Unit 7: Natural Selection focuses on the evolutionary processes that shape life on Earth. It accounts for 8–11% of the AP Biology Exam and connects genetics, ecology, and molecular biology into one unifying theme: evolution as the core of biology.
In this RevisionDojo Unit 7 Review, we’ll cover:
- Darwin’s theory and the modern synthesis
- Evidence for evolution
- Mechanisms of evolution
- Hardy–Weinberg equilibrium
- Speciation and phylogenetics
- Common AP exam question types
- Study tips to maximize your points
Unit 7 Overview
The College Board organizes Unit 7 into four major themes:
- Natural selection and adaptation
- Population genetics
- Speciation and phylogenetic trees
- Evidence supporting evolution
1. Darwin’s Theory of Natural Selection
Key Ideas:
- Variation exists in populations
- More offspring are produced than can survive
- Individuals with advantageous traits are more likely to survive and reproduce
- Over time, favorable traits increase in frequency
2. Evidence for Evolution
- Fossil record: Shows transitional forms
- Comparative anatomy: Homologous vs. analogous structures
- Molecular evidence: DNA and protein sequence similarities
- Embryology: Developmental similarities across species
- Biogeography: Geographic distribution of species supports adaptive evolution
3. Mechanisms of Evolution
- Natural selection: Directional, stabilizing, disruptive
- Genetic drift: Bottleneck effect, founder effect
- Gene flow: Movement of alleles between populations
- Mutation: Creates new alleles and variation
- Nonrandom mating: Sexual selection affects allele frequencies
4. Hardy–Weinberg Equilibrium
Purpose: Model to test if evolution is occurring
Conditions:
- Large population size
- Random mating
- No mutation
- No migration
- No natural selection
Equation:
p² + 2pq + q² = 1
p + q = 1
AP Tip: Be ready to calculate allele and genotype frequencies.
5. Speciation
- Allopatric speciation: Geographic isolation
- Sympatric speciation: No physical barrier; often due to polyploidy or niche differentiation
- Reproductive barriers:
- Prezygotic (temporal, behavioral, mechanical)
- Postzygotic (hybrid inviability or sterility)
6. Phylogenetic Trees and Cladograms
- Show evolutionary relationships
- Nodes = common ancestors
- Closer branches = more related
- Use shared derived characteristics to group species
7. Connections to Other Units
- Unit 5: Inheritance provides raw material for evolution
- Unit 6: Mutations and gene regulation impact evolution
- Unit 8: Ecology influences selection pressures
Common AP Biology Exam Questions for Unit 7
Multiple Choice Examples:
- Identify the type of selection in a given population graph
- Calculate allele frequencies from genotype counts
FRQ Examples:
- Explain how genetic drift impacts endangered species
- Use a cladogram to infer evolutionary relationships
Study Tips for Unit 7
- Practice Hardy–Weinberg Problems: Get comfortable with p and q calculations.
- Memorize Selection Types: Know their graphs and examples.
- Use Real Data: Interpret fossil records or genetic similarity tables.
- Draw Cladograms: Practice constructing them from traits or DNA data.
Frequently Asked Questions
1. How much of the AP Bio exam is Unit 7?
About 8–11% of the test.
2. Do I need to memorize the Hardy–Weinberg formula?
Yes, but you should also understand what each term represents.
3. Are phylogenetic trees always tested?
Almost every year—be ready to interpret and construct them.
4. What’s the most challenging part of Unit 7?
Applying multiple evolutionary mechanisms to a real-world scenario.
Call to Action
Unit 7 ties together genetics, molecular biology, and ecology—making it one of the most integrative units on the AP Biology Exam.
👉 Use RevisionDojo’s Hardy–Weinberg practice sets and cladogram builder to sharpen your Unit 7 skills and get closer to a 5.
