Big Beans, Little Beans Authors: Al Janulaw and Judy Scotchmoor

Overview: Students measure and note the variation in the lengths of lima beans. Students then compare the growth rate of different sized beans.

Lesson Concepts:

• Scientists pose, test, and revise hypotheses based on research outcomes.
• Scientists use only natural causes to explain natural phenomena.
• Evolution results from natural selection acting upon variation within a population.
• Individual organisms with advantageous traits are more likely to survive and have offspring.

Grade Span: 6–8

Materials:

• Nails of four different sizes
• Five small jars
• 1 kilogram of lima beans per class
• 1 small plastic bag for each group
• 2 mm rulers for each group
• 1 Lima Bean Recording Table per group
• 1 Lima Bean Graph per student
• Three pots of soil for each group
• Water
• A source of light

Advance Preparation:

— Fill four jars with nails, each with nails of a different size. Fill one jar with a mixture of nails of all four sizes.
— Make copies of Lima Bean Recording Table sheets (pdf file) and Lima Bean Graph sheets (pdf file).
— Prepare bags of lima beans for each group.
— Draw a class Lima Bean Recording Table and a Lima Bean Graph on the board.

Time: One or two class periods initially and about three weeks for the plants to grow following the initial lesson

Grouping: Groups of four and whole class

Teacher Background:

This lesson provides an opportunity to emphasize a number of ideas, in particular, the way scientists work. Students will emulate scientists as they form hypotheses (e.g., variation in bean size may affect the time of germination and the development of the seedling), test these ideas, and analyze the results. Students should also get the idea that individuals vary within species and that selection works on variation. Particular individuals may have an advantage in particular circumstances. This will provide a foundation for students’ later understanding about the importance of variation: variation provides flexibility (and a resulting advantage) for a population as a whole. More variable populations are more likely to include individuals able to survive changing environmental conditions, allowing those individuals to survive and reproduce. The big picture is that evolution depends on variation and selection of favored individuals within populations under particular situations.

Teaching Tips:

Prior to this activity, students should know that seeds germinate to produce the next generation of the plant and that the cotyledon provides both protection and food for the growing plant embryo. Class discussion will help students understand that there may be an advantage to individual variation within populations and that scientists perform experiments to test their ideas.

Vocabulary: variation

Procedure:

The first class period:

1. Show the class the four jars of nails. Allow them to notice that three jars have nails of uniform sizes and one contains a mixture of nails. Describe this scenario: You have a place in your home where tools and other useful items are kept. You know that sooner or later you will need a nail or two, but you cannot predict what the need will be. You can only have one jar. Which jar would you select? Hold a brief discussion, which hopefully leads to students realizing that the mixed jar is potentially more useful because it provides more options. Your needs vary with the tasks at hand, so having a variety of nail sizes may have an advantage.
2. Pass out the bags of lima beans to each group. Have students look at the beans. Are they all the same, or do they vary? How do they vary?
3. Pass out the rulers and recording sheets to the groups. Have the groups measure and record the lengths (to the nearest mm) of as many beans as they can in 10 minutes.
4. Have the groups add up the totals of each length of lima bean and individually graph their results, e.g., five beans 10 mm in length.
5. Ask students, within their groups, to discuss what they notice about their results.
6. Have a brief discussion with the whole class about what patterns they noticed.
7. Next, have a member of each group write their group’s totals on the table on the board.
8. Sum up the group totals and have students mark the data points on the graph on the board. Draw a “best fit” curve on the board graph.
9. Ask the class to compare their initial results with the class results. (The whole-class graph should look more like a normal curve.) Students should notice, both from the activity and from the graphs, that bean size varies and that there tend to be more in the middle than the top and bottom sizes.
10. Point out that there is variation within any population of living things, including beans. Ask: Might there be an advantage to beans being of any particular size? How could we find out? (Other possible questions to stimulate discussion: What advantage might a seed have if it is especially large? What advantage might there be in producing lots of small seeds? What problems might a plant have in producing especially large seeds?) Elicit one or more hypotheses from the students, e.g., larger beans will germinate more quickly.
11. Once the students have made their hypotheses, ask them what scientists do next? Elicit the response that they test their hypotheses and ask how they think they should test theirs. If students don’t suggest it on their own, inform them that they are going to plant beans of different sizes to test the proposed hypotheses about bean size and rate of germination and/or plant growth.


The second class period:

12. Have each group select two of the largest beans, two middle-size beans, and two of the smallest beans and plant each pair of beans in a pot. Have them label each pot with the size of the beans, the names of the members of the group, and the date planted.
13. Put the pots in a well-lit place and water regularly. Growth time for the beans will vary by the temperature, so plan flexibly.


During the next three weeks (or other appropriate length of time):
14. Have the students draw their plants on a daily basis in a journal, noting the date and any other observations, such as number and size of leaves, color, and height.


At the end of the growth period:

15. When the tallest beans have reached several centimeters, have students count the number of leaves and measure the height of the largest plant in each of their pots. Discuss the results. Is there any difference in rate of germination and/or growth between small, medium, and large lima beans? (Note: There is no way to predict the results of this experiment. So, remember that the point of the lesson is that scientists try things and learn from the results. Perhaps there will be a perceived growth advantage related to bean size, perhaps not.) Ask for students’ conclusions from the experiment. Which hypotheses were supported by this experiment? Which were not? Point out that it’s all right if their results are inconclusive. On the other hand, if there are differences in growth rate, ask such questions as Under what conditions in the wild might big beans or small beans have advantages? How might it be advantageous for the lima bean species to have variation in bean size?
16. Have students write what they have learned about variation from this activity.