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Teacher professional development and classroom resources across the curriculum

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Life Science: Field Population System

Assessing Variation

Almost all natural populations — plants, animals, and others — can be observed to vary in numerous ways. In “Assessing Variation,” you’ll observe how individuals in a population of Brassica rapa plants vary from one another by making observations that start with sprouting.

This population has been carefully composed of Fast Plants and the purple top white globe turnip — two varieties of Brassica rapa. A population of lettuce plants has also been included as a representative of a different species so that additional comparisons can be made. To assess variation, specific traits are observed, described, and measured, or quantified.

“Assessing Variation” was designed to be followed by “Selective Herbivory.” For this reason, we suggest you set up three Field Population Systems that can be used for both. However, you can do “Assessing Variation” with one system, and you can also use any seed type and follow the resulting plant population through an entire life cycle undisturbed.

Note: For Assessing Variation we planted the three seed types separately so that variation in the resulting plants would be easier to observe in photographs. We set up three different systems mixing the seeds as described for “Selective Herbivory.”

Materials Needed


  1. Start this activity with the timing of sprouting seeds. This is one trait that is likely to vary among individuals in the population.
  2. Use your “Assessing Variation” Data Sheet to track the growth of the plants over time. You’ll be looking at numerous traits that are likely to vary, and describing and quantifying each one.
  3. If you’re using these Field Population Systems for “Selective Herbivory,” you’ll be introducing butterfly larvae at about Day 10 after sprouting.

Activity Questions

Before the study period begins

  1. Which traits of a plant do you predict might vary during a life cycle?
  2. What do you need to do in order to assess variation in these traits?

After the study period ends

  1. Which of the traits that you made predictions about actually varied among the plants you observed?
  2. What was the range of variation in these traits?
  3. For which traits do you think there is a genetic basis for variation? An environmental basis? Explain your answer.
  4. Were there any other traits that varied? What were they and how did they vary?
  5. What might be the advantage for individuals possessing certain variations of a trait (e.g., lowest or highest values)? Pick one example and discuss.
  6. For at least one trait that varied, describe a scenario by which natural selection might occur, causing the population to evolve.

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