AP Biology Unit 7: Natural Selection — Evolution in Action

TLDR (Quick Summary)

• Natural selection drives evolution by favoring heritable traits that increase survival or reproduction.

• Evolution occurs when allele frequencies change in a population over time.

• Hardy-Weinberg equations predict equilibrium (no evolution).

• Mechanisms: natural selection, genetic drift, gene flow, mutation, sexual selection.

• ⚡ Download our AP Biology Unit 7 Cheat Sheet for graphs, formulas, and key mnemonics.

Why Unit 7 Matters

This is one of the most tested units on the AP Biology exam. Natural selection connects everything you’ve learned — genetics, ecology, and molecular biology — into one unifying concept: evolution explains biological diversity.

If you understand how traits spread or disappear over generations, you can handle any question about adaptation, allele frequencies, or population change.

Natural Selection: The Core Idea

Charles Darwin’s insight was simple yet powerful — individuals don’t evolve, populations do.

The Three Requirements for Natural Selection:

1. Variation: Individuals differ in traits.

2. Heritability: Traits can be passed to offspring.

3. Differential Survival/Reproduction: Those with favorable traits leave more offspring.

Mnemonic: “VHS → Variation, Heritability, Selection.”

Result? Over generations, advantageous traits become more common — this is evolution in action.

Evidence for Evolution

AP exams often ask you to connect evidence to mechanisms. Here’s what to know:

• Fossils: Show transitional species and gradual change.

• Homologous structures: Same origin, different function (bat wing vs human arm).

• Analogous structures: Different origin, same function (bird vs insect wing).

• Molecular data: DNA/protein similarities indicate relatedness.

• Biogeography: Species distribution aligns with continental drift and isolation.

Types of Selection

1. Directional Selection: favors one extreme (e.g., antibiotic resistance).

2. Stabilizing Selection: favors average (e.g., human birth weight).

3. Disruptive Selection: favors both extremes (e.g., rock pocket mice in mixed terrain).

Mnemonic: “D-S-D → Directional, Stabilizing, Disruptive.”

👉 These can be drawn as shifting, narrowing, or splitting bell curves — a favorite AP visual question.

Hardy-Weinberg: The Equation of No Evolution

If nothing changes, allele frequencies stay constant.

Formulas:
p + q = 1
p² + 2pq + q² = 1

• p = dominant allele frequency

• q = recessive allele frequency

• p² = homozygous dominant

• 2pq = heterozygous

• q² = homozygous recessive

Conditions: no mutation, no migration, random mating, large population, no natural selection.

Mnemonic: “M-M-R-L-S → My Mom Raises Large Snacks.”

Mechanisms of Evolution

1. Natural selection — traits improve survival/reproduction.

2. Genetic drift — random change (bottleneck or founder effect).

3. Gene flow — movement of alleles between populations.

4. Mutation — introduces new variation.

5. Sexual selection — mates choose based on traits.

Tutor Tip: Always link the mechanism to allele frequency change. AP graders want to see that connection explicitly.

Speciation: How New Species Form

• Allopatric speciation: geographic barrier splits populations (e.g., islands).

• Sympatric speciation: occurs without a barrier (e.g., polyploidy in plants).

• Reproductive isolation: prevents mating or producing fertile offspring.

Mnemonic: “Allo = Apart, Sym = Same.”

Real-World Examples

• Peppered moths: dark moths survived better in polluted cities (directional selection).

• Darwin’s finches: drought favored birds with larger beaks (natural selection in real time).

• Antibiotic resistance: bacteria evolve through selection pressure from medication.

Common Pitfalls

• Saying individuals evolve (only populations do).

• Forgetting that Hardy-Weinberg represents no evolution.

• Confusing gene flow (migration) with drift (random).

• Not mentioning allele frequency when defining evolution.

Start with our free Unit 7 Cheat Sheet and get personalized support from Ivy-League tutors. Book your free AP Biology strategy session today.