Unit 3: Diversity of Life
Module 1: Taxonomy and Introduction to Kingdoms
Learning Objectives: 1.4
Lesson 2: Dichotomous Keys
Lesson Objectives:
- To construct a dichotomous key based on the characteristics of various organisms.
- To use a dichotomous key to help identify organisms.
Evaluation:
- Assignment: Creating a Dichotomous Key (20 marks)
A dichotomous key is a device that can be used to easily identify an unknown organism. The word dichotomous comes from two Greek words that together mean, "divided in two parts". A dichotomous key consists of a series of two part statements that describe characteristic of organisms. At each step of a dichotomous key the user is presented with two choices. As the user makes a choice about a particular characteristic of an organism they are led to a new branch of the key. Eventually the user will be led to the name of the organism that they are trying to identify.
Read more about and view examples of dichotomous keys in a biology textbook.
For example:
Nelson Biology p. 421
Constructing Dichotomous Keys
As an example, we could construct a key using the following stationary supplies taken from a student's pencil case.
Some students find it easier to arrange their information in a spider key before setting it up in the traditional dichotomous key format.
The following key has been created as an example using the stationary supplies pictured above.
Key to Stationary Supplies
Spider Key:
Assignment: Creating a Dichotomous Key
Working individually or with a partner your will create a dichotomous key based on the characteristics of various organisms. Your key will be evaluated by other groups of students as they use the key to try and identify each of the ten organisms.
1. Obtain a group or list of ten different organisms.
Options:
- i) If you have access to a biology lab that contains preserved organisms, obtain a set of 10 different organisms. (Your teacher may make up the groups of organisms, or you may be free to choose 10 on your own.) You will need to keep your group of 10 organisms together because another group will use them when they evaluate your key. You should assign a name or a letter to your group as a way of identifying the 10 organisms that you are using. You should also assign each of the organisms numbers from 1-10 for ease of identification on your answer key.
- ii) If you do not have access to preserved organisms, choose one of the groups of 10 organisms from the groups listed at the link below. As mentioned above, in constructing dichotomous key you will distinguish between organisms on the basis of a variety of characteristics. If you are not familiar with the main characteristics if some of the organisms in the list you choose, find a picture of the organisms in a textbook, online or in any other suitable reference materials.
2. Carefully examine or think about the characteristics of the ten organisms and create a dichotomous key using some of these characteristics. Set up your dichotomous key on ONE PAGE with no answers on that page. Leave a blank where each of the answers should go so that another group can write in the answers. Put your names and the groups name or letter for the organisms you are using at the top of the page.
3. Make a second page as an answer key. Your answer key could be set up in the following way. If you are using preserved specimens and are not sure of the name of an organism ask your teacher or just use its number to identify it.
Example answer key:
2a. Organism #9 (frog)
3b. Organism # 3 (mushroom)
4. Hand in, or submit via email, your finished dichotomous key along with the answer key. A copy of your key will be distributed or emailed to two other groups of students for evaluation. This means that you should also receive two keys to evaluate.
5. When you receive a key that has been created by another group use the key along with the group of preserved organisms or the list of organisms to try and identify each of the organisms. Fill in the names and/or identifying numbers for each of the organisms in the blanks provided.
6. Assign each key that you work on a mark out of ten along with an explanation for the mark you are assigning. Use the questions listed below to help you evaluate each of the keys.
- What difficulties did I have with the key?
- What are the positive points about the key?
- What kind of improvements might be made to this key?
7. Hand in or submit evaluated keys via email to your instructor.
The idea for this assignment was adapted from an activity outlined in the Biology 20 Curriculum Guide.
Saskatchewan Education. (1992) Science: A Curriculum Guide for the Secondary Level Biology 20/30. Saskatchewan Education. p. 106
