Skip to main content Skip to main content

Essential Science for Teachers: Life Science

Bottle Biology | Bottle Basics Field Population System

The Field Population System

The Life Science Field Population System is designed to demonstrate concepts that are fundamental to understanding biological evolution. It is a companion to the videos for Session 5 (Variation, Adaptation, and Natural Selection) and Session 6 (Evolution and the Tree of Life).

The name is derived from the system that it is meant to simulate: a field in which one or more plant populations grow. In this type of natural setting, the individuals within such populations would be expected to vary in numerous ways. In the face of myriad factors that affect survival, some variants would be expected to survive better than others do. Through natural selection, these variants would be likely to leave proportionately more offspring. If this continued over many generations, the population would“evolve”— that is, the successful variants would become more common.

Life Science has suggested two activities to illustrate how these concepts are connected. The first, “Assessing Variation,” sets the stage for the second, “Selective Herbivory.” In the first activity, Fast Plants and turnips—both members of the same Brassica rapa species—are observed for variation as they grow. In “Selective Herbivory,” a population of both of these varieties of Brassica rapa is introduced to a voracious herbivore: the larvae of the cabbage white butterfly Pieris rapae. This activity involves observing feeding behavior of the larvae and hypothesizing about how variation in the plant population might cause certain variants to be selected over others.

“Salt of the Earth” models what happens to a population when its physical environment changes. In this case, plants are subjected to salt in their water. The impact of this treatment on survival and reproduction is assessed and followed by selective breeding (i.e., artificial selection) of those variants that survive better than the others. While this activity is specific for salt as a variable, it can be modified to test the impact of any number of environmental changes. And, while becoming familiar with selective breeding principles, you can continue to understand how evolution through natural selection works.

Note: All of these activities can be done using the offspring of Brassica rapa and Pieris rapae that result from studying their life cycles in the “Brassica & Butterfly System.”

You can follow along online and track your progress with “Assessing Variation” and “Selective Herbivory.”

Building Your Field Population System

One Field Population System requires a “Bottle Growing System” plus the appropriate seeds. If you follow “Assessing Variation” with “Selective Herbivory,” you can use the same Bottle Growing Systems. Both of these activities require Fast Plant, turnip, and lettuce seeds and the latter requires cabbage white butterfly eggs. In addition, you’ll need a Brassica Nursery* to rear the butterflies to the larval stage, a Light House* to grow all of the plants, and a Bee Stick* for pollination. “Salt of the Earth” can use any seeds — we recommend Fast Plants because of their rapid life cycle.

*Note: The instructions for the Brassica Nursery, Light House, and Bee Stick are provided as part of the Brassica & Butterfly System.

Materials Needed

For Building Bottle Growing Systems

  • One one-liter plastic bottle (provides a top unit and deep base unit)
  • One bottle cap (for deep base unit)
  • One 10-cm length of nylon craft cord (for wick)
  • China or non-permanent marker (for making marks)
  • Safety razor or utility knife (for starting bottle cuts)
  • Scissors (for finishing bottle cuts)
  • Soldering iron or drill (for making wick hole in bottle cap)

“Assessing Variation” requires at least one Bottle Growing System — you’ll want to prepare three if you follow it with “Selective Herbivory.” “Selective Herbivory” requires three Bottle Growing Systems. “Salt of the Earth” also requires three.

For Stocking and Maintaining (Assessing Variation and Selective Herbivory)

  • One package of Fast Plant (Brassica rapa) seeds
  • One package of purple top white globe turnip seeds
  • One package of lettuce seeds
  • Cabbage White Butterfly eggs
  • Potting soil mix (Jiffy-Mix©, Scott’s Redi-Earth©, or 1:1 peat moss and vermiculite)
  • Fertilizer solution (Peters Professional TM Fertilizer or Miracle Gro)


  • “Salt of the Earth” requires only Fast Plant seeds.
  • Each Field Population System for “Assessing Variation” and “Selective Herbivory” requires 10 – 15 seeds of each type. Be sure to order enough Fast Plant seeds for all of the activities you plan to do.
  • Fast Plant seeds and butterfly eggs can be ordered from Carolina Biological Supply Company (1-800-334-5551) or They can also be ordered from

Building Instructions

  1. Follow the instructions for making cuts, in “Bottle Basics,” to make a top unit and a deep base with each bottle.
  2. Melt or drill a hole in the bottle caps and screw onto each top unit.
  3. Insert wick through hole in bottle cap with approximately 5 cm on either side.
  4. Invert the top unit over the deep base.

Stocking Instructions

For Assessing Variation and Selective Herbivory

  1. Moisten the potting mix.
  2. Fill the top unit with potting mix to about 5cm from the rim.
  3. Apply water gently to the mix until dripping from the wick below.
  4. Add more potting mix until settled but be careful not to pack too heavily.
  5. Mix equal numbers of each seed together (10 – 15 seeds for each system).
  6. Scatter the seeds in the system, taking care to distribute them as evenly as possible.
  7. Cover the seeds lightly with a thin layer of potting mix.
  8. Fill the deep base about 3/4 full with fertilizer solution (follow instructions on fertilizer container)
  9. Place in Light House, elevating so that the plants will sprout about 10cm from the light.


For “Salt Of the Earth,” use only Fast Plant seeds. Sow 20 – 25 seeds in each system.

Maintenance Instructions

  1. Keep deep bases supplied with fertilizer solution.
  2. Thin seedlings as needed so that only single plants are growing from the same place.
  3. Adjust plants as they grow to maintain approximately 10cm distance to light.


Now that you’ve built the components you need for the Field Population System, try these activities to further your understanding. First, read the instructions and perform the activity. Then, for selected activities, view an example of our results in track our progress.

  • Assessing Variation… Observe how individuals in a population of Brassica rapa plants vary from one another. [track our progress]
  • Selective Herbivory… How selective are the larvae of the cabbage white butterfly on the plants that they eat?
  • Salt of the Earth… Do your own investigation and perhaps do some selective breeding!

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.

TRACK OUR PROGRESS: Assessing Variation

Selective Herbivory

How selective are the larvae of the cabbage white butterfly on the plants that they eat? These larvae prefer members of the Brassica family, which includes cabbage, broccoli, Brussels sprouts, kale, mustard, radish, and numerous others. One species — Brassica rapa — includes turnips, Chinese cabbage, pak choi, broccoli raab, and the Fast Plants that were selectively bred by Bottle Biology creator, Paul Williams. All of these are varieties of the same species, so they can be considered to be members of the same population when grown together.

Selective Herbivory tests whether or not cabbage white butterfly larvae feed selectively within a population of Brassica rapa. A population of lettuce has been sown with the other seeds to make additional comparisons. If larvae do feed preferentially, they could be considered agents of natural selection. Such agents would be expected to cause the population to evolve over many generations.

Materials Needed


  1. If you are using the same system for “Assessing Variation,” you’ll begin this study about 10 days after sprouting.
  2. To begin your study, gently gather numerous larvae from the Brassica nursery. You’ll need at least three for each system.
  3. Introduce equal numbers of larvae to each system, trying to spread them around and start them at the base of the plants’ stems.
  4. At least once each day for the duration of the study period (3 – 5 days), use your “Selective Herbivory” Data Sheet to make observations. You’ll be recording where the larvae are and what is happening to the plant populations.
  5. At the end of the study period, assess which plants were selectively fed upon, if any.

Activity Questions

Before the study period begins

  1. What traits vary among the plants in your Field Population System?
  2. Among the traits that vary for Brassica rapa, which do you think might affect feeding behavior of the butterfly larvae? Why?
  3. How will you be able to tell which variants the larvae select?
  4. What has to happen in order for selective herbivory to have an impact on the next generation of plants?

After the study period ends

  1. What occurred during the study period to the Fast Plants? The turnips? The lettuce?
  2. Was there a pattern to feeding behavior over time (i.e., an order to feeding choices)?
  3. Which of the two varieties of Brassica rapa, if either, did the larvae select preferentially?
  4. What implications does this have for the Brassica population in the next generation?
  5. Do you think that this is an example of natural selection? Why or why not?
  6. How might this result in evolution within the population?
  7. If selective herbivory was observed, which trait(s) do you think might have affected feeding choices? Why?
  8. One trait of Fast Plants that was discussed in the video during the Science Studio was hairiness of plants. Is there a way that the Fast Plants in your Field Population System might become hairier over time? Explain how this might occur.
  9. Is there a way that the larvae of the cabbage white butterfly might become more or less selective over time? Explain how this might occur.

Salt of the Earth

How does salt affect the growth of plants? In a population of plants, are some plants more resistant to the effects of salt than others? If so, how might such a population be selectively bred to tolerate higher concentrations of salt?

Salt sources in real-world situations include road salts, such as those that are applied during the winter to melt snow, and seawater. The salt in question may be common table salt — sodium chloride — or other salts such as calcium chloride (CaCl2) used in road salts.

“Salt Of The Earth” is an activity that is designed to find answers to the above questions. Using a population of Fast Plants, you’ll be able to do your own investigation and perhaps do some selective breeding — just like Paul Williams did to create Fast Plants! This is an advanced activity that is meant to help you apply the principles of selective breeding. However, you may find it appropriate for students in later elementary grades. Kathy Vandiver’s sixth-grade class — featured in Session 5 — was very successful doing an activity like this.

While this activity uses salt as a variable to affect plant growth, this same design can be used to vary any number of environmental conditions:

  • Nutrient levels
  • Temperature
  • Light
  • Water
  • Soil type
  • Soil moisture
  • Sound
  • Atmospheric conditions (e.g., smoke)
  • Pollutants (e.g., detergents, pesticides, oil, etc.)

Materials Needed

You can use seeds of any type for this activity. We suggest Fast Plants because of their rapid life cycle and predictable growth habits under the controlled conditions in a Light House. You can also substitute the Bee Stick with a cotton swab or fine paintbrush.


Effects of salt

  1. Prepare three different salt-water solutions: bottled water, 2% salt (2.0 grams table salt in 100 ml water — about 1/8 teaspoon) and 4% salt (4.0 grams salt in 100 ml water — about 1/4 teaspoon). The bottled water will act as a control, and the two concentrations of saltwater will allow you to compare how different levels of salt affect plant reproduction. Keep the extra solution for maintaining water levels.
  2. Add one solution to the deep base of each system. Do NOT use fertilizer solution in this activity.
  3. Plant an approximately equal number of seeds in each system (15-20). The study period starts at this point.
  4. Place each system in a Light House so that the seeds sprout about 10 cm from the light. Adjust this distance to the top of the plants as they grow.
  5. Once plants have sprouted, thin them to an equal number in each system (e.g., 10 –15 plants).
  6. Using your “Salt Of The Earth” Data Sheet, track the growth of plants in each system over time.
  7. Once the plants have begun flowering (about day 14 in the control population), use your Bee Stick to pollinate them. Gather pollen from the flowers of one plant and transfer it to the flowers of another. You can also use a fine paintbrush or cotton swab.
  8. Continue pollinating for 4 – 5 days until about day 18. After that, pinch any new flowers off at their bases.
  9. Continue to supply the appropriate salt-water solution to each Bottle Growing System for another 20 – 22 days to allow pods to develop and seeds to set.
  10. Remove water and allow the plants to dry until crispy brown — about one week.
  11. On the day you remove the water, note three individuals in each population that seem to have survived and reproduced better than the others and mark them in some way (e.g., a toothpick at the base). These will be the plants from which you will collect seeds for selective breeding.
  12. After about one week, the plants will be dry and the seeds will be ready for collection. Carefully remove the pods from the plants of the individuals you selected.
  13. Collect the seeds from these pods, keeping the seeds from each population separate.

Selective breeding

  1. Use the same set up for selective breeding.
  2. Sow the seeds collected from plants grown in each specific solution in a system containing that same solution.
  3. Observe through the life cycle to see how survival and reproduction is affected in the next generation.

Activity Questions

Before the study period begins

  1. What are some indicators of superior survival and reproduction of an individual plant among others in the same population?
  2. How might superior survival and reproduction become more common in subsequent generations of plants?
  3. How do you think the concentration of salt in a plant’s water supply might affect it?
  4. If you wanted to develop a population of salt-tolerant plants, how might you do this?
  5. How would you know if you were successful?

After the study period ends

  1. What occurred during the study period to the plants in distilled water? In the 2% salt solution? In the 4% salt solution?
  2. In each solution, what was the extent of variation with regard to plant survival and reproduction?
  3. If the plants in each population were left to reproduce undisturbed (i.e., without selecting superior survivors) what would the next generation of plants be like? Explain your reasoning.
  4. If you collected and sowed seeds from superior survivors, what do you expect to happen in the next generation? Explain your reasoning.
  5. Applying what you understand about heredity, account for variability in the plants’ salt tolerance. How does selective breeding for salt-tolerance impact the genome of a plant population?
  6. Could a salt-tolerant plant population develop in the natural world? Explain how this might occur.


Track Our Progress

Get a glimpse of our system at various points in selected activities. Remember that every system is unique: don’t expect yours to look exactly like ours!

Series Directory

Essential Science for Teachers: Life Science


Produced by Harvard-Smithsonian Center for Astrophysics. 2003.
  • Closed Captioning
  • ISBN: 1-57680-730-4