Materials Included In Kit

Safety Precautions

Toothpicks are sharp. Warn students to use good judgment when handling the toothpicks so that no one gets punctured with a toothpick.

Disposal

All materials may be reused many times. Have students sort the toothpicks at the end of the activity so that they are immediately reusable in your next class.

Teacher Tips

• Enough materials are provided in this kit for 8 groups of students. All materials are reusable. The activity can reasonably be completed in one 50-minute class period with follow-up discussions in succeeding class periods.

• A feeding time of 30 seconds is recommended. You can let students start and stop their own feeding times or you can direct the feedings yourself, keeping the entire class on the same time frame. This will depend upon your class and your goals. Watches or timers that can time to the nearest second will be required.
• You should determine how many generations you want students to complete. The trend of the natural selection might be obvious after several generations or it might take longer and vary from group to group. Time available and your student population might determine how many generations you select. The worksheet provides space for four generations. Four generations allows every student to make stick bug selections of every color.
• It will be important to discuss many topics after this introductory activity. The idea of natural selection influencing the gene pool of future generations is a central idea to biology. Discuss all of the assumptions made during this simulation.
• Calculations should be done to the nearest one-hundredth and then rounded off. Reproduction involves only whole individuals—no half stick bugs! Since the population should always start at 100 individuals, students will have to make decisions about how to round off and settle on whole individuals.
• This is an excellent activity to allow students to explore other environments and other variables. What happens if a different-colored forest is introduced? (Try a green piece of carpet or go out to a grassy area.)

Sample Data

Student data will vary considerably from the sample provided here. In this sample data the number selected of each color is held constant for each generation to illustrate the calculations and possible trend resulting from consistent selection over generations.

Answers to Questions

1. Did the population of stick bugs change through the generations? How?
   

   Student data will vary considerably from group to group but all should show a change in the numbers of each color through the generations. The most likely trend will be an increase in the number of tan and possibly yellow stick bugs.
   

2. What could cause a specific color of stick bugs to disappear? Is this likely to occur?
   

   The level of camouflage makes some individuals more susceptible to predation than others. Thus some individuals may be selected against. Even if the numbers of a particular type decrease, they may not disappear altogether.
   

3. What would happen if all of the red stick bug predators became extinct? What would likely happen to the stick bug population? How might this effect the tan stick bugs?
   

   If no other variable came into play, the red stick bug population would likely increase over time. The increase in red stick bugs might affect the number of other colors as they compete for space. The tan population might go down.
   

4. What happened to the gene pool of the stick bugs over the generations you studied?
   

   The number of total genes in the population stayed the same but the proportion of genes for various stick bug colors changed over the generations. The number of genes for tan stick bugs increased in the total gene pool.

Introduction

Predator/prey relationships can influence the nature and size of a population over time. Organisms that are killed before they reproduce do not contribute genes to the next generation! What can killing of selected individuals from a population do to the makeup of that population over time? How do populations change over time?

Concepts

• Carrying capacity

• Predator/prey
• Gene pool
• Population size
• Natural selection

Background

A species is a group of similar organisms capable of reproducing viable offspring of their kind. Those individuals that are involved in the production of offspring have their unique genetic material passed on to the next generation. Those individuals that are unsuccessful at reproducing do not pass their genetic traits to the next generation and their unique set of genes dies with them. All of the collective genes of a given population are referred to as the population’s gene pool. Since there is usually an uneven rate of reproduction among individuals in a population, the nature of the gene pool tends to vary from one generation to the next. If conditions during a given time period are detrimental to certain types of individuals and if the conditions affect the ability of the individuals to reproduce, then the gene pool of the population will be altered and changed over time. If “selection” for or against certain individuals affects their ability to reproduce offspring, it is referred to as natural selection. Biologists study populations and gene pools over time and study the trends in the makeup of populations resulting from natural selection.

This simulation activity will illustrate population changes resulting from selection pressures. Several generations of stick bugs will be tracked as they are being preyed upon by their enemy—“forceps birds.” After each generation, the individuals that have not been eaten will be allowed to reproduce in proportion to their group survival rate. The total number of individuals of a species that can be sustained indefinitely in a given area is called its carrying capacity. It will be shown that even if the total stick bug population is maintained at the carrying capacity, the genetic makeup of the population will change over time.

There are four color variations of stick bugs in the simulated population—red, yellow, green and tan. Stick bugs live in a forest of wooden sticks which affords them good camouflage and hiding places. The forceps birds (predator of stick bugs) have their favorite color stick bugs and search for their favorite color when they are feeding. In the simulation there will be periods of eating and capturing stick bugs, followed by a recovery and reproductive period. The nature of the total stick bug population will be tracked for several generations.

After each predator attack, the number of stick bugs of each color will be counted and then allowed to reproduce in proportion to the number that survived. The following formulas will be used to calculate the number of new stick bugs of each color that will be added to the forest. The letter abbreviations are used on the Stick Bug Worksheet.

(1) A = B/C where

A = Ratio of individuals of each color that will reproduce for the next generation.
B = Number of individuals (specific color) that survived. Count the number of individuals eaten (X). Subtract X from the initial number (Y). Then B = Y – X.
C = Total number of individuals (all colors) that survived.

(2) D = A (100 – C) where

D = Actual number of a specific color to add to the forest for the next generation.

(3) The new number of individuals of each color is the number where H = B + D.

(4) E = 100 where

E = Total starting population for each generation (carrying capacity).

Materials

Safety Precautions

Toothpicks have pointed ends and can cause punctures and slivers in the skin epidermis. Handle all the toothpicks carefully during this activity.

Procedure

1. Your teacher will assign you to a working team of four. Each team member will be assigned an initial stick bug color (yellow, red, green, tan).
2. Empty one box of flat wooden toothpicks into the forest pan. These flat toothpicks represent the background environment typical of the home of the stick bugs. Place the forest pan on the table in front of your team.
3. Carefully count 25 toothpicks of your assigned color and place them in the forest pan. Note: The tan stick bugs are different than the forest background—they are round and have pointed tips.
4. Locate a forceps for each team member. Your teacher will instruct you when to do the following:

1. Shake the forest pan without looking into it.
2. Set the pan where it can be reached equally by all team members (forceps birds).
3. Start to feed on your color of stick bug.
4. Stop feeding on the stick bugs.

5. Feeding Rules:

1. Start and stop precisely when the stop and start signals are given.
2. Only the forceps can be used to pick up and remove the stick bugs from the forest.
3. Only one stick bug can be removed at a time.
4. Stick bugs must be removed from the forest and placed on the table in front of you.

Warning: Be careful during the feeding activity—the toothpicks and forceps are sharp.

6. Count the number of each color of stick bug that was eaten during the specified feeding time (X). Record the number of survivors of each color (Y – X). (Subtract the number eaten from the 25 original bugs.) Calculate the ratio of reproduction (A) and the number of individuals of each color that should be added for the next generation (H). Record all of these calculations on the chart for the first generation on the Stick Bug Worksheet.
7. Count out the number of each offspring of each color needed to bring the population to 100 (carrying capacity) for the next generation. Add these newly reproduced individuals to the forest pan.
8. Each team member should select a different color stick bug for the next generation feeding.
9. Repeat steps 4–8 for the next generation at the direction of your instructor.
10. When data and calculations for all the studied generations have been recorded, answer the questions on the Stick Bug Worksheet.
11. Do not dispose of any of the materials. Your instructor will provide direction for how to recycle the materials.

Student Worksheet PDF

10407_Student1.pdf