AP Biology Unit 7 review: Natural Selection
Unit 7 explains changes in population allele frequencies, evidence for common ancestry and the origin of species. It is 13–20% of multiple choice. Expect to justify evolutionary claims with reproductive success, population data or branching relationships, and to distinguish selection from chance.
Unit 7 on the May 2027 exam
| 2027 exam | |
|---|---|
| Weight | 13–20% of the multiple-choice section |
| Exam date | Monday, May 3, 2027, afternoon session |
| Multiple choice | 60 questions in 90 minutes, worth 50% of the exam score |
| Free response | 6 questions in 90 minutes, worth 50%: two long questions (9 points each) and four short questions (4 points each). |
| Calculator | Both sections allow a four-function calculator with square root or a nongraphing scientific calculator. Bluebook includes Desmos scientific. Handheld graphing calculators are not allowed in 2027. |
| Class time | About 19–21 class periods in College Board's pacing guide |
| 2026 results | 15% of students scored a 5 on the whole exam, and about 71% scored 3 or higher |
Sources: College Board's course and exam description, exam page, 2027 exam dates and score distributions.
What Unit 7 covers
College Board's course description splits the unit into 12 topics. The second column is what you should be able to do for each one.
| Topic | What you should be able to do |
|---|---|
| 7.1 Introduction to Natural Selection | Connect inherited variation and differential reproduction to population change. |
| 7.2 Natural Selection | Predict how an environment changes the reproductive success of different phenotypes. |
| 7.3 Artificial Selection | Explain how human choices change inherited traits over generations. |
| 7.4 Population Genetics | Distinguish selection, drift, gene flow and mutation as causes of allele-frequency change. |
| 7.5 Hardy–Weinberg Equilibrium | Calculate allele and genotype frequencies and evaluate the assumptions of the equilibrium model. |
| 7.6 Evidence of Evolution | Use fossils, molecular similarities and anatomical evidence to support evolution. |
| 7.7 Common Ancestry | Explain conserved biological features as evidence of shared ancestry. |
| 7.8 Continuing Evolution | Analyze contemporary evidence that populations continue to evolve. |
| 7.9 Phylogeny | Read common ancestors and shared derived traits from a phylogenetic tree. |
| 7.10 Speciation | Explain reproductive isolation and the formation of separate species. |
| 7.11 Variations in Populations | Connect genetic diversity to a population’s ability to respond to environmental change. |
| 7.12 Origins of Life on Earth | Evaluate models and evidence for early organic chemistry and the origins of life. |
Identify the mechanism before explaining the frequency change
More common does not automatically mean better adapted. Ask what caused the change.
| Mechanism or model | Evidence | Inference |
|---|---|---|
| Natural selection | Heritable variants leave different numbers of surviving offspring | Alleles associated with greater reproductive success can increase. |
| Genetic drift | Random survival or sampling, especially after a bottleneck | Frequency changes need not improve adaptation. |
| Gene flow | Individuals or gametes move between populations | Migration can introduce alleles and reduce differences. |
| Mutation | A DNA sequence changes | New alleles arise without regard to what organisms need. |
| Hardy–Weinberg baseline | Large randomly mating population without selection, migration or mutation | p + q = 1; p² + 2pq + q² = 1. |
| Phylogeny | Two groups share a more recent common ancestor | They are closer relatives regardless of tip order. |
Common mistakes
Each of these loses points on questions you otherwise understand.
- Saying an organism evolved because it needed a trait. Heritable variants differ in reproductive success; populations change across generations. CED printed p. 127 flags this reasoning error.
- Using fitness as a synonym for strength. Specify relative reproductive success in the stated environment. CED p. 127.
- Confusing q with q². Under Hardy–Weinberg, the recessive allele frequency is q and the recessive homozygote frequency is q². CED p. 127.
- Combining all reproductive barriers into one explanation. A temporal barrier prevents mating; hybrid sterility acts after fertilization. CED p. 127 and topic 7.10.
6 practice questions with answers
Written by Ashore for this page, in the style of the multiple-choice section. Try each one before you open the answer.
Question 1 · 7.5
A population in Hardy–Weinberg equilibrium has a recessive homozygote frequency of 0.16. What fraction is heterozygous?
- A0.16
- B0.24
- C0.40
- D0.48
Show the answer
D. q² = 0.16 gives q = 0.4 and p = 0.6. Heterozygotes have frequency 2pq = 2(0.6)(0.4) = 0.48. The recessive phenotype frequency is not the recessive allele frequency.
Question 2 · 7.4
Among 100 diploid organisms, 25 are AA, 50 are Aa and 25 are aa. What is the frequency of A?
- A0.25
- B0.50
- C0.75
- D1.00
Show the answer
B. There are 200 allele copies. AA organisms contribute 50 A copies and heterozygotes contribute 50, giving 100/200 = 0.50. This count does not require assuming equilibrium.
Question 3 · 7.2
Before a pesticide is applied, a small fraction of insects carry an inherited resistance allele. Survivors leave offspring that are mostly resistant. What best explains the change?
- AResistant insects left more descendants after treatment
- BEach exposed insect acquired the same needed mutation
- CPesticide converted every susceptible allele into resistance
- DThe population stopped producing genetic variation
Show the answer
A. Resistance existed before treatment. Differential survival and reproduction changed allele frequencies. Selection acts on existing variation; the observation does not show directed mutation.
Question 4 · 7.4
A storm kills most island lizards without regard to color. Color-allele frequencies differ sharply among survivors. Which process best fits?
- AArtificial selection
- BDirectional selection
- CGenetic drift
- DAssortative mating
Show the answer
C. The survival event was random with respect to the trait and greatly reduced population size. Sampling by chance can change allele frequencies, a bottleneck form of genetic drift.
Question 5 · 7.9
A rooted tree has topology ((A,B),(C,D)). Which pair shares the most recent common ancestor?
- AA and C
- BB and D
- CA and D
- DC and D
Show the answer
D. C and D meet at an immediate shared node, while the cross-group pairs meet only at the older root. Rotating branches around a node would not change this relationship.
Question 6 · 7.10
Two frog populations breed in the same pond but in different months. What is the reproductive barrier?
- APostzygotic hybrid sterility
- BPrezygotic temporal isolation
- CPostzygotic hybrid inviability
- DPrezygotic mechanical isolation
Show the answer
B. Different breeding times prevent mating before a zygote forms. This is temporal isolation. Hybrid sterility and inviability require fertilization to have occurred.
How to study Unit 7
Write an evolutionary explanation in three steps: the inherited variation, the difference in reproductive success and the change over generations. If the scenario specifies random deaths, test a drift explanation before attributing the outcome to selection.
Practice both direct allele counting and Hardy–Weinberg predictions. Label every frequency as allele or genotype. On phylogenetic trees, circle shared nodes before comparing relatives, then redraw the same tree with rotated branches to check that your interpretation stays the same.
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