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Unit 1 Exam Review: Natural Selection

Scaffolded review of the Lessons 1–4 assessment

Lesson objective

Students will review the core ideas of natural selection (variation, heritability, selection pressure, fitness, and differential survival and reproduction) through scaffolded questions that build toward each exam item. By the end of the lesson, students will be able to analyze population data to calculate and describe changes in trait frequency, tell natural selection apart from chance, and correct common misconceptions. Students will then use claim–evidence–reasoning to explain how natural selection causes a population to become adapted to its environment over generations.

Answer each step in order, then try the Exam check. Answers save on this device as you work; each completed part also saves to the teacher dashboard. Use Next or the arrow keys to move between questions.

Key vocabulary

Use these words in every answer today.

1
Variation

Differences in a trait among individuals in a population

2
Heritable

Can be passed from parents to offspring through genes

3
Selection pressure

A factor in the environment that affects which individuals survive and reproduce

4
Fitness

How well an organism survives and produces offspring that also survive

5
Adaptation

A heritable trait that becomes more common because it helps survival and reproduction

6
Frequency

How common a trait is in a population, often as a percent

Part I: Natural Selection Foundations

Questions 1–8. Variation, inheritance, competition, selection pressure, fitness, and differential survival and reproduction.

Question 1: Identifying variation

Part I
  1. a
    In your own words, what does variation mean?
  2. b
    Variation is about differences in a trait (a physical feature like size, color or shape). Is "where a lizard lives" a trait of the lizard's body?
  3. c
    In a population of desert lizards, some have short, medium and long legs. What trait is different?
  4. d
    Is "all lizards need food and water" a difference or a similarity?

Exam check Which statement identifies the variation in the lizard population? Explain why the other choices are not variation.

Question 2: Naming the selection pressure

Part I
  1. a
    Define selection pressure:
  2. b
    Is a selection pressure part of the organism or part of the environment?
  3. c
    During a drought, only hard seeds are left for a bird population. What changed in the birds' environment?
  4. d
    Can a bird's "desire to survive" change which birds live? Why or why not?

Exam check In the drought, which factor is acting as the selection pressure?

Question 3: Why heritable variation matters

Part I
  1. a
    What does heritable mean?
  2. b
    A mouse loses its tail in an accident. Will its babies be born without tails? . Why?
  3. c
    If every individual were identical, could the environment "favor" some over others?
  4. d
    If a helpful trait is NOT passed to offspring, will it become more common in the next generation?
  5. e
    Does the environment create new traits when they are needed, or does it act on traits already present?

Exam check Explain why natural selection needs individuals that differ in traits that can be passed to offspring.

Question 4: Overproduction and competition

Part I
  1. a
    Most species produce more offspring than can support.
  2. b
    Name three resources that organisms need and that can run out: , ,
  3. c
    When there are more individuals than resources, what happens between individuals?
  4. d
    Will every offspring survive?

Exam check What is the most likely result when a population produces more offspring than the environment can support?

Question 5: What fitness really means

Part I
  1. a
    In everyday life, "fit" means strong or athletic. In biology, fitness measures success at .
  2. b
    Organism A lives 10 years and has 1 offspring. Organism B lives 3 years and has 8 offspring that survive. Which has higher fitness?
  3. c
    Why doesn't a long life alone mean high fitness?

Exam check In evolutionary biology, an organism has high fitness when it ….

Question 6: Correcting the "needs" misconception

Part I
  1. a
    Can an animal grow a new inherited trait just because it needs one?
  2. b
    Before the weather got colder, did all rabbits have the same fur thickness? (Think: variation.)
  3. c
    When it gets cold, which rabbits are more likely to survive the winter?
  4. d
    Which rabbits will have more babies? What fur will those babies tend to have?

Exam check A student says, "When the environment became colder, the rabbits grew thicker fur because they needed it." Rewrite this statement so it is correct.

Question 7: Evidence that selection happened

Part I
  1. a
    Does natural selection change one individual or a whole population?
  2. b
    Does it happen in one lifetime or over many generations?
  3. c
    When an individual gets bigger as it grows up, is that evolution?
  4. d
    If a helpful heritable trait shows up in 10% of a population, then 30%, then 60% over several generations, what is happening to its frequency?

Exam check Which observation would be the best evidence that natural selection has occurred in a population?

Question 8: Individuals vs. populations

Part I
  1. a
    Individuals (are selected / evolve). Populations (are selected / evolve).
  2. b
    Individuals with different traits survive and at different rates.
  3. c
    Because of (b), the offspring generation has more of the trait.
  4. d
    Do all individuals develop the same adaptation at once?

Exam check Describe how what happens to individuals causes a population to change over generations.

Part II: Desert Mice

Questions 9–15. Desert mice live near new dark volcanic rock. Fur color varies naturally. Hawks hunt by sight, so mice that stand out against the ground are easier to catch.

Light and dark desert mice on pale sand and dark volcanic rock, with a hawk overhead
Predators more easily spot mice that do not blend in with their environment.

Question 9: Reading the table

Part II
  1. a
    How many dark-fur mice are in Generation 1? Generation 4?
  2. b
    How many light-fur mice are in Generation 1? Generation 4?
  3. c
    Which number went up? Which went down?

Exam check Which fur-color trait increased in frequency from Generation 1 to Generation 4?

Question 10: Explaining the change

Part II
  1. a
    What color is the ground in this habitat?
  2. b
    Which mice blend in (are camouflaged) on that ground?
  3. c
    Which mice are easier for hawks to see and catch?
  4. d
    Mice that survive get to reproduce. What fur color will most of their offspring inherit?
  5. e
    Did any mouse change its fur color during its life? . Did the rocks create new fur colors?

Exam check Which explanation best accounts for the change shown in the table?

Question 11: Calculating percent

Part II
  1. a
    Write the formula: percent = ( ÷ ) × 100
  2. b
    Practice: a class has 30 students and 6 wear glasses. What percent wear glasses?
  3. c
    Generation 1: dark-fur mice = , total = . Percent =
  4. d
    Generation 4: dark-fur mice = , total = . Percent =

Exam check Calculate the percentage of dark-fur mice in Generation 1 and Generation 4. Show your work.

Question 12: Describing a trend

Part II
  1. a
    A trend describes the overall direction of data over time. List the dark-fur numbers in order: , , ,
  2. b
    Is the dark-fur number increasing, decreasing or staying the same?
  3. c
    What happens to light-fur mice at the same time?
  4. d
    Sentence frame: "From Generation 1 to Generation 4, dark-fur mice from % to %, while light-fur mice from % to %."

Exam check Describe the overall trend in fur color from Generation 1 to Generation 4.

Question 13: Selection pressure in the data

Part II
  1. a
    Look back at Question 2. A selection pressure is something in the that affects who survives.
  2. b
    In this scenario, what is killing some mice more than others?
  3. c
    Does the age of the rock or the number of generations decide which mouse gets eaten?

Exam check If hawks are the main predator, which factor most directly represents the selection pressure?

Question 14: Using numbers as evidence

Part II
  1. a
    A claim is a statement you are trying to prove. The claim here is: dark fur gave a survival advantage. What would you expect to happen to the number of dark mice if this claim is true?
  2. b
    Find one number (or one change between two numbers) that shows that happened:
  3. c
    Strong evidence includes a number AND what it is compared to. Sentence frame: "Dark-fur mice increased from in Generation to in Generation ."

Exam check Use one piece of numerical evidence from the table to support the claim that dark fur provided a survival advantage.

Question 15: Predicting Generation 5

Part II
  1. a
    How much did dark-fur mice increase each generation? Gen 1→2: Gen 2→3: Gen 3→4:
  2. b
    If hawks and dark rock stay the same, will dark fur still be an advantage?
  3. c
    Can the dark-fur number go above 100 in a population of 100? . So what is a reasonable range for Generation 5?
  4. d
    Will light fur likely disappear right away, or keep shrinking? Why?

Exam check If the habitat and predation stay similar, predict what could happen to the dark-fur trait in Generation 5. Explain your prediction using the data.

Part III: Adaptation and Explanation

Questions 16–20. Shorebird beaks, before-and-after data, selection vs. chance, and a full claim–evidence–reasoning response.

Question 16: Finding the advantageous trait

Part III
  1. a
    Where is the food now?
  2. b
    Which body part does a bird use to reach food in the sand?
  3. c
    Which version of that trait reaches deeper?
  4. d
    Do wing size or feather color help a bird dig deeper?

Exam check Which trait is advantageous after the drought?

Question 17: What adaptation looks like

Part III
  1. a
    Can a bird stretch its beak longer during its life and pass that on? (Hint: Question 3b.)
  2. b
    Which birds get more food and have more chicks?
  3. c
    What beak length will most chicks inherit?
  4. d
    Over several generations, what happens to the average beak length of the population?

Exam check Which outcome would best show that the shorebird population became adapted to the new conditions?

Question 18: Inferring from before-and-after data

Part III
  1. a
    Calculate the change for each beak size (after minus before). Small: Medium: Large:
  2. b
    Which beak size increased the most? Which decreased the most?
  3. c
    Think of tools: a small beak is like tweezers, a large beak is like a nutcracker. Which beak is better for cracking large, hard seeds?
  4. d
    An inference is a conclusion based on evidence. If large beaks were favored, what kind of seeds were probably left during the drought?
  5. e
    Sentence frame: "Large, hard seeds were probably favored because large beaks increased from % to %, while small beaks decreased from % to %."

Exam check Infer what type of food was probably favored during the drought and explain how the data support your inference.

Question 19: Selection vs. chance

Part III
  1. a
    In Population X, does beetle color affect which beetles survive?
  2. b
    In Population Y, does beetle color affect which beetles survive the flood?
  3. c
    Natural selection happens when a trait affects survival and reproduction in a consistent way.
  4. d
    When survival has nothing to do with traits, the change is due to .
  5. e
    Sentence frame: "Population shows natural selection because . Population changes by chance because ."

Exam check Which population is experiencing natural selection based on color, and which is changing mainly because of chance? Explain the difference.

Question 20: CER, Step 1

Part III

Return to the desert mice in Part II. Step 1: Gather the pieces of natural selection. Fill in each row for the desert mice.

Ingredient of natural selectionWhat it is for the desert mice
Variation
Heritability
Selection pressure
Differential survival
Differential reproduction
Change over generations

Question 20: CER, Steps 2–4

Part III

Step 2: Claim. Answer the question in one sentence. What happened to the population, and what caused it?

Sentence frame: "The desert mouse population became adapted to the dark-rock habitat through because ."

Claim:

Step 3: Evidence. Use at least two specific pieces of data.

• From the table (a number or percent):

• From the figure (what the picture or caption shows):

Step 4: Reasoning. Connect the evidence to the claim using science ideas. Use the Step 1 row words in order:

"Mice already varied in fur color. Hawks . Dark mice were , so they more often. They passed to offspring. Over generations, ."

Question 20: Put it together

Part III

Use your claim, evidence and reasoning from Steps 1–4 to write a complete answer.

Exam check Using evidence from the table and figure, explain how natural selection could cause the desert-mouse population to become adapted to the dark-rock habitat over several generations.

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