term dichotomous
means there will always be only two choices at each step in they key until the item is identified.
Rules to Follow When Using a Dichotomous Key
1. Always read both choices, even if the first seems to be the logical one.
2. Understand the meaning of the terms involved in the key.
3. When measurements are given, use a scale to measure the specimen. Do
not guess at a measurement.
4. Living things are always variable, so do not base your organism
identification on a single observation.
RESOURCE #2: How are Dichotomous Keys Represented?
Dichotomous keys can be represented graphically (see Figure A, below), but are also commonly represented
using a chart/table with instructions (see Figure B, below).
Figure A: Graphic Dichotomous Key
Figure B: Table Form Dichotomous Key
RESOURCE #3: Candy Dichotomous Key
Figure C: Candy Dichotomous Key
RESOURCE #4: Dichotomous Key to 10 Species of Bony Fish
1a. Has 2 separate dorsal fins.
b. Does not have 2 separate dorsal fins.
Yellow Perch
2
2a. Caudal fin forked.
b. Caudal fin not forked.
3
4
3a. Single dorsal fin.
b. 2 dorsal fins touching.
5
6
4a. Long anal fin.
b. Anal fin not long.
Starry Flounder
Rainbow Trout
5a. Axillary process at base of pelvic fin.
b. Axillary process absent.
Salmon
7
6a. End of pectoral fin rounded.
b. End of pectoral fin comes to a point.
Largemouth Bass
Bluegill
7a. Dorsal fin closer to anterior end.
b. Dorsal fin in middle or towards posterior end.
Catfish
8
8a. Dorsal fin in middle of dorsal side.
b. Dorsal fin toward posterior end.
Smelt
9
9a. Caudal fin symmetric.
b. Top of caudal fin longer than bottom.
Northern Pike
Sturgeon
Name: _______________________ Per: __ #
Reading and Creating a Dichotomous Key
DIRECTIONS: Use Resources #1 and #2 to to help you learn about what a dichotomous key is. Answer the
following questions with your group members.
RESOURCE #1: What is a Dichotomous Key?
1. In your own words and complete sentences, what is a dichotomous key?
2. Give one example of a situation in which a scientists might want to use a dichotomous key.
3. Why is it important to read both descriptions at a particular step in a dichotomous key?
RESOURCE #2: How are Dichotomous Keys Represented?
4. I have an organism that has legs, but no feathers. According to Figure A, what organism do I have?
5. Let’s practice using a dichotomous key to identify the birds shown above Figure B:
First, let’s identify Bird W.
Key
Action
1b. The beak is relatively stout and heavy.
Go to #2
2a. The bottom surface of the lower beak is flat and
straight.
This bird is a Geospiza
In this case, it only took us two steps to determine what species of bird we were examining. Create a chart like
the one above to identify Bird X:
Key
Action
RESOURCE #3: Candy Dichotomous Key
6. Using the dichotomous key found in Figure C, see if you can identify the multicolored candy shown to the right. Write the steps you followed in a chart form on
your own paper just like we did in the example above.
Here’s a start…
Key
Action
1. Packaged or Unpackaged?
Unpackaged
2. Soft or Hard?
Hard
RESOURCE #4: Dichotomous Key to 10 Species of Bony Fish
7. Use the dichotomous key to bony fish on Resource #4 and the diagram of the fish anatomical structures
below to determine the species of each of the fish diagrammed below. Write the name next to each fish.