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        Molecular Evidence for Evolutionary Relationships

        Students learn about the evolutionary relationship between humans and other organisms by comparing amino acid sequences in cytochrome-c from a variety of species.

        Lesson Summary


        In this activity, students learn about the evolutionary relationship between humans and other organisms, such as yeast. They compare the amino acid sequences in cytochrome-c for a variety of organisms and use this information to infer evolutionary relationships. They also learn about the role of homeobox genes in controlling animal body plans and what this indicates about the evolution of animals.


        • Recognize how comparisons of similarities and differences in molecular structure can suggest evolutionary relationships
        • Understand the importance of multiple lines of evidence in identifying evolutionary relationships
        • Understand how animal body plans are dictated by homeobox, master control genes
        • Recognize the evolutionary relationship between organisms based on similar genes

        Suggested Time

        • Two to three class periods

        Multimedia Resources


        Before the Lesson

        The Lesson

        Part I

        1. Show the video The Common Genetic Code and discuss the following:

        • What happened when researcher Paul Nurse added human DNA to mutant yeast cells?
        • What does this indicate about the evolutionary relationship between humans and yeast?

        2. Give students copies of the Molecular Connection Worksheet (PDF). Explain that cytochrome-c, an important enzyme found in virtually all organisms, is needed for the release of energy from food. Tell students they will compare the amino acid sequences for several different animals. From this information they will infer the evolutionary relationships between the animals. Then they will compare this molecular data to a cladogram, a branching diagram of possible evolutionary relationships based on the anatomical structures of the same animals.

        Have students work in pairs to do theMolecular Connection Worksheet (PDF). Then discuss the following:

        • Does the data from the amino acid sequences of different organisms agree with the anatomical data that was used to create the cladogram?
        • Based on the molecular data, how does the human-monkey relationship compare to the duck-chicken relationship?
        • Chickens and turkeys have the same sequence of amino acids in their cytochrome-c protein. How can the two birds have identical cytochrome-c and still be different species?
        • What line of evidence besides molecular and anatomical evidence might you consider in determining evolutionary relationships?

        3. Optional: Introduce the Chromosome Clues activity as a way to infer evolutionary relationships by comparing chromosomes. Give students a copy of theChromosome Clues Worksheet (PDF), and use the teacher's directions (Chromosome Clues Handout (PDF)) to guide the activity.

        4. Show the Genetic Tool Kit video. Then have students read Animal Body Plans: Homeobox Genes (Optional: Have students color the diagram following the directions, or make an overhead transparency of the diagram and color that.) Discuss the following:

        • What do homeobox genes code for in animals?
        • Why are homeobox (also called homeotic) genes called the "master control genes" or "master switches"?
        • What other organisms besides fruit flies have homeobox genes?
        • What does the presence of related groups of homeobox genes in fruit flies and mice indicate about their evolution?
        • How does Saint-Hilaire's "ridiculous" idea that vertebrates have the body plan of an upside-down insect have a sound basis in evolutionary genetics?
        • What do homeobox genes suggest about the evolution of different eyes in different organisms?

        5. To summarize, have teams of students write two thoughtful questions each about molecular evidence and evolutionary relationships. Collect the questions and hand them out to different teams to answer and report back to the class.


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