The Evolution of Shribles by Natural Selection

I. Introduction.

In this laboratory, you will explore the natural selection of a population of organisms called Shribles. The Shribles have different colors of fur, and this lets some of them blend into the background of their environment, but makes others very easy to see. You will act as a predator, hunting for Shribles while they live in their white paper box and reproduce. You will observe how the fur color of the population changes over three generations of Shribles as hunters continue to kill off some of the individuals. You will observe how certain traits are a good adaptation and allow individuals to survive and reproduce, while other traits are a bad adaptation, and make the individual more likely to die before it is able to have children.

II. Materials

White construction paper in the shape of a shallow box

Clear, pink, and green plastic beads (at least 30 per group of each color)

III. Procedure

- Using a WHITE paper box, create a population of 10 pink and 10 clear Shribles. Shribles usually live in grassy areas, so include at least 20 green grass shrubs (green beads).

- Hunter: Close your eyes.

- Agitator: Mix all the beads around evenly before the hunt begins. During the hunt, agitate the box by shaking it lightly (enough to move the beads around, but not so hard that they fall out).

- Hunter: On agitator's signal, open your eyes and try to catch as many Shribles as possible. However, you can only catch ONE SHRIBLE AT A TIME.

- Hunt until the teacher has counted to 3.

- Count the number of pink and clear Shribles still in the box and record the number of Shribles remaining as "Generation 1 after hunt."

- Return all green grass shrubs back into the box.

Now the population of Shribles will reproduce, and every Shrible will have two children which are the same color as the parent.

- Repopulate the group with the appropriate number of pink and clear baby Shribles.

- Record the total number of pink and clear Shribles in your box, and call this "Generation 2."

- Make sure that there are at least as many green grass shrubs as there are Shribles

- Repeat the hunt with two different hunters, and record the number of Shribles remaining as "Generation 2 after hunt."

The Shribles will reproduce again, with each Shrible having two children of the same color as the parents

- Repopulate the Shrible colony and record the number of Shribles as "Generation 3."

- Repeat the hunt with two more hunters, and record how many Shribles are left in "Generation 3 after hunt."

IV. Calculations

- For each generation, determine the percent of pink to clear Shribles before and after the

hunt. (Hint: In Generation 1, there were 50% of each before the hunt because there were 10 of one kind and 10 of the other).

- Create a bar graph of the % of pink Shribles left in each generation after the hunt. The x axis should have 3 categories (G1 after hunt, G2 after hunt, G3 after hunt), and the y axis should have % values.

- Create a bar graph of the % of white Shribles left after each hunt.

- Make sure that each graph has a title, and label each axis with the unit of measure and what the numbers represent.

V. Questions

1) Describe the results shown in each graph. What happened to Shribles of each color as they continued to be hunted through the generations?

2) Why did Shribles of one color survive better than those of the other?

3) Imagine this hunting continued for another ten generations. Would you expect to see any Shribles with pink fur left in the population at the end? Why or why not?

4) How does this laboratory demonstrate the mechanisms of Evolution by Natural Selection?

5) Now, imagine a population of only pink Shribles, living in a white paper box. Occasionally, some Shrible babies are born with clear fur instead of pink. The ones that have clear fur then go on to have babies with clear fur. After 10 generations, what would you expect the population to look like? Would you expect all pink Shribles, the way it began? A mix? All clear ones? Explain your predictions using the concepts of Evolution illustrated in this laboratory.

6) Imagine a mixed population of Shribles, half with pink fur and half with clear fur living in a white paper box. Suddenly, their habitat is flooded with pink water, and their box becomes pink forever. This does not disturb the hunters, who continue to look for dinner as before. What do you expect the population of Shribles to look like after 3 generations? Pink, clear, or mixed? Explain your prediction using the concepts of Evolution illustrated in this laboratory.