Sex and the Single Guppy:
Simulation of Natural Selection and Sexual Selection

Course: ISB 202: Applications of Environmental Science and Organismal Biology

Level: lower-level undergraduate non-science majors

Class Size: 180

Simulation Source: PBS Evolution Site

Annotation by: Diane Ebert-May and Debra Linton

Keywords: evolution; natural selection; sexual selection

Description: Provide a brief summary of and background for the inquiry-based activity (~100 words).

This inquiry is designed to help students understand the concepts of natural selection, sexual selection, and fitness. Included in the activity is a simulation based on John Endler's work with guppy populations in Trinidad [PBS Evolution website]. Endler found that male guppies are brightly colored in streams with no/few predators, but are drably colored in streams with many predators. In this activity, students create a testable hypothesis and design an experiment based on these observations. The experiments are run as simulations on the website, and students collect and analyze data and discuss the results in terms of natural selection, sexual selection, and fitness.

Objectives: Each of the student learning objectives must have a corresponding assessment.

Students will be able to...

  1. Design a testable hypothesis
  2. Analyze and interpret a simulation about an ecological problem
  3. Explain how evolution really works

Instructional Design: How did you implement the inquiry in class? What is the sequence of events? Roles of teacher and students?

  1. Introduce the question - details on PBS website
  2. Students
    • In your groups, develop a testable hypothesis to explain the observed pattern of fish coloration in the pools.
    • Design an experiment to test your hypothesis.
  3. Report out
    • Selected groups report their hypothesis
    • Class chooses a hypothesis to focus on
  4. Set up simulation (e. g. start with even mix of guppy coloration and few predators)
    • Students make prediction prior to simulation
  5. Run simulation
    • students collect data / or instructor provides
    • In large classes it may be difficult for students to see the actual numbers on the simulation. You may need to gather the data before class and provide a data table to the students.)
  6. Set up second simulation (e. g. even mix of guppy coloration and many predators)
    • Students make predication prior to simulation
  7. Run simulation - students collect data / or instructor provides
  8. Students plot data
  9. Questions for group discussion
    • Do these data match your prediction (support your hypothesis)? Why or why not?
    • What determines the color pattern of an individual guppy?
    • What role(s) does color play in guppy survival and reproduction?
    • Explain the results in terms of fitness and natural selection. Describe how sexual selection and natural selection push in opposite directions.
  10. Processing
    • Instructor facilitates whole class discussion of the questions.


  1. Students write a testable hypothesis - must include dependent and independent variables (in class on carbonless paper)
  2. In class in carbonless paper notebooks - students plot data, write answers to discussion questions, and random groups report out
  3. Exam - Extended responses, open-ended questions and multiple choice questions from the Conceptual Inventory of Natural Selection
    • Anderson DL, Fisher KM, Norman GJ. 2002. Development and Evaluation of the Conceptual Inventory of Natural Selection. Journal of Research in Science Teaching 39(10): 952-978


In guppy populations, what are the primary changes that occur gradually over time?

  1. The traits of each individual guppy within a population gradually change.
  2. The proportions of guppies having different traits within a population change.
  3. Successful behaviors learned by certain guppies are passed on to offspring.
  4. Mutations occur to meet the needs of the guppies as the environment changes.



Primary Sources:

Endler JA. 1980. Natural selection on color pattern in Poecilia reticulata. Evolution 34(1): 76-91.

Endler JA. 1978. A predator's view of animal color patterns. Evolutionary Biology 11: 319-364.


Secondary Sources:

PBS. Online lessons for students: learning evolution. Lesson 4: How does evolution work? Activity 2: Flashy fish. Retrieved 5 April 2003. <>