Key concepts
Natural selection

From National Science Education Standards: Organisms and environments

Have you ever wondered why some animals are so well camouflaged in their environments? Have you ever seen a walking stick? It's an insect that looks just like a twig! How did it come to look so much like a stick? If walking sticks lived on sandy surfaces, would they be as hidden from their predators?

In different environments, certain appearances are more useful than others, especially when hiding from predators. For instance, an insect hiding on tree bark might look very different from one that is hiding on a leaf. What colors would be best for each of these bugs?

Over millions and millions of years, all of the different species have come to look and act the way they do through a process called evolution by natural selection. Because this process is so slow, it is often difficult for people to imagine how it works. The main premise is very simple, however: individuals with certain characteristics, or traits, are better adapted to live successfully in their environment. Individuals with these traits live longer and have more offspring, thus increasing the chance that their traits are handed down to future generations.

As a result of living and having offspring in different environments, different populations develop varied appearances over time. Today we will explore the idea of evolution by natural selection and will simulate how a population can change over time.

•    Five to 10 different colors of construction paper
•    Hole puncher
•    Bowl
•    Colored fabric or different color surface (optional)
•    Timer (optional)

•    Using the hole puncher, punch out about 10 paper dots from each piece of paper. Each dot represents a different colored animal within the "dot" species.
•    Mix all of the paper dots in a bowl.
•    Spread the dots evenly over a surface in front of you. This surface represents the dots' environment, the habitat in which they live.

•     You are a predator that uses vision to find your prey: paper dots.
•    Close your eyes.
•    Open your eyes and pick up the first dot that gets your attention.
•    Close your eyes again.
•    Open them again and pick up the first paper dot that gets your attention. Keep the dots that you pick in a pile off to the side.
•    Pick dots until you have "preyed upon" (picked up) about three quarters of the paper dots.

•    What dots are left? These dots have escaped the predator. Because they have survived, they can reproduce. For each dot remaining on the surface, punch out three more dots of the same color. The three new dots represent their offspring, which have the same traits and the same color. There should again be as many dots on the surface (or "habitat") as when you started.
•    Place the new "offspring" and "parent" dots in the bowl. Mix them up.
•    Again, spread the dots evenly over the surface and repeat the activity.
•    By the time you have done this three to four times, you have preyed upon several generations of paper dots and allowed only some of the population to survive and reproduce. Does the population of dots look different now than when you first started the activity?
•    Alternative: For a quicker version of this activity, you can spread out all of the dots on the tabletop or floor. Start the timer and have the "predator" pick up as many dots as he or she can in fifteen seconds. The dots that remain should be those that blend in to the environment the best. You can repeat as above, allowing these dots to reproduce offspring for the next generation.
•    Extra: For an even greater understanding of evolution by natural selection, repeat this experiment on a different colored surface (try using fabric or paper) and see if the population changes in a different way. How does the environment affect the traits that are common in a population?


Read on for observations, results and more resources.

Observations and results
How did the population of paper dots change? Did dots of a certain color become more common than dots of other colors? Which dot colors died out? What do you notice about the dots who survived and reproduced? Why were they able to avoid you, their predator? If their "habitat," (the surface) was a different color, would the same dots have survived and reproduced?

Natural selection is the process by which certain traits in a population become more common because individuals with those traits survive and reproduce. They pass on their traits to their offspring, making them more common in the population. There is a species of moth in England called the peppered moth, whose story helps to illustrate the idea of evolution by natural selection.

Peppered moths lived among birch trees. There are both white peppered moths and black peppered moths. Before the Industrial Revolution, white peppered moths were much more common. They blended in well on the white bark of the birch trees. They remained hard for their predators, birds, to detect. As a result, they survived longer and reproduced more than the black moths, so the light-color traits were much more common in the peppered moth population. However, when the Industrial Revolution occurred, soot from factories covered the birch trees' white bark, turning them black. When this happened, the white peppered moths were no longer camouflaged against the bark. Now they were easily spotted—and eaten–by the birds while the black peppered moths were better camouflaged. After several generations, black peppered moths became much more common.

When England passed its version of the clean air act, decreasing the amount of soot that factories were allowed to emit, the birch trees eventually returned to having white bark again. With your knowledge from this activity, can you hypothesize which colored moth eventually became more common?

Share your color changing dots observations and results! Leave a comment below or share your photos and feedback on Scientific American's Facebook page.

Recycle the paper dots and clear off the surface.

More to explore
"Animal Mimics: More than just camouflage" from Scientific American
"Ugly Animals Need Love, Too" from Scientific American
"Animal Mimicry" overview from Alleghany County Public Schools
If You Can't Run, You've Got to Hide! activity from the University of Richmond
Claws, Coats and Camouflage: The ways animals fit into their world by Susan E. Goodman, ages 4–8
Living Color by Steve Jenkins, ages 9–12

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