To most students entering today’s biology classes, evolution is something that occurred long ago, and is therefore irrelevant to their lives. Examples of evolution that are important concerns in our modern world, such as the resistance of insects to pesticides and antibiotic resistance, do not match students’ concept of evolution. Finding an engaging way to teach students the basic principles of natural selection and how evolution is significant to their lives can be a difficult task.

Jelly beans and predators

In 1996, we implemented an activity published by Michigan State University (Smitley 1996) that used students and jelly beans to simulate the selection forces placed on populations by predators. To adapt this activity to the high school science classroom (Figure 1, p. 32), we created a fictional narrative and characters that demonstrate the effects of cryptic coloration and mimicry. Students play the role of competitive predators by collecting jelly beans in an effort to “survive”; as students progress through the activity, they learn that some jelly beans thrive while others become endangered or extinct. The activity’s defining moment comes when students are asked to evaluate how the results would be different if the environment changed, and they realize that diversity is the key to species survival. A follow-up exercise allows students to evaluate the jelly bean population using the Hardy Weinberg principle.

Before starting this activity, students need to be familiar with three basic principles of natural selection. The first is that more offspring are produced than can possibly survive. The second is that offspring have genetic variations. The third is that some of these variations provide an advantage. Offspring with an advantage usually survive longer and reproduce more offspring than those without the beneficial trait. Advantages come in many different forms, but in general provide the offspring with reduced competition and predation. Among these traits are cryptic and warning coloration, concepts students should also be familiar with before starting the activity.

Part 1: Cryptic coloration

Captain Dan and his crew were sailing from South America to Australia when they encountered a severe storm. The ship tossed in the sea for days before coming to rest on an island somewhere in the South Pacific. The ship was badly damaged and most of the supplies had fallen overboard during the storm. Dan and his crew were very hungry and the island didn’t promise much in the way of food.

After stepping foot on the unusual pine shaving terrain, the crew discovered tiny organisms hiding in the grass. They named them Jelly bellicus because the organisms reminded them of a sweet treat back home. Upon closer inspection the crew realized that there were eight different varieties of Jelly bellicus. The crew began gobbling up as many of the delicious critters as they could find. Soon the number of Jelly bellicus was reduced to just a few, and the crew began fighting for what was left.

Teachers should set aside 15 minutes for this part of the activity (Figure 1, Part 1, p. 32). Students are given the Jelly bellicus activity packet and follow along as the above passage is read aloud by the instructor. At the completion of the reading, each group of four to five students receives a tray filled with cedar chips (as shown on p. 30), sawdust or pine-shaving cage litter, and a bag of Jelly Belly jelly beans (the importance of using Jelly Belly–brand candies will become apparent in the second activity). The bags contain 10 jelly beans each of 8 different flavors, for a total of 80 jelly beans per bag. These candies can be found in almost any color, but at least three of the colors used should blend with the pine shavings. The colors that we use are tan, tan with spots, white, green, green with spots, red, red with spots, and maroon. After verifying that students have 10 of each type of jelly bean, they carefully mix the candies into the pine shavings.

Students are told that they will have 30 seconds to remove as many jelly beans as possible from the tray. All students forage at the same time and the instructor monitors the clock. Students may remove only jelly beans—grabbing a handful of shavings and then sorting through it is not allowed. To make foraging even more difficult, we sometimes instruct students to use only one hand to collect the jelly beans. Students are also told that the student from each group that collects the most jelly beans will earn extra credit at the end of the activity. At the teacher’s direction, students start searching for the jelly beans, quickly realizing that some of the jelly beans are easier to find than others. After 30 seconds, students stop and count their prey. Students calculate the number and type of jelly beans they have found and record their data along with the data of other members of their group (Figure 1, Part 1).

All of the jelly beans are returned to the tray, and students repeat the process two more times to establish a routine. Students then answer analysis questions about why some of the candies are easier to find while others are more difficult and evaluate predatory skills by comparing their results with others in the group (Figure 1, Part 1). Reasons for a particular student’s success range from physical speed to intimidation of group members. At the end of this activity, students have realized that individual differences in Jelly bellicus play a large role in survival, and that cryptic coloration is important to escaping predation.

Part 2: Mimicry

After eating mainly spotted red Jelly bellicus, several of the crewmembers became ill. Those that hadn’t eaten the spotted Jelly bellicus felt fine. The spotted critters must have been poisonous! It was difficult to determine which red Jelly bellicus had spots, so most of the crewmembers avoided red Jelly bellicus altogether.

This part of the activity also takes 15 minutes (Figure 1, Part 2). When buying Jelly Bellies, we select some with and some without spots to simulate mimicry. Using traditional jelly beans does not allow for this subtle difference in prey. Before starting the mimicry round, the class decides on a poisonous color (in this case, red). The poisonous color should be one of the brighter, easier-to-find jelly beans, which also has a spotted variety. The teacher should help students make a choice that will reflect this and simulate warning coloration. The teacher should indicate to students that the plain red jelly beans are harmless (mimic), but the spotted red ones will make students ill (model). If a student does pick up a spotted jelly bean, they are eliminated from the round and must stop collecting. Because natural predators can learn to avoid certain prey, students are allowed to rejoin the game for the next round.

The energy level of the competition changes as students become more cautious. Most avoid both the plain and spotted red candies because it takes too much time to carefully select jelly beans. Students who risk grabbing a red jelly bean and get one that is plain red are more likely to keep going after red jelly beans, while students who select a spotted one learn quickly to avoid all red jelly beans. This action simulates the learned behaviors displayed by natural predators. Students repeat this simulation three times and then complete questions about the phenomena of mimicry.

During the analysis phase (Figure 1, Part 2, p. 32), the teacher should initiate a conversation about how technique changed once one of the organisms became poisonous. By simulating the mimicry, students learn that warning coloration can be helpful for the poisonous model and that the mimic decreases the effectiveness of the model’s advertisement. They also evaluate how competition between students changed and how mimicry affects natural predation and survival. A well-known natural example of mimicry is provided by the Viceroy butterfly (Limenitis archippus) and the similarly colored Monarch (Danaus plexippus).

Part 3: Population shift

Captain Dan and his crew were stranded for months. As time went on, the Jelly Bellicus population began to change. Some of the organisms seemed to thrive and reproduce, while others became scarce, and some disappeared completely. Overall, it was becoming more difficult to find food. Captain Dan and his crew had to find something else to eat if they were going to survive.

This part of the activity (Figure 1, Part 3, p. 33), which requires 30 minutes, progresses much like the cryptic-coloration (Part 1), with one major difference: not all of the jelly beans are returned to the tray after round one. At the end of the first 30 seconds, students determine how many jelly beans survived and simulate reproduction based on this survival rate. For every pair of similar-colored jelly beans not captured left in the tray, two offspring are born. Students add only offspring back into the tray for the next round—the rest of the captured jelly beans are left out.

For example, if students collect 6 of the 10 green jelly beans, they would determine that there were 4 survivors still on the tray. Four survivors would produce 4 offspring, so only 4 green offspring jelly beans would be added to the tray. By contrast, if students collect only 2 tan jelly beans, 8 would have survived, and 8 offspring would be added to the tray. In the event that an odd number of jelly beans survive, students are instructed to calculate offspring based on the highest number of pairs that can be formed. For instance, if 7 red jelly beans survive, offspring would be calculated using 3 pairs (6 jelly bean “offspring”).

It is necessary to have additional jelly beans available during this part of the activity, as some of the populations will grow past the original 10 candies. The jelly beans that are easy to find will have few survivors and may become extinct due to their low reproduction rate. Those that are difficult to find survive, reproduce, and increase in total population. Students witness a decline in diversity as the population shifts toward primarily tan and white jelly beans.

At the end of three rounds students answer analysis questions about the evolution of the jelly bean population (Figure 1, Part 3, p. 33). We complete the activity by holding up a bag of litter that is a different color (green or cedar) and ask students how changing the jelly beans’ environment would change the results. We then discuss how the elimination of some of the colors might lead to extinction of all of the jelly beans because those that are left may not blend in with the new environment.

If time allows, the activity can be repeated with the changed environment (litter color). Students can apply this concept to real populations and environmental changes (Figure 1, Part 4), such as industrial melanism. Terms such as threatened, endangered, and extinct can all be applied to the plight of the jelly beans. Overall, students see how populations change over time and how those changes are driven by fitness and competition. They also see the importance of genetic diversity as it relates to long-term survival.

Extension: Do the math!

An advanced course extension involves the application of the Hardy-Weinberg principle to the jelly bean population to determine if the population is evolving with respect to a specific trait (Figure 1, Part 5). Students are told that the spotted jelly bean trait is recessive. They are given a total population size of 20 and are told that 5 of the 20 have spots. Using the formula p² + 2pq + q² = 1, students are asked to calculate the p and q values (the frequencies of each of the two alleles), as well as the number of jelly beans that must be heterozygous for the trait. The follow-up question suggests that due to their poisonous quality, the spotted jelly beans begin to flourish—instead of making up 25% of the population, they now comprise 64% of the total jelly beans. Students are asked to calculate the number of plain jelly beans and determine how many are carrying the spotted gene.

An effective tool

This activity has been a very effective tool in teaching students the principles of natural selection. Students love the competitive nature of the activity, and because it is easy to collect, sort, and count jelly beans, students of all levels get involved. The fictional story of shipwrecked sailors adds some interest to the activity and gives students a better understanding of how natural selection occurs. By playing the role of competitive predators, students find themselves immersed in the process of natural selection and they realize that humans and other predators can be the driving force for population change.

For the instructor, the activity requires minimal set-up time, and once the jelly beans and cage litter have been purchased, there is no additional cost. Because they have a written record of what is left in the box, participants can reset the game by finding all of the jelly beans and restocking the bag with the original 80 candies. The activity is then ready for the next class. Students are asked not to eat the jelly beans, and after being told how many dirty hands have touched the candies prior to their class, this is usually not an issue. [Safety note: Teachers must reinforce the “No eating in the lab” rule. In classrooms where this might be a particular concern, other objects like colored toothpicks might be substituted for the jelly beans.]

The original activity at Michigan State University was conducted outside, and although it was fascinating to see how natural lighting and environmental factors influenced the results, planning for the activity was difficult due to the unpredictability of weather. Additionally, finding all of the jelly beans outside was a problem even when the weather cooperated, and we were replacing the expensive Jelly Bellies on a yearly basis. While the outdoor activity simulated a field study, moving it indoors allows us to run the activity like a controlled experiment, and fewer jelly beans are lost.

Through the plight of Captain Dan and his crew, students gain a better understanding of predation, mimicry, and population genetics. Applying the Hardy Weinberg principle to the activity helps students understand how math can be used to analyze natural population shifts. Students are engaged and enjoy the activity, and we have found it to be a memorable part of our biology curriculum.

Deborah Tieman (Deborah.Tieman@fraser.misd.net) and Gary Haxer (Gary.Haxer@fraser.misd.net) are both high school instructors at Fraser High School in Fraser, Michigan.

Reference

Smitley, D. 1996. Predators as a selection force: The jelly bean lab. Natural Science 11(1): 13–16.