Paleobio Home Cassit Birds: Lessons on Genes, Inheritance,and Evolution
Lesson Four

Written by Jennifer A. Collins
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Concept: Natural selection acts on phenotypes and cause genetic and phenotypic changes in a population over time.

Overview: Students use their Cassit population to simulate how natural selection can affect a population over time by "killing off" birds with particular features so that they can no longer pass on their genetic material. Students discuss how this can lead to speciation/evolution.

Time:40-60 minutes

Grouping:partners, class

Vocabulary: natural selection, evolution

Advance Preparation:


  • Cassit bird population.
  • Cassit Bird Data Table and graph

    Procedure: 1. Explain to students that they are going to simulate a situation where certain features were selected for in a population. For example, explain to students that for some reason, birds with short/fat beaks were unable to get very much food. Ask students to hypothesize reasons why this situation would exist.

    2. Have all students with short fat beak birds stand-up. Select over 50% of those birds to be eliminated from the next mating. They should still hold onto their birds for another simulation.

    3. Repeat the mating process and record data so that students can see how such an event would affect the overall population. You can repeat this as many times as you see necessary to show how the population changes.

    4. You can repeat this process to show how the population could change due to other factors.

    Sample Scenarios:

  • Affects of predator/prey interactions: Tell students that birds with more (or less) spots are better camouflaged from predators. How will the population change over time?
  • Affects of population isolation: Tell students that a small group of birds gets blown off to an island (select a few birds from the class). What will the population look like compared to that on the mainland?
  • Affects of introduced species: Tell students that their bird population is on an island and a new predator (domestic cats) are introduced.

    5. Ask students the following questions:

  • How could such events lead to speciation/new species?
  • How does this explain how evolution occurs?
  • How does extinction of a group of organisms affect a population?

    Alternative Ideas:

  • Use the Bird Beak Game to demonstrate how birds with different beak shapes can take advantage of different food sources and how this could affect a population over time.
  • Compare the data generated by different classes to show variation between populations.
  • Have different classes experience different selection pressure and compare the changes in populations.

  • ©   Updated: June 2003   Contact: jen-AT-paleobio or allen-AT-paleobio