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Education on the Halfshell:
Creating a Dichotomous Key

By: Cassie Zanca

Download the entire lesson (PDF FILE: 2,790K)

Lesson Goal

The students will explore the benefits of creating dichotomous keys as a means of identifying an organism or object.

Lesson Objective

The student will create a dichotomous key to identify fossil oyster shells.

Background Information

A dichotomous key is a tool that allows the user to determine the identity of items and organisms in the natural world. It is the most widely used form of classification in the biological sciences because it offers the user a quick and easy way of identifying unknown organisms. Keys consist of a series of choices that lead the user to the correct name of a given item. "Dichotomous" means "divided into two parts." That is why dichotomous keys always give two choices in each step. In each step, the user is presented with two statements based on characteristics of the organism. If the user makes the correct choice every time, the name of the organism will be revealed at the end.

There are two kinds of descriptions that might be presented to the user of a dichotomous key: qualitative and quantitative descriptions. Qualitative descriptions concern the physical attributes, or qualities, of the item being classified. Examples of qualitative descriptions are such phrases as "contains green striations on top surface" or "feels slick on bottom surface." Quantitative descriptions concern values that correspond with the item being classified. Examples of quantitative descriptions are such phrases as "has 10 striations on top surface," "has 8 legs," or "weighs 5 grams". Knowing the difference between these two types of descriptions can be immensely beneficial for creators and users of dichotomous keys.

There are two ways to set up a dichotomous key. One way is to present the two choices together, and the other way is to group by relationships. When the dichotomous key is set up by presenting the two choices together, it is easy to distinguish between them. However, relationships between various characteristics are not emphasized. When the dichotomous key is grouped by relationships, the choices are separated, yet it is easy to see the relationships between them. While this method may prove to be more difficult to construct, many users prefer it because it gives them more information.

Teacher Preparation

In this activity, the teacher will need or collect five to six oyster shells for each student or student group. If oyster shells are not readily available, download the pictures of the oyster shells from the website. Enclosing the pictures in sheet protectors may be a good idea. From these shells or pictures, the students can now create their own dichotomous keys.

Student Preparation

You will create a dichotomous key in order to identify fossil oyster shells (using the images provided in Blackline Master 4) or a variety of shells provided to you by your teacher. Before writing your dichotomous key, read the background information on shells provided by your teacher (Blackline Masters 1- 3), as they will aid you in key construction. Continue to write the key until each individual shell can be identified. Make an answer key to accompany your dichotomous key.

Blackline Masters Alternative Assessments
  • Create a concept map from the key.
  • Students will design a dichotomous key using a particular item (examples: coke tabs, ketchup bottles, paper clips, other shells) and be graded on accuracy and completeness.
Answer Key(s)

Each student or student group will submit a dichotomous key and answer key; therefore, each answer key will be different.

Extension Ideas
  • Have students create a field guide using dichotomous keys to help with identification of a particular item (examples: plants on the school campus, or macro-invertebrates in a local stream, etc). Give students examples of field guides to generate ideas.
  • Using the key they have created, develop an outline of the format used in their English class, that students will understand what the divisions in an outline represent and the relationships among the terms.
Resources and Web Links

Timme, Stephen, 1991, Association for Biology Laboratory Education website, How to Construct and Use a Dichotomous Key, accessed 02/16/01, http://www.zoo.utoronto.ca/able/volumes/vol-12/7-timme/7-timme.htm

Description: An excellent web-based activity on the construction and use of a dichotomous key that also describes the use of a dichotomous key in the field and provides a key for prairie plants.

Frontier High School, Red Rock, OK, The Dichotomous Key, accessed 02/16/01, http://pc65.frontier.osrhe.edu/hs/science/hbotkey.htm

Description: Provides instructions on the two methods of constructing a dichotomous key as well as several online dichotomous keys. Grade level: High School.

Detka, Jon , California State University at Monteray Bay, Designing and Using a Dichotomous Key, accessed 02/16/01, http://www.monterey.edu/students/Students_D-H/detkajon/world/ron/dichotdesign.html

Description: Students first construct a simple dichotomous key and then use a basic key to identify some of the native plants and the most unwanted invasive weeds of California. Grade level: 3-5.

Santa Cruz Productions, Wastewater Filamentous Bacteria Dichotomous Key, accessed 02/16/01, http://home1.gte.net/vsjslsk1/gramstainflowchart.htm

Description: A completely web-based dichotomous key designed to assist students in identifying wastewater bacteria.

Correlations to the Louisiana & the National Science Education Standards

Louisiana Science Frameworks

Middle School Benchmarks

  • SI-M-A1-A7 
  • SI-M-B1-B7

High School Benchmarks

  • SI-H-A1-A6
  • SI-H-B1-B5
  • LS-H-C4,C6

National Science Education Standards

Middle School Standards

  • UC&P- Systems, order & organization
  • LS - Diversity & adaptations of organisms

High School Standards

  • UC&P - Systems, order, & organization

Download the entire lesson (PDF FILE: 2,790K)

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