PhET Activity for MS-LS2-1: Natural Selection

DIRECTIONS:
1. Click on the 'Intro' option in the center of the screen to begin.
2. Locate the single white rabbit in the equatorial environment.
3. Click the 'Add a Mate' button in the center of the simulation area to add a second rabbit.
Spend 2-3 minutes exploring the simulation. Press the play/pause button to see the rabbits move and reproduce. Observe the graph at the bottom of the screen but do not change any settings yet.

QUESTION: Are you acquainted with the tools in this simulation?

• Yes

• No

DIRECTIONS:
1. Make sure that you have two white rabbits in the environment from the previous step. (add mate).
2. Click the play button at the bottom center and let the simulation run until it reaches Generation 3. You can use the fast-forward button to speed this up.

QUESTION: Without any predators or food limitations, what happens to the rabbit population over the first three generations?

• The population grows exponentially.
• The population stays the same.
• The population decreases.
• The population grows and then crashes.

Answer explanation

The population grows exponentially because there are no limiting factors to control its growth. All offspring survive and reproduce.

DIRECTIONS:
1. Pause the simulation if it is running.
2. In the 'Add Mutation' panel on the right, click the 'Fur' button to introduce the brown fur mutation.

QUESTION: The introduction of brown fur into the population of white rabbits is an example of genetic (a)   .

• variation
• selection
• extinction
• competition

Answer explanation

The correct answer is 'variation'. Mutations are the ultimate source of new genetic variations in a population.

DIRECTIONS:
1. Press the play button and let the simulation run for two more generations (up to Generation 5).

QUESTION: In this environment with no predators, what do you predict will happen to the proportion of brown-furred rabbits compared to white-furred rabbits over many generations? Explain your reasoning.

Answer explanation

A good answer should predict that the proportions of brown and white rabbits will fluctuate randomly or stay relatively stable. The reasoning should state that since neither fur color provides a survival advantage (no camouflage is needed), both traits are equally likely to be passed on.

DIRECTIONS:
1. Pause the simulation.
2. In the 'Environmental Factors' panel on the right, check the box for 'Wolves'.
3. Press play and observe what happens.

QUESTION: In the equatorial environment, it is harder for white rabbits to ___, making them easier for wolves to spot. This environmental pressure causes the population of white rabbits to ___, while the brown rabbit population has a better chance of survival.

(a)  

Answer explanation

camouflage, decrease

DIRECTIONS:
1. Let the simulation run for several more generations with the wolves present.

QUESTION: What is the main cause for the increase in the brown rabbit population after wolves were introduced?

• The brown fur trait provides better camouflage, leading to higher survival and reproduction rates.
• The wolves prefer the taste of white rabbits.
• The brown rabbits reproduce faster than the white rabbits.
• The brown fur mutation started happening more frequently after wolves appeared.

Answer explanation

The correct answer is that brown fur provides a survival advantage through camouflage. This is a core concept of natural selection: a trait that helps an organism avoid predation is more likely to be passed on.

DIRECTIONS:
1. Pause the simulation.
2. In the panel to the right of the graph, select the 'Pedigree' radio button.
3. The graph area will now say 'Select a Bunny'. Click on one of the living rabbits in the simulation area.

QUESTION: Arrange the following steps in the correct order to view a rabbit's genetic history.

• Pause the simulation.
• Click on a specific rabbit in the environment.
• Analyze the displayed family tree and alleles.
• Select the 'Pedigree' radio button.

Answer explanation

1. Pause the simulation. 2. Select the 'Pedigree' radio button. 3. Click on a specific rabbit in the environment. 4. Analyze the displayed family tree and alleles.

DIRECTIONS:
1. If necessary, let the simulation run until you have at least one white rabbit born from two brown-furred parents.
2. Pause the simulation.
3. Select the 'Pedigree' view.
4. Click on the white rabbit that has two brown parents.

QUESTION: Using the evidence from the pedigree chart, explain how it is possible for two brown rabbits to have a white-furred offspring.

Answer explanation

A good answer will explain that both brown parents must be heterozygous, meaning they each carry one dominant brown allele (F) and one recessive white allele (f). Their phenotype is brown, but they are carriers of the white trait. There is a 25% chance that both parents pass on their recessive 'f' allele to an offspring, resulting in an 'ff' genotype, which expresses the recessive white fur phenotype.

DIRECTIONS:
1. In the panel to the right of the graph, select the 'Proportions' radio button.
2. Use the slider at the bottom of the proportions view to look at different generations.

QUESTION: In the Proportions view, you observe that the percentage of brown rabbits increases from the start to the end of a generation. What does this indicate?

• More brown rabbits were born than white rabbits.
• White rabbits were eaten by wolves at a higher rate than brown rabbits.
• The total population of rabbits increased.
• The total population of rabbits decreased.

Answer explanation

This indicates that within that generation, a higher proportion of white rabbits were removed from the population (eaten) than brown rabbits, causing the relative frequency of the brown rabbit trait to increase.

DIRECTIONS:
1. Ensure wolves are still present in the simulation.
2. At the top center of the simulation, click the 'Arctic' button to change the environment.
3. Ensure the 'Population' radio button is selected to view the graph.

QUESTION: Now that the environment is arctic, which fur color provides the best camouflage?

• Brown fur, because it is the dominant allele.
• White fur, because it blends in with the snow.
• Neither, because the wolves will eat both equally.
• Brown fur, because they are used to surviving.

Answer explanation

White fur now provides the best camouflage. The selective pressure has reversed; now brown rabbits are easier for predators to spot against the white snow.