AP Biology Unit 7: Natural Selection Study Guide

A complete, student-friendly guide to evolution, natural selection, genetic variation, population genetics, Hardy-Weinberg, speciation, phylogeny and AP-style application questions.

AP Biology • Unit 7 • Natural Selection • 2026

Quick Answer:

AP Biology Unit 7 focuses on evolution and natural selection. You need to understand how heritable variation within populations, environmental pressures, differential reproductive success and changes in allele frequencies can produce adaptation over generations.

Variation → Selection → Differential Reproduction → Evolution

The biggest AP Biology skill is not simply defining natural selection. You must be able to use evidence, graphs, population data and biological reasoning to explain why populations change over time.

1. AP Biology Unit 7: Understanding Evolution and Natural Selection

If you have completed AP Biology Unit 6: Gene Expression and Regulation, you already understand how DNA, mutations and gene expression contribute to biological variation. Unit 7 takes that foundation and asks a much bigger question:

How do populations change over generations?

The central concept is evolution by natural selection.

Evolution can be understood as a change in the genetic composition of a population across generations. Natural selection is one important mechanism that can produce evolutionary change.

What Is Natural Selection?

Natural selection occurs when individuals with heritable traits that improve survival or reproductive success tend to leave more offspring than individuals with less favorable traits under particular environmental conditions.

Over many generations, traits associated with greater reproductive success can become more common in the population.

A useful reasoning chain is:

Heritable variation → Environmental selection → Differential reproductive success → Change in population

Individuals Are Selected, Populations Evolve

This is one of the most important ideas to remember.

An individual organism does not evolve because it "needs" a particular trait. Natural selection acts on differences among individuals, but evolutionary change is observed across populations over generations.

For example, bacteria do not become resistant to an antibiotic because they decide that resistance is needed. If resistant variants are already present or arise through mutation, antibiotic exposure can favor their survival and reproduction.

Adaptation

An adaptation is a heritable characteristic that increases an organism's fitness in a particular environment.

Adaptations are population-level outcomes of evolutionary processes. They are not conscious choices made by organisms.

Fitness

In evolutionary biology, fitness refers to reproductive success relative to other individuals in a particular environment.

An organism that survives but produces no offspring may have lower evolutionary fitness than an organism that successfully reproduces and passes its alleles to the next generation.

AP Biology Tip:

Do not define fitness simply as "being strong" or "being healthy." In an AP Biology question, connect fitness to survival and reproductive success in a specific environment.

Sources of Genetic Variation

Natural selection requires variation. Important sources of genetic variation include:

Mutation
Meiosis and independent assortment
Crossing over
Random fertilization
Gene flow

Mutations introduce new genetic changes into populations. Sexual reproduction can then generate new combinations of existing alleles.

Mutation and Evolution

A mutation is a change in DNA sequence. Mutations can be harmful, neutral or beneficial depending on their biological context.

Natural selection does not create useful mutations because organisms need them. Instead, selection changes the frequency of existing heritable variation when different variants have different reproductive success.

Natural Selection Does Not Produce Perfect Organisms

Evolution works with existing variation and historical constraints.

An adaptation that is beneficial in one environment may provide little benefit—or even become disadvantageous—when environmental conditions change.

This is why natural selection should always be discussed in relation to an environment.

Artificial Selection

Humans can also influence which traits become more common by choosing organisms with particular characteristics for reproduction.

This process is called artificial selection.

Selective breeding in domesticated plants and animals provides examples of how changes in reproductive success can alter populations over generations.

Evidence for Evolution

Evolution is supported by multiple lines of evidence.

Evidence	What It Can Show
Fossils	Changes in organisms through geological time
Comparative anatomy	Structural similarities and differences among organisms
Embryology	Developmental similarities that can provide evidence of shared ancestry
Molecular evidence	DNA and protein sequence similarities
Biogeography	Geographic distribution patterns of organisms

Homologous Structures

Homologous structures share an underlying structural pattern because of common ancestry, even when their current functions differ.

For example, the forelimbs of humans, bats and whales contain similar basic skeletal elements but have become specialized for different functions.

Analogous Structures

Analogous structures can perform similar functions but evolved independently and do not necessarily indicate close evolutionary relationship.

When comparing structures, always distinguish similar function from shared ancestry.

Molecular Evidence

Comparisons of DNA, RNA and protein sequences can provide evidence about evolutionary relationships.

Generally, organisms with more similar molecular sequences are more closely related than organisms with greater sequence differences, assuming comparable genes and appropriate evolutionary analysis.

2. Population Genetics, Allele Frequencies and Hardy-Weinberg

One of the most important mathematical ideas in AP Biology Unit 7 is that evolution can be studied by measuring changes in allele frequencies within populations.

What Is a Population?

A population is a group of individuals of the same species living in the same geographic area and capable of interacting and reproducing.

Gene Pool

The gene pool consists of the collection of alleles present within a population.

If the frequencies of alleles in a population change across generations, the population is undergoing evolutionary change.

Allele Frequency

Allele frequency describes how common a particular allele is within a population.

For example, if a population contains 100 copies of a particular gene and 40 copies are allele A, the frequency of allele A is:

40 ÷ 100 = 0.40 = 40%

AP Biology questions may ask you to calculate allele frequencies from population data and determine whether those frequencies change over time.

Hardy-Weinberg Equilibrium

The Hardy-Weinberg principle provides a mathematical model for a population in which allele frequencies remain constant from generation to generation under specified assumptions.

The allele-frequency equation is:

p + q = 1

The genotype-frequency equation is:

p² + 2pq + q² = 1

Where:

p = frequency of one allele
q = frequency of the other allele
p² = expected frequency of one homozygous genotype
2pq = expected frequency of heterozygotes
q² = expected frequency of the other homozygous genotype

Conditions for Hardy-Weinberg Equilibrium

The model assumes conditions including:

Very large population size
Random mating
No mutation
No migration or gene flow
No natural selection

If one or more of these conditions is violated, allele frequencies can potentially change.

Hardy-Weinberg Example

Suppose the frequency of allele A is 0.70.

Then:

p = 0.70

Because p + q = 1:

q = 1 − 0.70 = 0.30

Expected genotype frequencies are:

AA = p² = 0.49
Aa = 2pq = 0.42
aa = q² = 0.09

The three expected genotype frequencies add to 1:

0.49 + 0.42 + 0.09 = 1.00

AP Exam Strategy:

Before calculating, identify what the question gives you. If it provides a recessive phenotype under Hardy-Weinberg assumptions, that frequency may represent q². Take the square root to obtain q, then calculate p.

Mechanisms That Change Allele Frequencies

Several evolutionary mechanisms can change allele frequencies.

Mechanism	Effect
Natural selection	Different reproductive success changes allele frequencies
Genetic drift	Random changes in allele frequencies, especially in small populations
Gene flow	Movement of alleles between populations
Mutation	Introduces new genetic variation

Genetic Drift

Genetic drift refers to random changes in allele frequencies.

Its effects can be especially strong in small populations.

Founder Effect

The founder effect occurs when a new population is established by a small number of individuals. The allele frequencies of the new population may differ from those of the original population simply because of which individuals founded it.

Bottleneck Effect

A bottleneck occurs when a population experiences a dramatic reduction in size.

The surviving individuals may not represent the original population's genetic diversity.

Gene Flow

Gene flow occurs when individuals or their gametes move between populations and introduce or remove alleles.

Gene flow can reduce genetic differences between populations.

3. Natural Selection, Speciation and Phylogeny

Types of Natural Selection

Natural selection can produce different patterns of phenotypic change.

Directional Selection

Directional selection favors individuals at one extreme of a phenotypic range.

The population distribution may shift toward that extreme.

Stabilizing Selection

Stabilizing selection favors intermediate phenotypes and can reduce the frequency of extreme phenotypes.

Disruptive Selection

Disruptive selection favors individuals at both extremes while selecting against intermediate phenotypes.

Selection Pattern	What Is Favored?
Directional	One extreme
Stabilizing	Intermediate phenotype
Disruptive	Both extremes

Sexual Selection

Sexual selection occurs when differences in mating success influence reproductive success.

Traits that improve an individual's ability to attract mates or compete for mates can become more common even if they carry other costs.

Speciation

Speciation is the formation of new species.

A major component of speciation is the development of reproductive isolation.

Reproductive Isolation

Reproductive isolation prevents populations from successfully producing fertile offspring with each other.

Isolation can occur before fertilization or after fertilization.

Prezygotic Barriers

Habitat isolation
Temporal isolation
Behavioral isolation
Mechanical isolation
Gametic isolation

Postzygotic Barriers

Reduced hybrid viability
Reduced hybrid fertility
Hybrid breakdown

Allopatric Speciation

Allopatric speciation occurs when populations become geographically separated.

Once separated, the populations can experience different mutations, selection pressures, genetic drift and other evolutionary processes.

Over time, reproductive isolation may develop.

Sympatric Speciation

Sympatric speciation occurs without a geographic barrier separating populations.

Differences in chromosome number, habitat preference, mating behavior or other factors can contribute to reproductive isolation.

Phylogenetic Trees

Phylogenetic trees represent hypotheses about evolutionary relationships among organisms or groups.

When reading a phylogenetic tree, focus on common ancestry rather than simply counting how far apart names appear on the page.

Common Ancestors

A branching point, or node, represents a hypothetical common ancestor of the lineages descending from that point.

Two organisms that share a more recent common ancestor are generally considered more closely related than organisms whose common ancestor is farther back in the tree.

Reading a Phylogenetic Tree

A common mistake is assuming that organisms positioned higher or lower on the page are more or less evolved.

That is not how phylogenetic trees should be interpreted.

Instead, trace the branches backward until you find the most recent common ancestor.

Cladograms and Derived Characteristics

Cladograms can be constructed using shared characteristics.

A derived characteristic is a trait that appears in a particular lineage after it diverges from an ancestral lineage.

Shared derived characteristics can help identify relationships among groups.

Evolutionary Relationships and Molecular Data

DNA and protein sequence comparisons can provide evidence for evolutionary relationships.

When two organisms have highly similar sequences for comparable genes, that similarity can provide evidence of shared ancestry.

AP Biology questions may combine molecular data with phylogenetic trees and ask you to determine which organisms are most closely related.

4. AP Biology Unit 7 Exam Strategy, Practice Questions, FAQs and Study Plan

How AP Biology Tests Unit 7

AP Biology questions frequently present evolution through data rather than simply asking for definitions.

You might see:

Population graphs
Allele-frequency calculations
Hardy-Weinberg data
Phylogenetic trees
Experimental evidence
Trait distributions
Environmental changes
DNA sequence comparisons

The best strategy is to connect the evidence to an evolutionary mechanism.

AP-Style Question 1: Natural Selection

A population of insects contains individuals with different levels of resistance to a pesticide. After repeated pesticide exposure, the proportion of resistant insects increases.

What is the best explanation?

Answer: Individuals with heritable resistance had greater survival and reproductive success under pesticide exposure, causing resistance-associated alleles to become more common in the population.

Notice the reasoning:

Variation → Selection pressure → Differential survival/reproduction → Allele-frequency change

AP-Style Question 2: Hardy-Weinberg

In a population assumed to be in Hardy-Weinberg equilibrium, the frequency of a recessive phenotype is 0.16. What is q?

Because the recessive phenotype represents q²:

q² = 0.16

Therefore:

q = √0.16 = 0.40

So the frequency of the recessive allele is 0.40.

AP-Style Question 3: Genetic Drift

A hurricane randomly kills most members of a small island population. The surviving population has very different allele frequencies from the original population.

What evolutionary mechanism best explains this change?

Answer: Genetic drift caused by a population bottleneck.

The important word is random. The allele-frequency change resulted from chance survival rather than differential survival based on a particular adaptive trait.

AP-Style Question 4: Phylogeny

Two species share a recent branching point on a phylogenetic tree. What does this suggest?

Answer: The two species share a more recent common ancestor with each other than with groups whose common ancestor with them occurs farther back in the tree.

AP-Style Question 5: Speciation

A population becomes separated by the formation of a mountain range. After many generations, the two populations can no longer successfully reproduce with one another.

What process may have occurred?

Answer: Allopatric speciation may have occurred because geographic isolation was followed by evolutionary divergence and reproductive isolation.

Common AP Biology Unit 7 Mistakes

Mistake 1: Saying individuals evolve because they need to

Evolution occurs at the population level over generations. Individuals do not evolve because they consciously need a trait.

Mistake 2: Confusing natural selection with genetic drift

Natural selection involves differential reproductive success associated with heritable traits. Genetic drift involves random changes in allele frequencies.

Mistake 3: Calling every useful trait an acquired adaptation

Adaptations are heritable characteristics shaped by evolutionary processes. Acquired traits are not automatically inherited genetically.

Mistake 4: Forgetting the environment

A trait's effect on fitness depends on environmental conditions.

Mistake 5: Misreading phylogenetic trees

Do not interpret vertical position as evolutionary advancement. Trace branches to common ancestors.

Mistake 6: Using Hardy-Weinberg equations without checking assumptions

The equations describe expected frequencies under specific assumptions. Always identify what the problem is asking before calculating.

How to Study AP Biology Unit 7

1. Master the natural-selection story

Practice explaining:

Variation → Heritability → Selection → Reproduction → Population change

2. Practice Hardy-Weinberg calculations

Memorize the equations, but also understand what p, q, p², 2pq and q² represent.

3. Compare evolutionary mechanisms

Create a quick comparison of natural selection, genetic drift, mutation and gene flow.

4. Practice phylogenetic trees

Find common ancestors and identify sister taxa rather than judging organisms by their position on the page.

5. Practice graph interpretation

Look for changes in allele frequency, phenotype distributions and reproductive success.

6. Explain your evidence

When answering an AP-style question, do not stop after stating a conclusion. Explain how the evidence supports your conclusion.

AP Biology Unit 7 Study Checklist

☐ I understand evolution as population-level change across generations.
☐ I understand natural selection.
☐ I understand adaptation and fitness.
☐ I understand sources of genetic variation.
☐ I understand mutation and recombination.
☐ I can calculate allele frequencies.
☐ I understand Hardy-Weinberg equilibrium.
☐ I can use p + q = 1.
☐ I can use p² + 2pq + q² = 1.
☐ I understand genetic drift.
☐ I understand founder and bottleneck effects.
☐ I understand gene flow.
☐ I can distinguish directional, stabilizing and disruptive selection.
☐ I understand reproductive isolation.
☐ I understand allopatric and sympatric speciation.
☐ I can interpret phylogenetic trees.
☐ I understand molecular evidence for evolution.
☐ I can analyze evolutionary data.
☐ I can support conclusions with evidence.

Related AP Biology Study Guides

Build your AP Biology preparation as a connected study system. These articles form the wider AP Biology content cluster.

Related Article	Why Read It?
AP Biology Study Guide: Complete 8-Unit Guide	Complete AP Biology pillar resource
AP Biology Unit 1: Chemistry of Life	Molecular foundation for Biology
AP Biology Unit 2: Cells	Cell structure and function
AP Biology Unit 3: Cellular Energetics	Energy and metabolism
AP Biology Unit 4: Cell Communication and Cell Cycle	Cell signaling and cell division
AP Biology Unit 5: Heredity	Genetics and inheritance
AP Biology Unit 6: Gene Expression and Regulation	DNA, RNA, proteins and gene regulation
AP Biology Unit 8: Ecology	Population, community and ecosystem biology
AP Biology FRQ Guide	Practice evidence-based AP responses

Frequently Asked Questions About AP Biology Unit 7

What is AP Biology Unit 7 about?

AP Biology Unit 7 focuses on evolution and natural selection. Major concepts include genetic variation, natural selection, population genetics, Hardy-Weinberg equilibrium, genetic drift, gene flow, speciation and phylogenetic relationships.

What is natural selection?

Natural selection occurs when heritable differences among individuals lead to differences in survival or reproductive success, causing associated traits or alleles to change in frequency across generations.

What is biological fitness?

Biological fitness refers to reproductive success relative to other individuals in a particular environment.

What is the Hardy-Weinberg equation?

The Hardy-Weinberg model uses p + q = 1 for allele frequencies and p² + 2pq + q² = 1 for expected genotype frequencies under specified equilibrium assumptions.

What are the five Hardy-Weinberg assumptions?

The model assumes a very large population, random mating, no mutation, no migration or gene flow, and no natural selection.

What is genetic drift?

Genetic drift is a random change in allele frequencies. Its effects are often particularly significant in small populations.

What is the founder effect?

The founder effect occurs when a new population is established by a small number of individuals whose allele frequencies may differ from those of the original population.

What is the bottleneck effect?

A bottleneck occurs when a population experiences a severe reduction in size, potentially causing random changes in allele frequencies and loss of genetic diversity.

What is speciation?

Speciation is the formation of new species and commonly involves the development of reproductive isolation between populations.

What is the difference between allopatric and sympatric speciation?

Allopatric speciation involves geographic separation, whereas sympatric speciation occurs without geographic separation.

How should I read a phylogenetic tree?

Trace branches backward to identify common ancestors. Organisms sharing a more recent common ancestor are generally more closely related than organisms whose common ancestor occurs farther back.

Hardy-Weinberg Calculator for AP Biology

Use this simple educational calculator to explore Hardy-Weinberg relationships.

Enter q²:

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Variation → Selection → Reproduction → Population Change

Understand the evolutionary mechanism first. Then use the evidence to solve the AP Biology question.

Final Thoughts

AP Biology Unit 7 becomes much easier when evolution is treated as a connected process rather than a list of vocabulary terms.

Start with genetic variation. Then ask what environmental conditions favor particular heritable traits. Next, determine which individuals have greater reproductive success and finally ask how allele frequencies change across generations.

For population genetics, practice the Hardy-Weinberg equations and understand what each variable represents. For phylogeny, focus on common ancestry. For speciation, focus on reproductive isolation.

Most importantly, practice using evidence. AP Biology questions often give you the data first and expect you to build the biological explanation from it.

Don't just memorize evolution. Follow the change in the population.

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