AP Biology, AP exams, 13–20%

Unit 7: Natural Selection

Unit 7 carries the widest weighting range in the course. It covers the mechanisms of evolutionary change, population genetics and Hardy-Weinberg calculations, the evidence for common ancestry, phylogenetic tree reading, and speciation and extinction. Quantitative work here is heavier than in most other units.

Position: 7 of 8 units
Published weighting: 13–20% in the official framework
Topics covered: 7 topics in this guide

What is covered

Topics inside this unit.

These topics describe the scope students most often review for this part of AP Biology. Treat them as a study checklist rather than an exhaustive list of what a single exam form can sample.

Natural and artificial selection: The conditions required for natural selection, and how artificial selection demonstrates the same mechanism.
Population genetics: Allele and genotype frequencies, genetic drift, gene flow, and how population size affects variation.
Hardy-Weinberg equilibrium: The five conditions, the p and q equations, and using a departure from equilibrium as evidence of evolution.
Evidence of evolution and common ancestry: Fossil, morphological, biochemical, and molecular evidence, including conserved genes across distant taxa.
Phylogeny and cladistics: Reading and constructing trees from shared derived characters, and what node placement does and does not imply.
Speciation and extinction: Reproductive isolation, allopatric and sympatric speciation, adaptive radiation, and the conditions driving extinction.
Origin of life on Earth: The major hypotheses and the experimental evidence used to evaluate abiotic synthesis of organic molecules.

Study tips

How to practice Natural Selection.

Section-specific guidance that follows from how this material is usually tested.

Do Hardy-Weinberg problems daily in short bursts; the algebra is simple but the setup and interpretation are where points move.
Practice building a cladogram from a character table, then defend one node in a sentence.
Give this unit the largest share of review time; it carries the widest published weighting range of the eight units.

Where this fits

Unit 7: Natural Selection is 7 of 8 units in the AP Biology guide, and it carries a published weighting of 13–20% in the official framework. Use the full guide to balance it against everything else you have to review.

Back to the AP Biology guide Official exam information

Sample Natural Selection questions

Practice questions, not yet expert-reviewed

Original practice questions written to the published framework for this section. No expert has reviewed them, and they are not taken from any official exam. Try each one before you open the answer.

Question 1Foundational
In the Hardy-Weinberg equation, what does the term 2pq represent for a population at equilibrium?
1. AThe frequency of the dominant allele
2. BThe frequency of homozygous dominant individuals
3. CThe frequency of homozygous recessive individuals
4. DThe frequency of heterozygous individuals
Show answer
Answer: D. The frequency of heterozygous individuals
Genotype frequencies are p squared for homozygous dominant, 2pq for heterozygous, and q squared for homozygous recessive, and the factor of two accounts for the two ways a heterozygote can be assembled. Confusing 2pq with the dominant allele frequency is the common error: p by itself is an allele frequency, while 2pq counts individuals.
Key insight: p and q count alleles; p squared, 2pq, and q squared count individuals and must sum to 1.
Question 2Foundational
What is the term for a change in allele frequencies from one generation to the next caused purely by chance, with the largest effect in small populations?
1. AGenetic drift
2. BGene flow
3. CNatural selection
4. DAdaptive radiation
Show answer
Answer: A. Genetic drift
Genetic drift is random sampling error in which alleles get passed on, so it can eliminate or fix alleles regardless of whether they are beneficial. Gene flow is the tempting distractor, but it involves alleles moving between populations rather than random sampling within one. Natural selection is nonrandom by definition, since it depends on differences in fitness.
Key insight: Drift is random and strongest in small populations; selection is nonrandom and depends on fitness.
Question 3Intermediate
Dolphins and sharks are only distantly related, yet both evolved streamlined bodies and paddle-shaped fins in the open ocean. What term describes features that are similar in function but were not inherited from a shared ancestor?
1. AHomologous structures
2. BVestigial structures
3. CAnalogous structures
4. DShared derived characters
Show answer
Answer: C. Analogous structures
Convergent evolution produces analogous structures when similar selective pressures shape unrelated lineages in the same way. Homologous structures are the tempting distractor, but those are inherited from a common ancestor and may serve very different functions, as with a bat wing and a whale flipper. Only homologous features are evidence of shared ancestry, which is why analogous features are excluded when building a phylogeny.
Key insight: Shared function is not evidence of relatedness; shared underlying structure is.
Question 4Intermediate
What type of speciation occurs when a new river divides one population into two groups that then diverge until they can no longer interbreed?
1. ASympatric speciation
2. BAllopatric speciation
3. CArtificial selection
4. DConvergent evolution
Show answer
Answer: B. Allopatric speciation
Allopatric speciation begins with geographic isolation, which stops gene flow so that drift and different selective pressures can push the two groups apart. Sympatric speciation is the tempting distractor, but it happens without any physical barrier, through mechanisms such as polyploidy or habitat specialization within the same range. Convergent evolution describes unrelated lineages becoming similar, which is the opposite pattern.
Key insight: Allopatric needs a barrier; sympatric happens in the same place through reproductive isolation.
Question 5Advanced
In a population of 500 individuals in Hardy-Weinberg equilibrium, 20 individuals are homozygous recessive for a trait. How many individuals are expected to be heterozygous?
1. A32
2. B80
3. C160
4. D320
Show answer
Answer: C. 160
The homozygous recessive frequency is 20 divided by 500, which is 0.04, so q is the square root of 0.04, or 0.2, and p is 0.8. Heterozygote frequency is 2pq, or 2 times 0.8 times 0.2, which equals 0.32, and 0.32 of 500 is 160 individuals. Answering 320 reports the homozygous dominant group instead, because p squared is 0.64 and 0.64 of 500 is 320.
Key insight: Always start Hardy-Weinberg problems from q squared, because the recessive phenotype is the only genotype you can count directly.

Continue through AP Biology

Practice Natural Selection deliberately.

Create your Arcade Academy workspace to turn this topic list into spaced-repetition reviews, linked notes, and progress you can act on.

Create your account

Unit 7: Natural Selection