Identify the population
Which group of organisms is being studied?
What is population genetics in AP Biology?
Population genetics is the study of allele frequencies and genotype frequencies in populations. In AP Biology, evolution is measured as a change in allele frequencies in a population over generations.
Short answer
Population genetics = tracking allele frequency change in populations.
In one sentence
Population genetics connects inheritance to evolution by measuring how allele frequencies change across generations.
AP exam tip: On population genetics AP Biology prompts, ask whether allele frequencies changed—not whether an individual adapted during its lifetime.
Population Genetics Key Takeaways
- A population is a group of the same species in the same area.
- A gene pool includes all alleles in a population.
- Allele frequency measures how common an allele is.
- Genotype frequency measures how common a genotype is.
- Populations evolve when allele frequencies change over generations.
- Evolutionary mechanisms include natural selection, genetic drift, gene flow, mutation, and nonrandom mating.
Population Genetics AP Shortcut
Compact reference
- Population = same species, same area.
- Gene pool = all alleles in the population.
- Allele frequency = percent of allele copies.
- Genotype frequency = percent of individuals with a genotype.
- Evolution = allele frequency change over generations.
AP exam clue: If the allele frequencies changed, the population evolved.
Population Genetics Reasoning Ladder
Identify the gene or trait
Which allele, genotype, or phenotype is being tracked?
Calculate or compare frequencies
Find allele frequency or genotype frequency.
Compare generations
Look for frequency change over time.
Identify the mechanism
Selection, drift, gene flow, mutation, or nonrandom mating.
Explain evolution
If allele frequencies changed, the population evolved.
AP exam clue: Individuals do not evolve. Populations evolve when allele frequencies change.
Why do populations evolve, not individuals?
Direct answer: Individuals do not evolve because their allele combinations do not change in response to need. Populations evolve when allele frequencies change across generations.
- Individuals can survive, reproduce, or die.
- Populations contain genetic variation.
- Selection or other mechanisms can change which alleles become more common.
- Evolution is measured at the population level.
See the natural selection AP Biology guide for how selection acts on individuals while populations change.
What is a gene pool?
Direct answer: A gene pool is the complete set of alleles in a population.
- Includes all alleles for all genes in a population.
- Gene pools can change over time.
- Mutation can add alleles.
- Gene flow can move alleles.
- Natural selection and drift can change allele frequencies.
What is allele frequency?
Direct answer: Allele frequency is how common an allele is in a population.
allele frequency = number of copies of an allele / total number of allele copies
For a diploid population, each individual has two allele copies for a gene. If there are N individuals, there are 2N allele copies.
Example
In a population of 100 diploid organisms, there are 200 total allele copies. If 60 copies are allele A, the frequency of A is 60/200 = 0.30.
What is genotype frequency?
Direct answer: Genotype frequency is how common a genotype is in a population.
genotype frequency = number of individuals with a genotype / total number of individuals
Example
If 25 out of 100 individuals are AA, the AA genotype frequency is 25/100 = 0.25.
Allele Frequency vs Genotype Frequency
Direct answer: Allele frequency counts allele copies, while genotype frequency counts individuals with each genotype.
| Feature | Allele Frequency | Genotype Frequency |
|---|---|---|
| What is counted | Allele copies | Individuals with each genotype |
| Denominator | Total allele copies (2N in diploids) | Total individuals (N) |
| Example | 110 A copies / 200 = 0.55 | 30 AA / 100 = 0.30 |
| AP exam clue | Evolution is measured by allele frequency change | Phenotype proportions depend on genotype frequencies |
| Hardy-Weinberg connection | p and q are allele frequencies | p², 2pq, and q² are genotype frequencies |
How to Calculate Allele Frequency
Count total individuals.
Multiply by 2 for total allele copies in diploid organisms.
Count copies of the allele.
Divide allele copies by total allele copies.
Convert to percent if needed.
Check that allele frequencies add to 1.
Worked example — population: 30 AA, 50 Aa, 20 aa
Calculate A: AA contributes 60 A alleles. Aa contributes 50 A alleles. Total A = 110. Total alleles = 200. A frequency = 110/200 = 0.55.
Calculate a: Aa contributes 50 a alleles. aa contributes 40 a alleles. Total a = 90. a frequency = 90/200 = 0.45.
Check: 0.55 + 0.45 = 1.
How to Calculate Genotype Frequency
Using the same population: 30 AA, 50 Aa, 20 aa out of 100 individuals.
- AA frequency = 30/100 = 0.30
- Aa frequency = 50/100 = 0.50
- aa frequency = 20/100 = 0.20
AP trap: Do not confuse genotype frequency with Hardy-Weinberg expected genotype frequency unless the question says the population is in equilibrium.
How does population genetics connect to Hardy-Weinberg?
Direct answer: Hardy-Weinberg equilibrium is a population genetics model that predicts genotype frequencies from allele frequencies when no evolution is occurring.
- p and q are allele frequencies.
- p², 2pq, and q² are genotype frequencies.
- If observed frequencies differ from expected frequencies, the population may be evolving.
- Hardy-Weinberg is a no-evolution baseline.
Deep dive: Hardy-Weinberg equilibrium and Hardy-Weinberg practice.
What changes allele frequencies in a population?
Direct answer: Allele frequencies can change because of natural selection, genetic drift, gene flow, mutation, or nonrandom mating.
| Mechanism | How it changes population genetics | AP exam clue |
|---|---|---|
| Natural selection | Alleles linked to higher fitness become more common | Nonrandom reproductive success |
| Genetic drift | Allele frequencies change by chance | Strongest in small populations |
| Gene flow | Alleles move between populations | Migration |
| Mutation | New alleles can appear | Source of new variation |
| Nonrandom mating | Genotype frequencies change | Mate choice or inbreeding |
Natural Selection and Allele Frequency Change
Direct answer: Natural selection changes allele frequencies when heritable traits affect reproductive success.
- Selection acts on phenotypes.
- Phenotypes can be linked to alleles.
- Higher fitness increases reproductive success.
- Alleles linked to higher fitness may increase over generations.
Read natural selection and evolutionary fitness for mechanism depth—this page focuses on measuring allele change.
Genetic Drift and Gene Flow
Direct answer: Genetic drift changes allele frequencies by chance, while gene flow changes allele frequencies when individuals or gametes move between populations.
- Drift is random.
- Drift is strongest in small populations.
- Bottleneck and founder effects are drift examples.
- Gene flow can add or remove alleles.
- Gene flow can make populations more similar.
Mutation and Genetic Variation
Direct answer: Mutation can introduce new alleles into a gene pool.
- Mutation is a source of new genetic variation.
- Mutation itself is random with respect to need.
- Selection can act on variation created by mutation.
- Mutation connects Unit 6 gene expression to Unit 7 evolution.
Review mutations in Unit 6 and genetic variation in Unit 5.
AP Biology Data Patterns for Population Genetics
Data pattern: Allele frequency changes from one generation to the next.
What to do: State that evolution occurred.
Data pattern: Genotype counts are given.
What to do: Calculate genotype frequencies or allele frequencies.
Data pattern: Small population changes randomly.
What to do: Consider genetic drift.
Data pattern: Individuals migrate into a population.
What to do: Consider gene flow.
Data pattern: Phenotype affects offspring number.
What to do: Consider natural selection.
Quick Check
Quick Check
Test yourself in 5 seconds
A population has 40 AA individuals, 40 Aa individuals, and 20 aa individuals. What is the frequency of the A allele?
Answer: C — There are 100 diploid individuals, so 200 total allele copies. AA contributes 80 A alleles and Aa contributes 40 A alleles, for 120 A alleles total. 120/200 = 0.60.
Common Population Genetics Mistakes
Mistake: Individuals evolve.
Fix: Populations evolve when allele frequencies change.
Mistake: Allele frequency and genotype frequency are the same.
Fix: Allele frequency counts allele copies; genotype frequency counts individuals.
Mistake: Forgetting diploid organisms have two allele copies.
Fix: Total allele copies = 2 × number of individuals.
Mistake: Assuming all frequency change is natural selection.
Fix: Drift, gene flow, mutation, and nonrandom mating can also change frequencies.
Mistake: Saying mutation happens because organisms need it.
Fix: Mutation is random with respect to need.
Mistake: Confusing observed frequencies with Hardy-Weinberg expected frequencies.
Fix: Hardy-Weinberg predicts expected values under no evolution.
Population Genetics FRQ Strategy
Direct answer: For population genetics FRQs, calculate or compare allele frequencies, identify whether evolution occurred, and explain which mechanism likely changed the population.
The frequency of allele ____ changed from ____ to ____. Because allele frequencies changed across generations, the population evolved. The likely mechanism is ____ because ____.
Scoring checklist
- Identifies allele or genotype frequency.
- Shows correct calculation.
- Compares generations.
- States whether evolution occurred.
- Identifies a mechanism.
- Supports the mechanism with evidence.
More practice: Unit 7 FRQ practice and Unit 7 practice questions.
Mini FRQ: Allele Frequency Change
Prompt
A population of 100 diploid insects has two alleles for body color: B and b. In generation 1, the frequency of B is 0.30. After ten generations in a new environment, the frequency of B is 0.55.
- (a) Identify whether evolution occurred. (1 pt)
- (b) Explain your reasoning. (2 pts)
- (c) Describe one mechanism that could cause the change. (2 pts)
- (d) Explain why this change is measured at the population level. (2 pts)
Scoring checklist
- Identifies allele or genotype frequency.
- Shows correct calculation.
- Compares generations.
- States whether evolution occurred.
- Identifies a mechanism.
- Supports the mechanism with evidence.
Point rubric
- (a) Identify whether evolution occurred. (1 pt)
- (b) Explain your reasoning. (2 pts)
- (c) Describe one mechanism that could cause the change. (2 pts)
- (d) Explain why this change is measured at the population level. (2 pts)
Model answer
- Evolution occurred.
- The B allele frequency changed from 0.30 to 0.55.
- Natural selection could cause the change if B is linked to a phenotype with higher reproductive success.
- Evolution is measured in populations because allele frequencies change across generations, not within a single individual.
Common mistake: Do not say individual insects evolved because their alleles changed during life.
Population Genetics Flashcards
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Population Genetics Practice Questions
Population Genetics FAQ
What is population genetics in AP Biology?
Population genetics is the study of allele frequencies and genotype frequencies in populations. Evolution is measured as a change in allele frequencies in a population over generations.
What is a gene pool?
A gene pool is the complete set of alleles in a population.
What is allele frequency?
Allele frequency is how common an allele is in a population.
What is genotype frequency?
Genotype frequency is how common a genotype is in a population.
How do you calculate allele frequency?
Divide the number of copies of an allele by the total number of allele copies. In diploids, total allele copies = 2 × number of individuals.
How do you calculate genotype frequency?
Divide the number of individuals with a genotype by the total number of individuals in the population.
What is the difference between allele frequency and genotype frequency?
Allele frequency counts allele copies in the population, while genotype frequency counts individuals with each genotype.
Why do populations evolve but individuals do not?
Individuals do not change their allele combinations in response to need. Populations evolve when allele frequencies change across generations.
How does population genetics connect to Hardy-Weinberg?
Hardy-Weinberg equilibrium predicts genotype frequencies from allele frequencies when no evolution is occurring. Departures suggest a mechanism is changing frequencies.
What changes allele frequencies in a population?
Natural selection, genetic drift, gene flow, mutation, and nonrandom mating can all change allele or genotype frequencies.
How does natural selection change allele frequencies?
When heritable traits affect reproductive success, alleles linked to higher fitness may become more common over generations.
How should I explain population genetics on an AP Biology FRQ?
Calculate or compare allele frequencies, state whether evolution occurred, and explain which mechanism likely changed the population with supporting evidence.