Although most middle school students are familiar with a variety of classification systems, they may not have considered that these "human constructs" can help us identify similarities and differences between organisms. In this lesson, students learn how classification schemes are used to illustrate the relationships among organisms and, ultimately, the unity of life. They begin by classifying an assortment of organisms, using their own criteria, then explaining why they created the categories they did. Next, students explore developmental and genetic similarities among various life forms -- similarities that support the theory of common ancestry. Lastly, students explore a Web activity to learn more about the kinds of evidence used to determine evolutionary relationships and to apply their understanding of classification systems.
- Develop an awareness of the similarities and differences between organisms
- Discover developmental and genetic similarities among life forms that suggest a common ancestry and the unity of life
- Gain an understanding of classification systems and the evidence used to create them
- Two class periods
- The Common Genetic Code QuickTime Video
- Common Past, Different Paths QuickTime Video
- All in the Family Flash Interactive
- Samples of organisms or living material for students to examine
Before the Lesson
- Read the background essay that accompanies each resource to gain information that will help you facilitate class discussion.
- Gather a variety of living things, such as leaves, seeds, mushrooms, snails, and preserved specimens. Include samples or photographs of organisms from all five kingdoms. (Ten to 20 specimens for each pair of students)
- Alternatively, you can ask students to bring organisms from home or have them collect specimens in the field. If the latter, provide plastic bags or plastic jars to transport materials. Have students label their containers with the name of the organism inside (if known) and where they found it. Supplement the students' samples with preserved materials, classroom cultures, slides, or photographs.
1. Explain to students that humans rely on classification systems to bring order to many everyday activities. For example, imagine how difficult it would be to grocery shop if the items were placed randomly throughout the store. Instead, grocery store items are grouped according to their similarities. In other words, they are categorized. Explain that scientists categorize organisms in the same way.
Have students work in pairs, and provide each pair with 10 to 20 of the collected specimens. Ask students to group their specimens according to similarities they observe. Tell them that they can use whatever criteria they want but that they must be able to explain to the class why they grouped the specimens as they did.
During the discussion of each pair's categorization, ask them to consider the following:
- What similarities exist among the organisms within each category?
- What differences exist between different categories of organisms?
- What can be learned about organisms by classifying them?
- Are some organisms in a particular group more similar to each other than to any other organism in that group? Might these organisms form a subgroup?
2. Have students watch the The Common Genetic Code video, which illustrates the genetic similarity among all living things. The video highlights the research of scientist Paul Nurse, who demonstrated that a human gene could function in a yeast cell. Discuss the following:
- What happened when researcher Paul Nurse added human DNA to mutant yeast cells?
- If human genes can repair yeast cells that have lost their ability to replicate, what does that tell us about the two organisms?
- What does it mean to "ronserve" a gene?
- What are "junk genes"?
- Because we share the gene pool with "yeast and the lowly worm," Paul Nurse believes that we should have greater respect for all living things. Do you agree? Why or why not?
3. Show the Common Past, Different Paths video and discuss the following:
- How do the similarities in the developing embryos support the theory of common ancestry?
- What might the developmental similarities seen in the video suggest about the genetic similarities among these animals?
4. Have students work in pairs as they explore the All in the Family Web activity. In this activity they learn how different types of evidence are used to determine evolutionary relationships. Out of a group of three animals, students choose the two they think are most closely related. They can base their choice on appearance, anatomical similarities, developmental morphology, or molecular evidence. As they discover, looks can sometimes be deceiving.
Instruct student pairs to make classification decisions only after discussing reasons for and against the other options. Afterward, have students go back and dig for more evidence to support (or correct) their answers. Remind students that this is the same process scientists go through: forming a hypothesis on the basis of available evidence, then refining or affirming their hypothesis based on the discovery of new evidence.
5. Finish the lesson by discussing the concept of unity among all organisms. Stress the following points:
- All organisms, even those that are distantly related, share many of the same genes -- genes that scientists can now transfer from one organism to another, where they produce the same effect as in the original organism.
- As a result of these shared genes, organisms share many physiological, chemical, and behavioral traits, as well as many developmental processes. These similarities can be observed to varying degrees among all organisms.
Check for Understanding
Have students discuss the following:
- How do classification schemes help to illustrate the unity of life?
- Although they diverged about a billion years ago, humans and yeast cells still share thousands of genes. What could explain this genetic relatedness?
- Given that humans and chickens lack tails when they are born or hatch, what explains the presence of tails on the embryos of these organisms?