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Student Lab
N at u r al S el ec ti o n I n s t r uc ti o n a l C a s e : A s er i e s o f 3 s t u d e n t -c e nt er e d s c i e nc e l es s o ns
Lesson 2
Goldfish vs. Cheez-its
Summary
Students engage in an activity that demonstrates how natural
selection occurs by predation. Students set up a simulated lake
habitat populated by a species of fish (the cheese cracker fish,
Pisces cheddarensis) that has two variants, or morphs (Goldfish
cracker and Cheez-it cracker) that differ in body type and
swimming speed. Students examine how the proportions of
Goldfish and Cheez-its change over successive generations as the
predator population (the students) catch and eat their prey.
Objective
Suggested Timeline
45-50 minutes
Materials
For each lab group:

One colored die with stickers
numbering the sides 1-2-2-3-3-4
(the Goldfish +die)

A second different color die
with stickers numbering the
sides 5-6-6-7-7-8 (the Cheez-it
die)

Fish-shaped crackers (like
Goldfish crackers) preferably
orange in color

Square crackers (like Cheez-its)
preferable orange in color

One paper plate (colored blue if
possible to represent a lake)

demonstrate how a predator can change the size and
composition of a prey population

predict potential changes in a prey population over successive
generations

collect and analyze data to test predictions

express ratios and use graphs to represent the data gathered
during the simulations
Teacher Background Knowledge
Predation is one of the major drivers of natural selection.
Genetic variation exists in all populations. Predators exert
selective pressure on the genetic variation in prey populations by
eating particular individuals, e.g., those that are easiest to capture
and eat. Prey that are not captured and eaten will survive to
reproduce, and so pass on their genes and have higher fitness.
This lesson provides an interactive example of this process.
Predators (represented by the students) feed on a population of
fish (cheddar crackers) with genetic variation (two different types
of cheddar crackers). There are slow swimming, box-shaped
individuals (Cheez-its) and fast swimming, torpedo-shaped
individuals (Goldfish). Cheez-its are more easily captured by the
predators than Goldfish. Over successive generations, the
proportion of Cheez-its declines as that of Goldfish rises.
Integrated Middle School Science Partnership
Natural Selection Instructional Case
A series of 3 student-centered science lessons
Lesson 2 Goldfish vs. Cheez-its

Two paper cups (5 ounces or
more)

One cup filled with fish shaped
crackers

One cup filled with square
crackers

One plastic spoon (to remove
crackers from cups and onto the
plate

For each student:

Copy of lab sheets
Insightful students may wonder what will happen to the predator
population when there are no more Cheez-it individuals left in the
prey population. In this lesson, predators are able to eat Goldfish
to get their “fill,” but if this wasn’t allowed, the predators would
quickly run out of prey fish, thereby sealing the fate of the
predator population. The size and variation in predator
populations are influenced by the availability of prey. This
brings up an important point about predator-prey relationships in
the real world—they have co-evolved* over many generations.
While the sizes of prey populations may go up and down in
dynamic synchrony with the sizes of predator populations,
population sizes in general remain fairly stable on an
evolutionary time scale.

Co-evolution involves two or more species that reciprocally affect each
other’s evolution. Predator-prey co-evolution can lead to an
evolutionary “arms race” of adaptations in both the predators and prey.
The evolution of a trait that gives one population an advantage will put
selective pressure on the other population, so that individuals with the
ability to overcome the defense will be favored and have higher fitness.
Standards
California Science Content Standards, 7th Grade: 3.a., 7.a., 7.b.
California’s Common Core Standards for Literacy in Science, and
Technical Subjects (Grades 6–8), Reading: 3, 4, 9; Writing: 7, 8, 9.
Preparation


Put the numbered stickers on
the sides of the two differently
colored dice

The other color with
the numbers 5-6-6-77-8 (the Cheez-it die)

One color with the numbers 1-22-3-3-4 (the Goldfish die)

Fill two paper cups,
one with each type of
cracker for each lab
group
Integrated Middle School Science Partnership
Page 2 of 4
Plus 3-Page
Student Lab
Natural Selection Instructional Case
A series of 3 student-centered science lessons
Lesson 2 Goldfish vs. Cheez-its
Lesson

Tell the class that today they will be studying a fish population in a habitat. This is the Cheese
Cracker (Pisces cheddarensis) species of fish! There is variation in body type in this population that
corresponds to the swimming speed of the fish--Goldfish are the torpedo-shaped, fast swimming
fish, and Cheez-its are the square-shaped, slow swimming fish. There is one predatory fish
population (represented by each student group) that finds the slow-swimming Cheez-it's the easiest
to catch and eat.

Pass out plates, cups with crackers and spoons to each group.

Have students observe the characteristics of the two different types of crackers.

Remind the students that the paper plate represents the lake habitat for the prey population; the
cheese crackers represent the prey population which is made up of two different morphs, the
Goldfish (torpedo-shaped) and Cheez-its (square-shaped); and the students represent the predator
population.

Have students populate the pond by having students roll each die to see how many of each type of
fish to start with. The die with the faces 1-2-2-3-3-4 tells how many Goldfish crackers to take out of
the cup and add to the paper plate (symbolically putting the “fish” in the “lake”). The other die (with
sides reading 5-6-6-7-7-8) determines the number of Cheez-its to place in the pond.

Have students record the number of each morph of fish in this first generation on the lab sheet, and
calculate the ratio of the Goldfish to Cheez-its. [If students do not remember how to write a ratio,
teachers might have to give them a quick re-teach of ratios].

Continue the story of the lake habitat fish population by explaining that the Cheez-it fish are very
slow swimmers. They are similar to the Puffer Fish whose boxy, rigid body makes it a slower
swimmer. Show a short video of a puffer fish swimming. (One video can be found at
http://www.youtube.com/watch?v=8cbAWkpTZXo). Have students observe that puffer fish swim
mainly by moving their tail and fins, and their body is more rigid. The Goldfish are faster swimmers.
In contrast to the Puffer Fish, Goldfish swim using their fins, tail, and body, which allows them to
swim faster. (This can be seen in a video at http://www.youtube.com/watch?v=iZzCrcKhkaY). The
predator species find it easier to catch fish that swim slower. Students represent the predators.

On the lab sheet, have students predict what will happen to the numbers of Cheez-it and Goldfish by
having each student record their prediction on the graph at the bottom right hand corner of the lab
sheet (remind students that the predators will eat more of the Cheez-its, but there are more Cheezits to start with).

Have students get to the second generation by eating the three slowest fish. If there are at least
three Cheez-its, all three should be eaten. If there are less than three Cheez-its, students should add
Goldfish to bring the total number of prey to three. Students remove the three fish from the paper
plate, and may eat them.

Have students double the amount of all surviving fish with the same proportion of Cheez-its and
Goldfish crackers by adding fish from the cups (where fish eggs are stored) into the lake. Students
will record the numbers of each fish type for Generation 2, calculate the new ratio of Goldfish to
Cheez-its, and repeat the predation cycle three more times for a total of four generations.
Integrated Middle School Science Partnership
Page 3 of 4
Plus 3-Page
Student Lab
Natural Selection Instructional Case
A series of 3 student-centered science lessons
Lesson 2 Goldfish vs. Cheez-its

Have students record the data from their four generations on the graph provided, choosing one
color to draw the Goldfish data, and another color to record the Cheez-it data. Students will refer to
their graph and the ratios of the two fish types and answer the two questions under the graph.

Discuss the findings of the different student groups with the whole class.
Teacher Actions While Students are Working
Teachers should perform their own trial runs of this activity before
using it in class. Teachers will notice that there are a number of
outcomes (scenarios), i.e., over successive generations, Cheez-its will
decline in numbers, reach a plateau in numbers, or go extinct. In this
last scenario (Cheez-it extinction), predators must eat Goldfish to
survive. In all scenarios, the Goldfish increase in numbers. Ask
students if they feel this class activity demonstrates “survival of the
fittest.”
Further Extensions
For an additional scenario, teachers can ask students to consider another trait of Cheez-its: the fact that
they are like Puffer Fish and have excellent defenses against predation. They may be slow swimmers, but
their rigid, spiny bodies can puff up to a large size if attacked, making them very difficult to get a hold of
and bite, AND they are poisonous when eaten. Have students predict the outcome (numbers of Goldfish
and Cheez-its) if the predators can’t eat the Cheez-its if they are caught, or if the predator eats a Cheezit, the predator dies. Have students predict what will happen to the numbers of predators (the size of
predator population) under this scenario.
Modifications for English Learners
Add modifications? Clarifications?
Pitfalls to Watch Out for When Teaching

Students need to work collaboratively in their group and should not eat too many crackers or they
won’t get through four generations. This needs to be clarified!

Misconceptions: Focus on the fact that natural selection acts on populations NOT individuals.

Make sure teachers understand that the Cheez-its and Goldfish represent genetic (and phenotypic)
variants of a population of a single species.

Define population vs. species (another misconception). By setting up Cheez-its and Goldfish as
separate species, teachers are not demonstrating the processes of both natural selection and
ecosystem dynamics.
Vocabulary
prey, predator, traits, characteristic, species, population, natural selection, fitness, survival of the fittest,
generation, body morph, genetic variation
Integrated Middle School Science Partnership
Page 4 of 4
Plus 3-Page
Student Lab
Natural Selection Instructional Case
A series of 3 student-centered science lessons
Lesson 2 Goldfish vs. Cheez-its
Data Table
Generation
Number of
Goldfish
Number of
Cheez-its
Ratio of
Goldfish:Cheez-its
1
2
3
4
Predict: What will happen to the two fish types over the course of four generations?
Integrated Middle School Science Partnership
Student Lab
Page 1 of 3
Natural Selection Instructional Case
A series of 3 student-centered science lessons
Lesson 2 Goldfish vs. Cheez-its
Graph Name ___________________
Number of Generations
How did the 2 types of fish change over 4 generations?
How do you think these 2 fish types would change over 10 generations?
Integrated Middle School Science Partnership
Student Lab
Page 2 of 3
Natural Selection Instructional Case
A series of 3 student-centered science lessons
Integrated Middle School Science Partnership