About the Activity

In the last activity, Morphological Evidence, students focused on how scientists use structural evidence to determine evolutionary relationships among organisms. Students realized that although structural evidence is important, it can also be misleading. In this activity, students learn how scientists use molecular evidence to confirm, or more accurately determine, evolutionary relationships. Students explore evidence from a variety of sources in order to answer the question: Are flightless birds an example of convergent or divergent evolution? In the next activity, Representing Relationships, students will examine shared characteristics and use a Tree of Life based upon morphological, fossil, and molecular evidence to illustrate evolutionary relationships among all living things.

Learning Objectives

After completing this activity, students will be able to:

  • Use protein sequences to determine evolutionary relationships.
  • Explain the variety of methods used by scientists to determine evolutionary relationships.
  • Discuss the importance of using multiple lines of evidence to support scientific theories.

During the Activity

Activity Sequence in Brief

    Day One

    Engage
    Students review convergent and divergent evolution and the pitfalls of morphological evidence in determining evolutionary relationships.

    Explore
    Students analyze morphological, fossil, and geological evidence for ratite birds.

    Explain
    Student teams present arguments supporting their opinions about the evolution of ratite birds.

    Evaluate
    Students write arguments for whether they believe the evidence supports the recent or distant ancestry of ratite birds.

    Day Two

    Engage
    Students review morphological, fossil, and geological evidence for the evolution of ratite birds.

    Explore
    Students compare protein sequences for amphibians.

    Explain
    Students analyze their data for the protein sequences of amphibians.

    Explore
    Students apply what they have learned about molecular evidence to ratite birds.

    Explain
    Students review all the evidence to conclude that ratite birds are closely related.

    Evaluate
    Students write explanations for how molecular evidence supports the argument that ratites are closely related.

Day One

Engage (5 minutes)

  1. Review the previous activity. Ask students for examples and/or definitions of convergence and divergence. (Convergent evolution = distant ancestry, similar morphology in response to similar environmental pressures. Divergent evolution = recent common ancestor, different morphology in response to differing environmental pressures.) Ask students to discuss the pitfalls of using structural (morphological) evidence to determine evolutionary relationships. (Structural comparisons can be helpful, but they can also be misleading since unrelated organisms may have similar form due to environmental pressures, and other related organisms may have very different forms due to living in different environments.)
  2. Have students turn in their Patterns of Form and Function Student Activity Sheet from the previous day, including their answers to the final two questions on the activity sheet, which they completed for homework.

Explore (30 minutes)

  1. Tell students that today and the following class day they will explore several kinds of evidence in order to determine the evolutionary relationship among a group of birds called ratites.
  2. On the computer, display the image: Ratite Bird Distribution. Ask students what they know about ostriches. (They are relatively large, heavy bodied, flightless birds.) Explain that these characteristics are typical of a group of birds called ratites. The map in this image shows the distribution of ratite birds and pictures of present-day species. Give students an opportunity to look at the images of the birds and their present locations. Have students point out the countries and bird names. Explain that two other types of ratite birds, the Moa and the Elephant Bird, are now extinct. Moas lived in New Zealand and Elephant Birds lived on the island of Madagascar.
  3. Ask students to describe some of the obvious similar features of the birds. (They all have heavy bodies, wings too small for flying, thick legs and larger feet, small heads, and small brains.) Ask them to describe some of the differences they observe in the birds. (They are different in color, length of neck, and length of legs.)
  4. Can students identify any commonalties about the location of these birds? (They live mostly in the Southern Hemisphere.) What is the climate of the areas in which these birds live? (Warm.)
  5. Explain that from examining the map, two questions come to mind:  (a) How closely are these birds related? and (b) How did they get to the locations in which they are currently living?
  6. Ask students to suggest explanations that could answer these questions. After guiding them to the following two options, write them on the board:  
    • The birds have a relatively recent common ancestor, but look different because they have undergone changes through time in response to local environments. The birds are examples of divergent evolution.

      OR
    • The birds are unrelated, but have developed similar characteristics as a result of similar environmental pressures. The birds are examples of convergent evolution.
  7. Ask students to decide which of the two options they think provides the best explanation. Next to each option on the chalkboard, write the number of students who support it.
  8. What types of evidence might scientists need to examine in order to determine the best explanation? (Morphological, geological, dating techniques, fossil evidence, examination of current species, molecular evidence.)
  9. Explain that in this activity, students will be using the same types of evidence that scientists would use to answer questions about ratite bird ancestry. They will begin with morphological and geological evidence.
  10. Display the image: Ratite Birds Questions. Tell students that they will use several fact sheets to try and answer these two questions.
  11. Divide students into teams of three or four. Distribute the first four pages of the Ratite Birds Fact Sheets, which provide morphological, fossil, and geological evidence. Do not distribute the fifth page, on molecular evidence, of the Ratite Birds Fact Sheets at this point.
  12. Tell students that they have 20 minutes to develop their answers to the two projected questions. Ask them to be prepared with an argument for whether they believe the ratite birds are closely or distantly related. Instruct students to write their ideas and arguments on their own paper, not on the fact sheets.

Explain (15 minutes)

  1. Bring the teams together as a class. Have students present and discuss their arguments. (Typical arguments might be that the fossil and geological evidence does not support divergent evolution. The oldest fossils for ratite birds are about 60 million years old and were discovered in South America. Assuming that South America is where these birds originated, the continents had already separated by 65 mya [although South America was still attached to Australia through Antarctica]. From a morphological perspective the skeletons and descriptions of extant birds suggest they are related. Expect conflicting views.)
  2. Ask if any students have changed their opinions about whether ratite birds are closely or distantly related after looking at this evidence. Write the new numbers of how many students support each option on the chalkboard below the previous numbers.

Evaluate

  1. For homework, ask students to use their notes to write a summary of their argument for whether they believe the evidence supports the idea that ratites have a recent common ancestor or are only distantly related.

Day Two

Engage (5 minutes)

  1. Ask students to turn in their homework from the previous day.
  2. Review the previous day's activities with students: What types of evidence did they examine? (Morphological, fossil, and geological evidence.) What question were they trying to answer? (Ratite bird ancestry.)
  3. Display the image: Fossil and Geological Evidence. As a class, review the evidence in this image. Do students think the evidence supports a recent common ancestor originating in South America, or not? Why or why not? (Students will probably have conflicting answers. Some students may say yes, arguing that the oldest fossil was found in South America. Others may say no, arguing that by 65 mya South America and Africa were separated - and by what route could a flightless bird travel to get to Africa?)
  4. Point out that disagreement is common within the scientific community. Debates are often settled by gathering more evidence. Explain that, in order to help settle the question, students now will examine another type of evidence to help them determine the ancestry of ratite birds.

Explore (15 minutes)

  1. Explain that in addition to fossil, geological, and morphological evidence, scientists use molecular evidence to determine evolutionary relationships. Tell students that in the following activity, they will learn how to analyze molecular evidence. They will then apply this skill to the question of whether ratite birds have a recent common ancestor.
  2. Divide the class into teams of three or four students. Distribute a copy of the Amphibian Protein Student Activity Sheet to each student. Explain that students will examine the analysis of a protein sampled from several frogs, a toad, and a salamander. The protein is called cytochrome b, and it is important in determining how cells use oxygen. (Note: Make sure that students understand that the letters in these sequences represent amino acids, not bases in DNA. You may wish to remind students that have studied protein synthesis that three bases code for an amino acid.)

    EXTENSION: To provide background or review on the names, one letter codes, and molecular structures of the 20 amino acids, display the image: Amino Acid Table.
  3. Ask a volunteer to read the directions on the activity sheet. Make sure students understand the task, and then tell them that they have 10 minutes to complete the activity sheet. Instruct the team members to divide up the task so that each team member compares about two sequences.

Explain (5 minutes)

  1. Display the transparency: Sequence Comparison: Frogs, Toad, and Salamander. Ask volunteers to report on their team's results. Determine which results are accurate and write the data on the transparency.
  2. Ask students which organisms had the most differences from Frog A. (The toad and the salamander.)
  3. Ask students what they think it means when there are few differences between the cytochrome b sequences in two amphibian species? (It suggests that the organisms are closely related.)

Explore (10 minutes)

  1. Have students meet in their teams from the previous day. Redistribute the first four pages of the Ratite Birds Fact Sheets, and now also distribute the last page on molecular evidence. Explain that the table on this last page represents comparisons of the differences among the cytochrome b sequences of present-day ratite birds, as well as the alligator.
  2. Once again, display the image: Ratite Birds Questions. Ask students to examine the molecular evidence on the fact sheet and make a determination as to whether the molecular evidence supports a recent common ancestor (divergent evolution) or whether it supports a more distant common ancestor (convergent evolution).

Explain (15 minutes)

  1. Ask students to discuss their conclusions. Did the molecular evidence cause any students to change their original arguments? Why or why not? After discussion, explain that the molecular evidence strongly supports a recent common ancestor.
  2. Some students might still have concluded, even with the molecular evidence, that ratite birds are not closely related. Ask these students what additional evidence might help reconcile the two conflicting lines of evidence? If a prompt is necessary, ask how newly discovered fossils might change the story. (One obvious answer is that the crucial fossils of ratite birds from more than 60 million years ago may not be discovered yet. If older fossils were found in South America, then the existence of ratite ancestors could be documented prior to the breakup of the continents.)
  3. On the computer, display the image: Cladograms. Explain that in order to understand relationships among organisms, scientists use several sources: they compare amino acid sequences in proteins (as the students just did), RNA sequences from ribosomes, and different parts of the DNA molecule. Many scientists also incorporate morphological information that adds to their understanding about relationships among fossils and present-day animals. Explain that this image depicts two possible interpretations of the different types of data for ratite birds, and hypothesizes evolutionary relationships among the birds.
  4. Ask students to describe how these two cladograms are different and what they each say about the evolution of the ratite birds. (The molecular evidence suggests that the flightless birds shared a common ancestor with the tinamou [a weakly flying bird], and that the emu and cassowary share a more recent common ancestor than do the emu and the kiwis or emu and rhea, for example. The fossil and geological evidence suggest the emu, cassowary, and kiwi are more closely related to each other and more distantly related to the rhea and ostrich.)
  5. Emphasize to students that when answering complex questions about the nature of evolutionary relationships, it is best to use more than one line of evidence. Biologists use a combination of molecular, morphological, geological, paleontological, and/or biochemical evidence to arrive at a conclusion.

Evaluate

  1. For homework, have students interpret the molecular evidence for ratite birds.  Ask them to write a paragraph explaining how the number of differences in amino acid sequences of cytochrome b among ratite birds is evidence for divergent evolution. How did the cytochrome b sequences for the tinamou and alligator help to confirm this conclusion?

Materials

Preparation

For Each Student

  • None

For Each Student Team

  • Scissors

For Teacher

Student Handouts

Student Reader Articles

  • None

Media
  1. Prepare any necessary handouts and transparencies. Familiarize yourself with the media. For background information on the topics covered in this activity, review "The Science & Resources" section (accessed from the menu bar above).
  2. If you are having students turn in their scavenger hunt organisms (from Activity 1.1) within these two class periods, set up 40 boxes or "areas" in your classroom (numbered 1-40) for them to deposit their samples. The next activity, 5.4 Representing Relationships relies on these samples.


NOTE: All Teacher CD-ROMs in the complete Voyages Through Time curriculum (not this SAMPLE) provide both MS WORD and PDF versions of items such as student activity sheets and tests. In the complete VTT curriculum, teachers may use MS WORD (or other word processor) to modify any of the printable items if they wish to do so.