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How Evolution Works

Students learn about natural selection, the mechanism that drives evolution.


Lesson Summary


In this lesson, students learn about natural selection, the mechanism that drives evolution. They begin by viewing short videos about the evolution of physical and behavioral adaptations in various organisms. This is followed by a discussion of the natural selection mechanism, which has, over many generations, given rise to these and other extraordinary traits. Students then watch a video about peacocks that explores the interplay between two evolutionary imperatives: the need to survive and the need to reproduce. Lastly, students pair up to perform experiments with simulated populations of guppies, another species whose evolution is driven by the interplay between the forces of predation and sexual selection.


  • Explore and begin to understand the meaning of natural selection
  • Explore and begin to understand the meaning of sexual selection
  • Discover the role that genetic variation, adaptation, and sexual selection play in natural selection
  • Learn some experimental techniques and how to analyze experimental results
  • Understand that there is an interplay between different selection factors and that these factors sometimes have competing outcomes

Suggested Time

  • 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: A Pressure-Filled World

1. Show the Hummingbird Species in the Transitional Zones video and discuss the following:

  • What is the relationship between the length of the hummingbirds' beaks and the flowers from which they feed?
  • If the size and shape of the flowers available to a group of hummingbirds were to change dramatically over a short period of time, would individual hummingbirds have the ability to change the size and shape of their beaks to adapt?
  • Although individual members of a species may look very much alike, there is often a great deal of variation among them. What role might this variation play in natural selection and evolution?

2. Show the Evolution of Camouflage video and discuss the following:

  • What is the praying mantis's strategy for self-defense?
  • The mantis is nearly invisible sitting on a leaf in the forest, but when the scientist places the insect on his blue shirt, it becomes very obvious. What does this suggest about how well this species of mantis would survive in a different environment -- a desert or a short-grass prairie, for example?
  • Would an individual mantis be able to transform its appearance if it were placed in another type of environment? Why or why not?
  • If the forest were to dry out and turn to grassland, or if this species of mantis began to expand its range to an area with fewer trees, what might happen to the species over time?

3. Show the Floral Arrangements video and discuss the following:

  • Some plant species rely on the wind for pollination. What other pollination strategies do plants use?
  • Insects are often critical to a plant species' survival. What reward do plants usually offer in return for the insects' services?
  • The orchid in the video promises a different kind of payoff. What is that false promise?

Part II: Competing Pressures

4. Show the Tale of the Peacock video and discuss the following:

  • From the research discussed in the video, what appears to be the most important factor in a male peacock's ability to attract mates and successfully reproduce?
  • What happened when the scientists altered the peacocks' tails by cutting them short?
  • Aside from providing the egg, what role do peahens play in the reproductive success or failure of a would-be mate?
  • How might this trait have begun, and how might it have evolved over millions of years?

5. Lead students through the introductory screen and the section titled "Read about Endler's discovery" of the Sex and the Single Guppy Web activity. During this time, remind students that scientific research begins with systematic observations, followed by the formulation of a hypothesis that explains the observations, and then experimental testing of the hypothesis. Explain to students that they are going to follow these same steps in search of a scientific explanation for the differences in coloration among different guppy populations.

6. After reading and discussing the two introductory sections listed above, ask students to work in pairs and begin their investigation of guppies and their habitat. One student should use the computer while the other takes notes on what they both observe. Recommend that students spend time clicking on the pools on the "Endler's discovery" screen to learn more about the conditions in each. Students can also spend time in the Guppy Gallery to learn more about the guppies, their predators, and real-life stream habitats. At the end of the observation period, students should consider the given hypotheses -- and any they may have come up with on their own -- and choose the one they think best explains the differences in guppy coloration. Ask students to record their chosen hypothesis.

7. When students are finished with their preliminary observations, tell them to trade places. Ask students to set up their first experimental simulation by selecting the distribution of each guppy color type as well as the number and type of predators that their experimental pool will have. Recommend that students run the simulation for 10 to 15 guppy generations to yield observable results. Then ask students to run three or four more simulations and experiment with different sets of variables to test their hypothesis. Students should record and/or print out their results.

8. Following the experiments, have students consider the following questions and present their answers to the class.

  • What hypothesis did you test?
  • What conclusions did you reach based on your simulation results?
  • How do sexual selection and predation interact in this guppy simulation? How might this interaction differ in a natural environment?
  • What other questions did the guppy simulation raise? How might you test for answers?

Check for Understanding

Have students discuss the following:

  • Explain the statement that predation and sexual selection pressures have competing outcomes. Give examples from the media you've seen.
  • What would likely result from a sudden, dramatic change in the praying mantis's environment?
  • How might the outcome of a more gradual change -- one that takes place over thousands or millions of years -- differ from the outcome of a sudden change?


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