Practice a College Board-style free response question on Natural Selection. Write your response, then reveal the model answer to see exactly what earns each point.
Free Response Question · Unit 7 · Hardy-Weinberg Equilibrium and Selection
In a population of 1,000 beetles, body color is determined by a single gene with two alleles: B (dark, dominant) and b (light, recessive). A survey finds 360 light-colored (bb) beetles in the population. Researchers assume the population is in Hardy-Weinberg equilibrium for this gene.
Genotype
Phenotype
Number observed (of 1,000)
BB
Dark
?
Bb
Dark
?
bb
Light
360
A
Using the data and the Hardy-Weinberg equations, calculate the frequency of the recessive allele (q) and the dominant allele (p) in this population. Show your work.
✓ Model answer (earns the point)
The frequency of the bb genotype (q²) = 360/1000 = 0.36. So q = √0.36 = 0.6. Since p + q = 1, p = 1 − 0.6 = 0.4.
Why it scores: Correctly identifies q² = 0.36 from the observed bb frequency, correctly takes the square root to find q = 0.6, AND correctly uses p + q = 1 to find p = 0.4. Showing each step explicitly earns full credit even if presented slightly differently.
B
Using your answer from Part A, calculate the expected number of heterozygous (Bb) beetles in this population of 1,000, assuming Hardy-Weinberg equilibrium.
✓ Model answer (earns the point)
The heterozygous frequency is 2pq = 2(0.4)(0.6) = 0.48. In a population of 1,000, the expected number of Bb beetles is 0.48 × 1,000 = 480 beetles.
Why it scores: Correctly calculates 2pq using the p and q values from Part A, AND correctly multiplies that frequency by the total population size (1,000) to get a count (480), not just a frequency.
C
A new predator moves into the beetles' habitat and preferentially eats light-colored (bb) beetles because they are easier to spot against the dark soil. Predict how the allele frequencies of B and b will change over the next several generations, and explain why, in terms of natural selection.
✓ Model answer (earns the point)
The frequency of the b allele will decrease, and the frequency of the B allele will increase over successive generations. Because the predator selectively removes light-colored (bb) individuals before they can reproduce, those individuals contribute fewer b alleles to the next generation's gene pool. Dark-colored individuals (BB and Bb) survive and reproduce at a higher relative rate, passing on more B alleles. This is natural selection: a heritable trait (color, determined by genotype) is affecting differential reproductive success in a way directly tied to the new selective pressure (the predator), shifting allele frequencies away from Hardy-Weinberg equilibrium over time.
Why it scores: Correctly predicts the direction of allele frequency change for BOTH alleles, AND explains the mechanism (bb individuals removed before reproducing → fewer b alleles passed on), AND explicitly names this as natural selection acting on heritable variation.
How to score points on AP Biology FRQs
Use the data when it's provided. If there's a table or graph in the stimulus, reference specific values. The graders look for it.
Show your work on math questions. Hardy-Weinberg FRQs award partial credit for correct setup even if the final number is off — write out p, q, p², 2pq, and q² explicitly.
Answer the verb in the prompt. "Describe" needs an observation. "Explain" needs a mechanism. "Predict and justify" needs a prediction AND the biology behind it.
Trace cause to effect. Don't just state outcomes — connect them. "Predator removes bb beetles → fewer b alleles passed on → b frequency decreases" is a strong causal chain.
For selection questions, name the trait, the pressure, and the mechanism. AP graders want to see WHAT trait is selected, WHAT is doing the selecting, and HOW that changes allele frequencies — not just "evolution happens."