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        How Evolution Works

        Students learn about natural selection by discussing the evolution of the eye, genetic variation, adaptation, sexual selection, and the Galapagos finches.

        Lesson Summary


        In this three-part lesson, students learn about natural selection, the mechanism that drives evolution. They begin by discussing the evolution of the eye and how even a complex organ can evolve through natural selection. Then they divide into groups to learn about genetic variation, adaptation, and sexual selection and report their findings back to the class. Finally, students analyze data to determine how the beak length of Galápagos finches evolves according to environmental factors.


        • Understand natural selection
        • Learn about the role genetic variation, adaptation, and sexual selection play in natural selection
        • Learn how to interpret graphical data

        Suggested Time

        • Three class periods

        Multimedia Resources

        After the Lesson

        The Lesson

        Part I: Evolution of the Eye

        1. Show the Evolution of the Eye video and have students read the corresponding backgrounder.

        2. Share with your class the following quote from Darwin's On the Origin of Species.

        Organs of Extreme Perfection and Complication
        To suppose that the eye with all its inimitable contrivances for adjusting the focus to different distances, for admitting different amounts of light, and for the correction of spherical and chromatic aberration, could have been formed by natural selection, seems, I freely confess, absurd in the highest degree. When it was first said that the Sun stood still and the world turned round, the common sense of mankind declared the doctrine false; but the old saying of Vox populi, vox Dei, as every philosopher knows, cannot be trusted in science. Reason tells me, that if numerous gradations from a simple and imperfect eye to one complex and perfect can be shown to exist, each grade being useful to its possessor, as is certainly the case; if further, the eye ever varies and the variations be inherited, as is likewise certainly the case; and if such variations should be useful to any animal under changing conditions of life, then the difficulty of believing that a perfect and complex eye could be formed by natural selection, though insuperable by our imagination, should not be considered as subversive of the theory. From Charles Darwin, On the The Origin of Species (New York: The Modern Library, 1993), 227-228.)

        3. Discuss with your class the following topics:

        • Why were people resistant to the idea that the eye evolved through natural selection?
        • What evidence can you give to show that evolution did occur through natural selection? (For example, some animals alive today have simpler stages of the eye; Nilsson's work shows how the evolution could take place through small changes, etc.)
        • Why has the flatworm not evolved a more complex eye with a lens?
        • Why did Darwin compare his theory of natural selection to Copernicus's theory that Earth orbited the Sun?

        4. If time allows, have students visit the Life's Grand Design interactive feature. This illustrated essay by Ken Miller discusses how a complex organism such as the eye could evolve through natural selection and refutes the "intelligent design" concept. This detailed essay provides a good follow-up assignment after the in-class discussion.

        Part II: Components of Natural Selection

        5. Divide the class into three groups. Assign each group a different component of natural selection: genetic variation, adaptation, and sexual selection. (If you have only one computer in your classroom, keep the class as one group and choose one video from each topic to show and discuss.)

        • Have the genetic variation group watch the The Red Queen and A Mutation Story videos and visit the Microbe Clock interactive activity. Tell students to take notes as they view the segments. Then instruct them to write short summaries for homework and reconvene as a group the next day. Ask them to prepare a short presentation for the class about the factors that provide genetic variation (sexual reproduction, mutation), why genetic variation is important for the survival of a species, and how variation makes natural selection possible. Tell students to cite specific examples from the sources they viewed.
        • Have the adaptation group view the Evolution of Camouflage video, the Shape of Trees: The Frustration Principle image, and Mimicry: The Orchid and the Bee and read the corresponding backgrounders. Tell students to take notes on the segments and readings, then write paragraph summaries for homework. Then have them prepare a short presentation for the class about the role adaptation plays in the process of natural selection. Ask them to consider external influences such as predators, competitors for food, and access to mates (or, in the orchid's case, pollinators).
        • Have the sexual selection group view the Tale of the Peacock video, the The Mating Game interactive feature, and the Sweaty T-shirts and Human Mate Choice video. Tell students to take notes while they view the videos and try out the interactive feature, then have them prepare a short presentation for the class about the role sexual selection plays in the process of natural selection. Ask them to consider why male competition and female choice have evolved as a selection means in many species and the importance partner selection plays in genetic diversity.

        Part III: Interpreting the Data

        6. Copy the Finch Beak Data Sheet for your class. Read the backgrounder in class and talk about the importance of the Grants' work. Ask students to identify specific data (from the data sheet) that supports each of the following claims:

        • Some of these variable traits are heritable (passed on to offspring).
        • More offspring are produced than can survive because of limited resources such as food and nesting sites.
        • Individuals with advantageous traits are more likely to survive and reproduce. Print the Answer Key to Darwin's Finches (PDF) for your own reference.

        7. Discuss the following questions as a class:

        • How do you know that finches' beak length is heritable?
        • How did the finch population change from before the drought to after?
        • Why do you think the average beak length of the birds increased?

        8. If time allows, expand your discussion on finches to include the topic of adaptive radiation. Print out the Adaptive Radiation: Darwin's Finches graphic. Talk about how environmental influences on the Galápagos Islands led to the evolution of thirteen distinct species of finches.

        Optional Activity: Sex and the Single Guppy

        Part IV

        9. Have students visit the Sex and the Single Guppy interactive feature and ask them the following questions:

        • What hypotheses did you test?
        • What conclusions did you reach based on your simulation results?
        • How do sexual selection and natural selection interact in this guppy simulation? In nature?
        • What other questions did the guppy simulation raise? How might you test for answers?


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