AP Biology 7.5 Hardy-Weinberg Equilibrium Study Notes - New Syllabus Effective 2025
AP Biology 7.5 Hardy-Weinberg Equilibrium Study Notes- New syllabus
AP Biology 7.5 Hardy-Weinberg Equilibrium Study Notes – AP Biology – per latest AP Biology Syllabus.
LEARNING OBJECTIVE
Describe the conditions under which allele and genotype frequencies will change in populations.
Key Concepts:
- Hardy-Weinberg Equilibrium
7.5.A – Conditions That Cause Allele & Genotype Frequencies to Change
🌱 What’s Meant by Frequency?
- Allele frequency = how common an allele is in a gene pool
- Genotype frequency = how common a specific combination of alleles is (like AA, Aa, or aa)
- If these frequencies change over generations, that’s evolution at the population level!
🔁 When Do These Frequencies Change?
Allele and genotype frequencies will change when one or more of the Hardy-Weinberg conditions are violated:
| ❌ Condition Violated | 🔍 Effect on Population |
|---|---|
| 1. Small population size | Leads to genetic drift (random changes in genes) |
| 2. Mutations occur | Introduce new alleles into the gene pool |
| 3. Migration happens | Causes gene flow, adding/removing alleles |
| 4. Non-random mating | Certain traits become more common (sexual selection) |
| 5. Natural selection | Favors certain alleles over others |
📌 Examples:
- A mutation in a flower color gene creates a new red color → allele frequency shifts.
- A drought kills only small-beaked birds → big-beak allele becomes more common.
- A few birds fly to a new island and form a new population → allele frequencies change.
✅ Summary:
Allele and genotype frequencies will only stay constant if:
- No mutations
- No migration
- No genetic drift
- Random mating
- No natural selection
Since these ideal conditions almost never exist in real life, populations are always evolving!
7.5.A.1 – Hardy-Weinberg Equilibrium: A Model for Non-Evolving Populations
📘 What Is Hardy-Weinberg Equilibrium?
It’s a mathematical model that says:
If no evolutionary forces act on a population, then allele frequencies will stay constant across generations.
In short: 🔁 No change in p and q = no evolution happening!
🧬 The Conditions Required (The “Perfect World”)
To stay in Hardy-Weinberg equilibrium, a population must meet 5 conditions:
| ✅ Condition | 🔎 Why it matters |
|---|---|
| 1. Large population | Prevents genetic drift (random changes in small populations) |
| 2. No migration | No gene flow – alleles aren’t added or removed |
| 3. No mutations | No new alleles introduced |
| 4. Random mating | No sexual selection – all individuals are equally likely to mate |
| 5. No natural selection | All traits are equally fit – no advantage or disadvantage |
These are idealized conditions – real populations never meet all of them.
⚠️ So What’s the Point?
Even though the conditions are unrealistic, Hardy-Weinberg is useful because:
- It provides a null hypothesis (a reference to test whether evolution is occurring)
- If allele frequencies change, we know something evolutionary is affecting the population
🔑 Summary:
- Hardy-Weinberg is a theoretical baseline: If all 5 conditions are met, no evolution occurs.
- Real populations always evolve, but this model helps us measure how and why allele frequencies shift.
7.5.A.2 – Calculating Allele Frequencies in Non-Evolving Populations (Hardy-Weinberg Equation)
🧠 What’s the Main Idea?
In a non-evolving population, we can predict and calculate allele and genotype frequencies using the Hardy-Weinberg equation.
This equation works only when the population is in Hardy-Weinberg equilibrium, meaning there is:
- No evolution
- No mutations, migration, natural selection, etc.
📐 Hardy-Weinberg Equations
🔹 Allele Frequency Equation: p + q = 1
p = frequency of dominant allele
q = frequency of recessive allele
This just means: all alleles for a gene in a population = 100% (1.0)
🔸 Genotype Frequency Equation: p² + 2pq + q² = 1
p² = frequency of homozygous dominant genotype (e.g., RR)
2pq = frequency of heterozygous genotype (e.g., Rr)
q² = frequency of homozygous recessive genotype (e.g., rr)
All genotypes together also make up 100% of the population.
🌼 EXAMPLE 
Imagine wildflowers with:
- Red allele = CR
- White allele = CW
Given:
p = 0.8 (CR)
q = 0.2 (CW)
Now apply the equation:
| Genotype | Formula | Frequency |
|---|---|---|
| CRCR (red) | p² = 0.8² | 0.64 (64%) |
| CRCW (pink) | 2pq = 2(0.8)(0.2) | 0.32 (32%) |
| CWCW (white) | q² = 0.2² | 0.04 (4%) |
✅ The allele frequencies in the next generation stay the same (as long as the population stays in Hardy-Weinberg equilibrium).
🧪 Why This Matters in Evolution:
Hardy-Weinberg equilibrium gives us a “null model” – if real data deviate from it, evolution is happening.
Scientists compare observed data to H-W predictions to see if natural selection, drift, or gene flow are affecting the population.