Variation exists
Individuals in a population have different phenotypes.
What is natural selection in AP Biology?
Natural selection is a mechanism of evolution in which individuals with heritable traits that improve reproductive success leave more offspring, causing those traits and alleles to become more common in the population over generations.
Short answer
Natural selection = heritable traits affecting reproductive success.
In one sentence
Natural selection acts on heritable variation and can change allele frequencies in populations over generations.
AP exam tip: On natural selection AP Biology prompts, trace variation → selection pressure → differential reproductive success → allele frequency change. Avoid need-based language.
Natural Selection Key Takeaways
- Natural selection requires variation in a population.
- The variation must be heritable.
- The environment creates selection pressures.
- Individuals with advantageous traits often leave more offspring.
- Fitness means reproductive success.
- Populations evolve when allele frequencies change over generations.
Natural Selection Reasoning Ladder
Variation is heritable
Some trait differences are genetic and can be passed to offspring.
Selection pressure acts
The environment favors some phenotypes over others.
Differential reproductive success occurs
Individuals with favored traits leave more surviving offspring.
Allele frequencies change
Alleles associated with higher fitness become more common over generations.
Adaptation may result
The population becomes better matched to that environment over time.
AP exam clue: Do not say organisms evolve because they need to. Say heritable variation already existed and affected reproductive success.
The Natural Selection Logic Formula
Heritable variation + selection pressure + differential reproductive success = allele frequency change
This is not a math formula like Hardy-Weinberg equilibrium. It is the AP Biology explanation pattern for natural selection.
Does natural selection act on individuals or populations?
Direct answer: Natural selection acts on individual phenotypes, but populations evolve.
- Individuals survive and reproduce differently.
- Individuals do not change their genes because they need to.
- Evolution is measured as allele frequency change in populations.
- The population changes over generations.
AP trap: Never write that an individual evolved during its lifetime.
Why Natural Selection Requires Heritable Variation
Direct answer: Natural selection can only cause evolutionary change if the trait differences are heritable.
- Selection can favor a phenotype.
- But if the phenotype is not genetic, it will not reliably pass to offspring.
- Heritable variation can come from mutation, recombination, meiosis, and sexual reproduction.
- Natural selection acts on existing variation.
Review genetic variation in Unit 5 and mutations in Unit 6 for where new alleles originate.
What is a selection pressure?
Direct answer: A selection pressure is an environmental factor that affects survival or reproductive success.
Examples include predators, antibiotics, drought, food availability, temperature, disease, competition, and mate choice.
| Selection Pressure | Favored Trait | Why Fitness Changes |
|---|---|---|
| Predators | camouflage | Less visible individuals survive and reproduce. |
| Antibiotics | resistance | Resistant bacteria survive treatment. |
| Drought | deeper roots | Plants access water and reproduce. |
| Cold climate | insulation | Organisms survive and reproduce in cold conditions. |
| Mate choice | attractive signal | Individuals gain more mates. |
What does fitness mean in natural selection?
Direct answer: Fitness means reproductive success, or how effectively an organism passes genes to the next generation.
- Fitness does not mean strongest.
- Fitness does not mean healthiest in every case.
- Fitness depends on environment.
- A trait can be helpful in one environment and harmful in another.
Shortcut: Fitness = more viable offspring. See the full evolutionary fitness guide.
What is an adaptation?
Direct answer: An adaptation is a heritable trait that increases fitness in a specific environment.
- Adaptations are population-level results of natural selection.
- Adaptations do not appear because organisms need them.
- Adaptations depend on the environment.
- A trait can stop being adaptive if the environment changes.
How does natural selection change allele frequencies?
Direct answer: Natural selection changes allele frequencies when individuals with certain heritable alleles reproduce more successfully than others.
- If a phenotype increases reproductive success, alleles linked to that phenotype may increase.
- If a phenotype lowers reproductive success, alleles linked to that phenotype may decrease.
- The change happens across generations.
Connect to population genetics and Hardy-Weinberg equilibrium when a prompt asks you to quantify allele change.
Different types of natural selection (directional, stabilizing, disruptive) change trait distributions in distinct ways—see that guide for selection graph interpretation.
Natural Selection vs Evolution
| Feature | Natural Selection | Evolution |
|---|---|---|
| Meaning | Nonrandom sorting of heritable traits by reproductive success | Allele frequency change in a population over time |
| Level | Acts on individual phenotypes | Measured in populations across generations |
| What changes | Which individuals survive and reproduce | Allele and genotype frequencies |
| AP exam clue | Explain variation, pressure, fitness, and offspring | State that allele frequencies changed in the population |
| Example | Dark insects leave more offspring on dark bark | The allele for dark coloration increases over generations |
Direct answer: Evolution is allele frequency change over time; natural selection is one mechanism that can cause that change.
Natural Selection vs Genetic Drift
Direct answer: Natural selection is nonrandom because traits affect reproductive success, while genetic drift is random allele frequency change.
Natural selection
Trait-based, fitness-related, nonrandom.
Genetic drift
Chance-based, strongest in small populations, random.
Natural Selection Examples in AP Biology
Antibiotic resistance in bacteria
Resistance alleles already exist or arise by mutation. Antibiotics kill susceptible bacteria, resistant bacteria survive and reproduce, and resistance alleles increase.
Peppered moths
Camouflage affects predation risk. The better-camouflaged phenotype has higher survival and reproductive success in that environment.
Finch beak size
Food availability can favor certain beak sizes, causing related alleles to become more common.
Sickle cell and malaria
In malaria regions, heterozygotes may have a fitness advantage, affecting allele frequencies.
The sickle cell example connects to Hardy-Weinberg equilibrium when heterozygote advantage keeps an allele at high frequency despite selection against homozygous recessives.
Does natural selection create new traits?
Direct answer: Natural selection does not create traits because organisms need them. Mutation and recombination create variation; selection changes which variants become more common.
Major AP mistake: Need-based language is one of the most common errors on natural selection FRQs.
| Weak wording | AP Biology wording |
|---|---|
| Animals evolved longer necks because they needed food. | Some individuals already had heritable neck-length variation, and those with advantageous traits reproduced more. |
| Bacteria became resistant because they wanted to survive. | Resistant variants survived antibiotics and reproduced more. |
| The environment caused the trait to appear. | The environment selected among existing heritable variation. |
AP Biology Data Patterns for Natural Selection
Trait frequency changes over generations.
What to do: Identify the selection pressure and explain allele frequency change.
Survival differs by phenotype.
What to do: Connect phenotype to fitness.
Reproductive output differs by phenotype.
What to do: Explain differential reproductive success.
Allele frequency graph shifts.
What to do: State that evolution occurred in the population.
Environment changes.
What to do: Predict which phenotype has higher fitness in the new environment.
Quick Check
Quick Check
Test yourself in 5 seconds
A beetle population contains green and brown beetles. Birds more easily see green beetles on dark bark. Over several generations, brown beetles become more common. Which explanation best describes natural selection?
Answer: B — Natural selection acts on existing heritable variation. Brown beetles had higher fitness in that environment, so alleles associated with brown coloration became more common over generations.
Common Natural Selection Mistakes
Mistake: Individuals evolve.
Fix: Individuals are selected; populations evolve.
Mistake: Organisms adapt because they need to.
Fix: Selection acts on existing heritable variation.
Mistake: Natural selection creates mutations.
Fix: Mutation creates variation; selection sorts it.
Mistake: Fitness means strongest.
Fix: Fitness means reproductive success.
Mistake: Evolution always improves organisms.
Fix: Evolution changes populations; it does not aim for perfection.
Mistake: All traits are adaptive.
Fix: Some traits are neutral, harmful, or only useful in certain environments.
Natural Selection FRQ Strategy
Direct answer: For natural selection FRQs, explain variation, heritability, selection pressure, differential reproductive success, and allele frequency change.
The population has heritable variation in ____. The selection pressure is ____. Individuals with ____ have higher fitness because ____. Therefore, alleles for ____ become more common over generations.
Scoring checklist
- Identifies heritable variation.
- Identifies selection pressure.
- Connects phenotype to survival or reproduction.
- Uses reproductive success / fitness.
- Predicts allele frequency change over generations.
- Avoids need-based language.
More practice: Unit 7 FRQ practice and Unit 7 practice questions.
Mini FRQ: Natural Selection in a Population
Prompt
A population of insects contains light and dark individuals. The insects live on dark tree bark. Birds eat more light insects than dark insects. After ten generations, the dark phenotype is more common.
- (a) Identify the selection pressure. (1 pt)
- (b) Explain why dark insects have higher fitness in this environment. (2 pts)
- (c) Predict how allele frequencies may change over generations. (2 pts)
- (d) Explain why this is not an example of individual insects evolving. (2 pts)
Scoring checklist
- Identifies heritable variation.
- Identifies selection pressure.
- Connects phenotype to survival or reproduction.
- Uses reproductive success / fitness.
- Predicts allele frequency change over generations.
- Avoids need-based language.
Point rubric
- (a) Identify the selection pressure. (1 pt)
- (b) Explain why dark insects have higher fitness in this environment. (2 pts)
- (c) Predict how allele frequencies may change over generations. (2 pts)
- (d) Explain why this is not an example of individual insects evolving. (2 pts)
Model answer
- Birds are the selection pressure.
- Dark insects are less visible on dark bark and are less likely to be eaten.
- Dark insects survive and reproduce more, so alleles associated with dark coloration may increase.
- Individual insects do not evolve; the population changes across generations.
Common mistake: Do not say insects became dark because they needed camouflage.
Natural Selection Flashcards
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Natural Selection Practice Questions
Natural Selection FAQ
What is natural selection in AP Biology?
Natural selection is a mechanism of evolution in which individuals with heritable traits that improve reproductive success leave more offspring, causing those traits and alleles to become more common in the population over generations.
How does natural selection cause evolution?
Natural selection causes evolution when differential reproductive success changes allele frequencies in a population across generations.
Does natural selection act on individuals or populations?
Natural selection acts on individual phenotypes, but populations evolve when allele frequencies change over time.
Why does natural selection require heritable variation?
Selection can only cause evolutionary change if trait differences are genetic and can be passed to offspring.
What is differential reproductive success?
Differential reproductive success means some individuals leave more surviving offspring than others because of heritable traits.
What does fitness mean in natural selection?
Fitness means reproductive success, or how effectively an organism passes genes to the next generation.
Does natural selection create new traits?
No. Mutation and recombination create variation; natural selection changes which variants become more common.
How does natural selection change allele frequencies?
When individuals with certain heritable alleles reproduce more successfully, those alleles become more common over generations.
What is a selection pressure?
A selection pressure is an environmental factor that affects survival or reproductive success.
What is an adaptation?
An adaptation is a heritable trait that increases fitness in a specific environment.
What is an example of natural selection?
Antibiotic resistance in bacteria is a classic example: resistant variants survive treatment and reproduce more, increasing resistance alleles.
How should I explain natural selection on an AP Biology FRQ?
Identify heritable variation, the selection pressure, differential reproductive success, fitness, and predicted allele frequency change over generations.