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        Evolutionary Arms Race

        Students examine the interactions among different types of organisms and the importance of these relationships to the evolution of species.

        Lesson Summary


        This lesson explores the connection between an organism's behavioral and physical traits and its environment. In particular, students examine the interactions among different types of organisms and the importance of these relationships to the evolution of species. Students begin the activity by discussing adaptations that have evolved from generations of predator-prey interactions. Next, they investigate a classic evolutionary "arms race" between two species: newts and garter snakes. Then they explore the complex symbiotic relationship between leaf-cutter ants, the fungus they grow for food, and the bacteria they employ to protect their crop from mold. Finally, they go online and explore a virtual coral reef and document the different types of relationships they find among the organisms that live there.


        • Recognize that a connection exists between an organism's physical and behavioral traits and its environment
        • Understand that an important part of an organism's environment are the other organisms that live there
        • Explore and define different types of interactions that can occur between organisms
        • Observe that there are variations among individuals in a population and that, depending on environmental conditions, these variations may confer advantages or disadvantages
        • Discover that environmental pressure can select for some individual organisms in a population and against others, depending on the traits those individuals possess.
        • Consider that humans can play an important role in an organism's environment

        Suggested Time

        • Two to three class periods


        Before the Lesson

        Read the background essay that accompanies each resource to gain information that will help you facilitate class discussion.

        The Lesson

        Part I: Life-Changing Relationships

        1. Show the Animal Defenses and Masters of Disguise videos and discuss the following:

        • What are some of the defenses animals use to avoid being eaten by predators?
        • What are some of the defenses that plants have to keep from being eaten?

        2. Explain to students that some differences among individuals in a population -- variations that can be passed from one generation to the next -- provide survival and/or reproductive advantages under certain environmental conditions. In other words, variation in a population means that some individuals have a better chance than others of surviving and reproducing. Ask students:

        • How might some of the defense strategies in the videos have evolved?
        • How do species that lack any type of defense strategy continue to exist?

        3. Show the Toxic Newts video and discuss the following:

        • What do the rough-skinned newts in the video use to protect themselves from predators?
        • What has caused the poison in the skin of the newts to become increasingly toxic? (the snakes' growing resistance to the newts' toxicity)
        • What might happen if the garter snakes' resistance failed to keep pace with the toxicity of the newts? (The snakes, at least those that eat newts, may eventually go extinct.)
        • What keeps the newts from winning the arms race? (The newts' toxicity grows stronger very gradually, which allows the snakes' resistance to keep pace.)

        4. Show the Ancient Farmers of the Amazon video and discuss the following:

        • What do leaf-cutter ants grow in their colonies?
        • Much like human farmers, ant farmers must deal with a pest that threatens their crops. What is this pest?
        • It was long thought that leaf-cutter ants meticulously "weeded" the pests from their crops. What did one student discover on the ants themselves that challenged this belief? (bacteria that produce an anti-mold chemical)
        • Why hasn't the mold that infests the leaf-cutter ants' crops simply grown resistant to the antibiotics produced by the ants' bacteria? (The mold probably has evolved resistance. However, the bacteria have also continued to evolve.)

        Part II: Exploring Interactions on the Great Barrier Reef

        5. Some of the most important types of relationships that can occur between organisms include predation, parasitism, competition, commensalism, and mutualism. Discuss each one and provide examples, either from the videos listed above or from other sources, including books and the Internet. Suggest to students that these types of interactions can be as important to the evolution of species as the physical environment.

        6. Have students work in pairs to explore theCoral Reef Connections Web activity. Tell students to search for an example for each type of relationship that can occur between species: predation, parasitism, competition, commensalism, and mutualism. Have students take turns exploring and taking notes. In their notes ask them to describe the type of relationship, the organisms involved, and the evolutionary effect the relationship might have on one of the two species. Then have the student pairs present their observations to the class.

        Check for Understanding

        Ask students to choose four organisms from any of the four videos or the Web activity, and describe the relationship of each organism to another organism in its environment. Tell students to describe the adaptations -- physical or behavioral traits -- that help each organism survive there.


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