In this lesson, students learn about the coevolution of species. They watch a video about the leafcutter ant, whose evolution is influenced by not one, but three other species. Then students try out the Coral Reef Connections Web activity and explore the different types of symbiotic relationships that exist between species (commensalism, mutualism, predator and prey, and competition). After learning about how different species influence one another's evolution, students study how humans influence the evolution of bacteria through the development of antibiotics.
- Understand coevolution
- Explore the four types of symbiotic relationships between organisms
- Learn about antibiotic resistance
- Two to three class periods
- Ancient Farmers of the Amazon QuickTime Video
- Toxic Newts QuickTime Video
- Coral Reef Connections Flash Interactive
- Microbe Clock Flash Interactive
- Evolving Ideas: Why Does Evolution Matter Now? QuickTime Video
- Complex Relations HTML Document
Before the Lesson
- Review natural selection
Part I: Introduction to Coevolution
1. Have students watch the Ancient Farmers of the Amazon video.
2. Ask students to draw a picture that includes the ants, fungal gardens, mold, and bacteria. Tell them to label their drawings and add arrows to show how the four organisms interact.
3. Discuss with your class how this is an example of the symbiosis of four organisms. Ask:
- What is the relationship between the ants and the fungus? How does this relationship influence the way they evolve?
- What is the relationship between the ants and the mold? What kind of evolution does this initiate? (evolutionary arms race)
- Why was Cameron Currie's discovery of the bacteria on the ants' bodies so amazing? (Ants were using antibiotics long before humans.)
Ask students to think of other examples of species that have coevolved.
Part II: Coral Reef Connections
4. Divide your class into four groups. Ask each group to visit theCoral Reef Connections interactive and read the introduction and the instructions. Assign each group a relationship: commensalism, mutualism, predator and prey, or competition.
5. Instruct each group to identify all the species that are in the category they are studying and to take notes on how they interrelate. Make sure students visit all four reef zones.
6. Have each group draw a representative member of each species and cut them out. Divide the chalkboard into four segments, one for each reef zone. Then have students come forward, tape their organisms to the board, and draw lines to show the organisms' connection to members of other species. Tell them to describe the relationships, briefly, to their classmates. (Note: Some species fall into more than one category.)
7. Choose some organisms at random and ask students to name the other species that influence each organism's evolution. Then ask students to describe the types of relationships each organism has with the other species and how students expect the organism to evolve.
Part III: Evolutionary Arms Race
8. Have students view the Toxic Newts video and read the backgrounder. Discuss the following questions with your class:
- Why has the newt developed a level of toxicity much higher than that necessary to kill most predators?
- What is the selective pressure on the newt?
- What is the trade-off for the snake in developing greater resistance to toxicity?
Like the garter snake, which develops resistance to the increasingly toxic newt, bacteria evolve resistant strains that can live in the presence of antibiotics. Since bacteria are single-celled organisms that reproduce quickly, they evolve much more rapidly than more complex organisms. This isn't true coevolution, since the antibiotics do not evolve naturally. But one can compare the parallel evolution of a natural organism -- bacteria -- with the evolution of modern medicine. Have your students visit the Microbe Clock Web activity to see the alarming rate at which bacteria evolve.
9. Have students view the video titledEvolving Ideas: Why Does Evolution Matter Now?. Discuss in class the following questions:
- How do humans influence the evolution of microbes?
- How does natural selection work to produce multi-drug-resistant tuberculosis?
- Why is understanding evolution especially important today, now that people can travel easily?
10. Ask the students to apply what they have learned about antibiotic resistance in the Microbe Clock interactive and the Evolving Ideas: Why Does Evolution Matter Now? video to the recent anthrax attacks. Have them conduct research on the Web and in newspapers and magazines to learn more about anthrax. (They can visit the NOVA Web site at http://www.pbs.org/wgbh/nova/bioterror/ .) Then have them write a short paper giving their opinion about how the government should handle bioterrorism threats. Students should use the following questions to guide their research:
- Why is it important to make sure patients take the full course of antibiotics prescribed to them?
- Why shouldn't everyone take Cipro as a preventive measure?
- How long does it take to develop an antibiotic? How long do bacteria take to become resistant?
- How does the anthrax vaccine work?
11. Students who want to learn more about coevolution may wish to read an excerpt from Darwin's On the Origin of Species. Have them list examples Darwin gives of commensalism, mutualism, predator and prey, or competition.