---
title: "AP Bio Unit 7 Review: Natural Selection | Fiveable"
description: "Review AP Biology Unit 7 with study guides, practice questions, and key terms on natural selection, population genetics, Hardy-Weinberg, and speciation."
canonical: "https://fiveable.me/ap-bio/unit-7"
type: "unit"
subject: "AP Biology"
unit: "Unit 7 – Natural Selection"
---

# AP Bio Unit 7 Review: Natural Selection | Fiveable

## Overview

Unit 7 covers the mechanisms and evidence of evolution, from Darwin's natural selection to Hardy-Weinberg math, phylogenetic trees, speciation, and the RNA world hypothesis. At 13-20% of the exam, it is one of the highest-weight units in AP Bio.

## AP CED Alignment

This unit hub is organized around AP Course and Exam Description topics, skills, and exam task types when they are available in the source data.
- 7.1: Introduction to Natural Selection
- 7.2: Natural Selection
- 7.3: Artificial Selection
- 7.4: Population Genetics
- 7.5: Hardy-Weinberg Equilibrium
- 7.6: Evidence of Evolution
- 7.7: Common Ancestry
- 7.8: Continuing Evolution
- 7.9: Phylogeny
- 7.10: Speciation
- 7.11: Variations in Populations
- 7.12: Origins of Life on Earth
- guide: Origin of Life on Earth Review
- 7.1-7.2: How Natural Selection Works
- 7.4: Population Genetics and Random Processes
- 7.6-7.7: Evidence for Evolution and Common Ancestry
- Science Practice 2 - Visual Representations
- Science Practice 1 - Concept Explanation
- FRQ 1 – Interpreting and Evaluating Experimental Results (Long)
- FRQ 2 – Interpreting and Evaluating Experimental Results with Graphing (Long)
- FRQ 4 – Conceptual Analysis (Short)

## Topics

- [7.1: Introduction to Natural Selection](/ap-bio/unit-7/intro-natural-selection/study-guide/v9Lf9qQpmpSXvd2ZUOqH): Natural selection is a major mechanism of evolution. Competition for limited resources causes differential survival; individuals with favorable phenotypes reproduce more and pass those traits on. Fitness is measured by reproductive success, and biotic and abiotic environments can shift which traits are favored.
- [7.2: Natural Selection](/ap-bio/unit-7/natural-selection/study-guide/Nc1t327OihZEnIVHHYtC): Selection acts on phenotypic variation in populations. Environments apply selective pressures that can change over time. Sickle cell anemia and DDT resistance illustrate how the same allele can be beneficial or harmful depending on context. Molecular variation inside cells also affects fitness.
- [7.3: Artificial Selection](/ap-bio/unit-7/artificial-selection/study-guide/YdhzRk9EPvFMpXZ8Cthc): Humans direct breeding to favor specific traits, changing allele frequencies in domesticated species. Dog domestication and maize development from teosinte are key examples. Artificial selection demonstrates the same mechanism as natural selection but with human preference as the selective pressure.
- [7.4: Population Genetics](/ap-bio/unit-7/population-genetics/study-guide/W2p2XxaDmtKBRhLnXkYM): Random processes including mutation, genetic drift, the bottleneck effect, the founder effect, and gene flow all change allele frequencies. These forces are distinct from natural selection and are strongest in small populations. Changes in allele frequencies across generations are evidence of evolution.
- [7.5: Hardy-Weinberg Equilibrium](/ap-bio/unit-7/hardy-weinberg-equilibrium/study-guide/DQK5SLWcKmZatpBgPXmV): The Hardy-Weinberg model predicts allele and genotype frequencies in a non-evolving population using p + q = 1 and p squared + 2pq + q squared = 1. Five conditions must hold for equilibrium. Deviations from expected frequencies indicate that at least one evolutionary force is acting.
- [7.6: Evidence of Evolution](/ap-bio/unit-7/evidence-evolution/study-guide/Vy9P6fJvRt1ZTEWg31KI): Evidence comes from fossils dated by rock strata and isotope decay, morphological homologies and vestigial structures, and comparisons of DNA and protein sequences. Multiple independent lines of evidence from different disciplines all support evolution and common ancestry.
- [7.7: Common Ancestry](/ap-bio/unit-7/common-ancestry/study-guide/FNiYICtpxNBjLu17IWjK): All eukaryotes share membrane-bound organelles, linear chromosomes, and genes containing introns. These shared structural and molecular features are evidence that all eukaryotes descended from a single common ancestor.
- [7.8: Continuing Evolution](/ap-bio/unit-7/continuing-evolution/study-guide/fb67fTvqhnbBXkLYOazP): Evolution is ongoing. Antibiotic resistance in MRSA, pesticide resistance in insects, herbicide resistance in weeds, and the emergence of new pathogen variants all demonstrate natural selection acting in real time on existing genetic variation.
- [7.9: Phylogeny](/ap-bio/unit-7/phylogeny/study-guide/jpSuwEfOUXMb3aXNAeBk): Phylogenetic trees and cladograms represent hypotheses about evolutionary relationships. Cladograms show branching order based on shared derived characters; phylogenetic trees add time calibration. Nodes represent common ancestors, and the outgroup is the least related lineage used as a reference.
- [7.10: Speciation](/ap-bio/unit-7/speciation/study-guide/EvkCBpDW4LggHrVIepHo): Speciation requires reproductive isolation. Allopatric speciation involves geographic separation; sympatric speciation does not. Prezygotic barriers prevent mating or fertilization; postzygotic barriers reduce hybrid fitness. Evolution can be gradual or proceed in bursts during adaptive radiation.
- [7.11: Variations in Populations](/ap-bio/unit-7/extinction/study-guide/CpKuTxKrClQmBnYbY5tb): Genetic diversity determines a population's ability to respond to environmental change. Low-diversity populations like California condors and black-footed ferrets face higher extinction risk. An allele adaptive in one environment may be harmful in another, making variation essential for resilience.
- [7.12: Origins of Life on Earth](/ap-bio/unit-7/variations-population/study-guide/yLpYxsJWp5GUp1WgzReR): Earth formed 4.6 bya; earliest fossil evidence for life dates to 3.5 bya. The RNA world hypothesis proposes RNA as the first genetic material, capable of both storing information and catalyzing reactions. Geological and fossil evidence supports the timeline for life's origin.
- [guide: Origin of Life on Earth Review](/ap-bio/unit-7/origin-life-on-earth/study-guide/piE4nORgEvSV69tUm5nI): AP Biology origin of life on Earth review: early Earth timeline (4.6, 3.9, 3.5 bya), abiogenesis vs panspermia, organic chemistry, and the RNA world hypothesis.

## Hardest Topics And Analytics

Snapshot: practice snapshot
This snapshot uses Fiveable practice activity to show where students tend to miss questions and which review moves are worth prioritizing first.
- **67% average MCQ accuracy** (Across 36k multiple-choice practice attempts for this unit.)
- **36k MCQ attempts** (Practice activity included in this snapshot.)
- **66% average FRQ score** (Across 133 scored free-response attempts for this unit.)
- **7.4: Population Genetics**: 35% MCQ miss rate across 3641 attempts. Review Population Genetics with attention to how the concept appears in AP-style source and evidence questions.
- **7.7: Common Ancestry**: 33% MCQ miss rate across 2520 attempts. Review Common Ancestry with attention to how the concept appears in AP-style source and evidence questions.
- **7.12: Origins of Life on Earth**: 32% MCQ miss rate across 2372 attempts. Review Origins of Life on Earth with attention to how the concept appears in AP-style source and evidence questions.
- **7.11: Variations in Populations**: 32% MCQ miss rate across 2097 attempts. Review Variations in Populations with attention to how the concept appears in AP-style source and evidence questions.

## Review Notes

### 7.1-7.2: How Natural Selection Works

Natural selection requires heritable phenotypic variation, competition for limited resources, and differential reproductive success. Individuals with favorable phenotypes leave more offspring, so those alleles increase in frequency over generations. Environments apply selective pressures that can change direction as conditions shift. Selection can be directional (one extreme favored), stabilizing (intermediate favored), or disruptive (both extremes favored). Molecular variation inside cells, such as differences in enzyme activity or protein isoforms, also contributes to fitness differences.

- **Evolutionary fitness**: Measured by reproductive success, not physical strength. An organism that survives but leaves no offspring has zero fitness.
- **Selective pressure**: Any biotic factor (predation, disease, competition) or abiotic factor (temperature, drought) that causes differential survival and reproduction.
- **Heterozygote advantage**: When heterozygotes have higher fitness than either homozygote, as in sickle cell anemia where HbS/HbA individuals have malaria resistance without severe anemia.
- **Directional selection**: One extreme phenotype is favored, shifting the population mean over time. Example: antibiotic resistance increasing in a bacterial population.
- **Phenotypic variation**: The raw material for selection. Without variation, all individuals have equal fitness and selection cannot change allele frequencies.

**Checkpoint:** Can you explain why DDT resistance spread through insect populations and connect that to differential reproductive success?

Type of Selection | Which phenotype is favored | Effect on variation | Example
--- | --- | --- | ---
Directional | One extreme | Decreases | Antibiotic resistance in bacteria
Stabilizing | Intermediate | Decreases | Human birth weight
Disruptive | Both extremes | Increases | Beak size in African seedcrackers

### 7.3: Artificial Selection

Artificial selection works by the same logic as natural selection, but humans choose which individuals reproduce based on desired traits. Over generations, this shifts allele frequencies and can dramatically change a species. Dog domestication from wolves and the transformation of teosinte into modern maize are classic examples. Artificial selection demonstrates that heritable variation exists and that selecting for specific phenotypes changes populations, which supports the broader mechanism of natural selection.

- **Artificial selection**: Human-directed breeding that favors specific phenotypes, changing allele frequencies in domesticated species over generations.
- **Genetic diversity reduction**: Intensive selective breeding can reduce genetic variation in a population, increasing vulnerability to disease or environmental change.

**Checkpoint:** How does artificial selection in dogs or crops illustrate the same mechanism as natural selection in wild populations?

### 7.4: Population Genetics and Random Processes

Evolution is not driven by selection alone. Mutation adds new alleles to a population at random. Genetic drift causes random allele frequency changes that are most pronounced in small populations. The bottleneck effect occurs when a population is drastically reduced, leaving survivors with a non-representative sample of the original gene pool. The founder effect occurs when a small group colonizes a new area. Gene flow from migration adds or removes alleles, connecting or separating populations. Changes in allele frequencies across generations are direct evidence that evolution is occurring.

- **Genetic drift**: Random change in allele frequencies due to chance, not selection. Most powerful in small populations and can fix or eliminate alleles.
- **Bottleneck effect**: A sharp reduction in population size reduces genetic diversity; surviving alleles may not represent the original population.
- **Founder effect**: A small group that colonizes a new area carries only a subset of the original gene pool, leading to different allele frequencies in the new population.
- **Gene flow**: Movement of alleles between populations via migration. Prevents divergence when populations exchange individuals; promotes divergence when it stops.
- **Mutation**: The ultimate source of new genetic variation. Mutations are random with respect to fitness and provide the raw material on which all other evolutionary forces act.

**Checkpoint:** Explain why genetic drift has a larger effect on allele frequencies in a population of 20 individuals than in a population of 20,000.

### 7.5: Hardy-Weinberg Equilibrium

Hardy-Weinberg equilibrium describes allele and genotype frequencies in a population that is not evolving. The equations are p + q = 1 and p squared + 2pq + q squared = 1, where p is the frequency of one allele and q is the frequency of the other. Five conditions must all be met for equilibrium: large population size, no migration, no new mutations, random mating, and no natural selection. Because these conditions are never fully met in nature, Hardy-Weinberg serves as a null hypothesis. A mismatch between observed and expected frequencies signals that at least one evolutionary force is acting. On the AP exam, you are expected to use the equations to calculate allele and genotype frequencies from given data.

- **p + q = 1**: The frequencies of all alleles at a locus must sum to 1. If q is given, p = 1 minus q.
- **p squared + 2pq + q squared = 1**: Expected genotype frequencies under equilibrium. p squared = homozygous dominant, 2pq = heterozygous, q squared = homozygous recessive.
- **Null hypothesis**: Hardy-Weinberg equilibrium is the baseline expectation of no evolution. Deviations from it indicate that selection, drift, mutation, migration, or nonrandom mating is occurring.
- **Genotype frequencies**: The proportion of each genotype in a population. Under Hardy-Weinberg, these are predictable from allele frequencies alone.

**Checkpoint:** If 9% of a population shows a homozygous recessive phenotype, what are the frequencies of p and q, and what percentage of the population is heterozygous?

Condition violated | Evolutionary force acting
--- | ---
Small population | Genetic drift
Migration occurs | Gene flow
New mutations arise | Mutation pressure
Nonrandom mating | Inbreeding or assortative mating
Natural selection acts | Differential reproductive success

### 7.6-7.7: Evidence for Evolution and Common Ancestry

Evidence for evolution comes from multiple independent fields. Fossils can be dated using the age of surrounding rock strata, carbon-14 decay, and geographic data. Morphological homologies, including vestigial structures like the human coccyx and whale pelvic bones, indicate shared ancestry. Comparisons of DNA nucleotide sequences and protein amino acid sequences show that more closely related species have more similar sequences. All eukaryotes share membrane-bound organelles, linear chromosomes, and genes containing introns, which is structural and molecular evidence for a single common eukaryotic ancestor.

- **Fossil dating**: Age is estimated from rock layer position, carbon-14 decay rates, and geographic context. Provides a timeline of evolutionary change.
- **Morphological homologies**: Structural similarities across species that reflect common ancestry, such as the pentadactyl forelimb in mammals.
- **Vestigial structures**: Reduced or non-functional structures inherited from ancestors in which they were functional, such as whale pelvic bones.
- **DNA sequences**: More similar DNA sequences between species indicate more recent common ancestry. Used alongside morphological data to infer evolutionary relationships.
- **Membrane-bound organelles**: Shared by all eukaryotes, including mitochondria and chloroplasts, supporting common eukaryotic ancestry.

**Checkpoint:** Name three independent lines of evidence for evolution and explain what claim each one supports.

Evidence type | What it shows | Example
--- | --- | ---
Fossil record | Change over time and transitional forms | Tiktaalik as fish-to-tetrapod transition
Morphological homology | Common ancestry through shared structures | Pentadactyl forelimb in mammals
DNA/protein sequences | Degree of relatedness between species | Cytochrome c similarity across eukaryotes
Vestigial structures | Descent with modification | Human coccyx, whale pelvic bones
Shared eukaryotic features | Single common eukaryotic ancestor | Introns, linear chromosomes, membrane-bound organelles

### 7.8: Continuing Evolution

Evolution is ongoing. Genomic changes accumulate over time, the fossil record shows continuous change, and resistance to antibiotics, pesticides, herbicides, and chemotherapy drugs demonstrates rapid evolution in real time. MRSA evolving resistance to methicillin and insects evolving resistance to DDT are direct examples of natural selection acting on existing variation. Pathogens also evolve, causing emergent diseases as new variants arise. These examples follow the same logic as natural selection: variation exists, selective pressure is applied, resistant individuals reproduce more, and resistance spreads through the population.

- **Antibiotic resistance**: Bacteria with resistance alleles survive antibiotic treatment and reproduce, increasing the frequency of resistance in the population. MRSA is a key example.
- **Pesticide resistance**: Insects with detoxification alleles survive pesticide application and pass those alleles on, shifting population allele frequencies.
- **Emergent diseases**: Pathogens evolve new variants that can infect new hosts or evade immune responses, demonstrating ongoing molecular evolution.

**Checkpoint:** Explain why antibiotic resistance in bacteria is evidence that evolution is still occurring, using the logic of natural selection.

### 7.9: Phylogeny

Phylogenetic trees and cladograms are diagrams that represent hypotheses about evolutionary relationships. Cladograms show branching order based on shared derived characters but do not indicate time or amount of change. Phylogenetic trees add time calibration using fossils or a molecular clock. Nodes represent the most recent common ancestor of the lineages that branch from them. The outgroup is the lineage least related to the others and is used as a reference point. Shared derived characters, called synapomorphies, are the most informative traits for grouping organisms. Molecular data, such as DNA and protein sequences, generally provide more reliable evidence than morphological traits alone because convergent evolution can make unrelated species look similar.

- **Cladogram**: A branching diagram grouping organisms by shared derived characters. Branch order indicates relative relatedness; branch length has no meaning.
- **Phylogenetic tree**: Like a cladogram but calibrated for time or amount of change using fossils or a molecular clock.
- **Shared derived characteristic**: A trait inherited from a common ancestor that is present in two or more lineages. Used to group organisms into clades.
- **Outgroup**: The lineage that diverged earliest, used as a reference to identify which traits are ancestral and which are derived.
- **Molecular clock**: Uses the rate of DNA or protein sequence change to estimate when two lineages diverged from a common ancestor.

**Checkpoint:** On a cladogram, how do you identify which two species are most closely related, and what does a node represent?

Feature | Cladogram | Phylogenetic tree
--- | --- | ---
Shows time | No | Yes, with calibration
Branch length meaning | No quantitative meaning | Represents amount of change or time
Based on | Shared derived characters | Morphological and molecular data
Represents | Relative branching order | Evolutionary history with time scale

### 7.10: Speciation

Speciation occurs when populations become reproductively isolated and can no longer exchange genes. The biological species concept defines a species as a group that can interbreed to produce viable, fertile offspring. Allopatric speciation involves geographic isolation; sympatric speciation occurs in overlapping ranges. Prezygotic barriers prevent mating or fertilization before a zygote forms, including habitat, temporal, behavioral, and mechanical isolation. Postzygotic barriers reduce the fitness of hybrids after fertilization, including hybrid inviability and hybrid sterility. Evolution can proceed slowly through gradualism or in bursts separated by long stable periods in punctuated equilibrium. Adaptive radiation, as seen in Hawaiian Drosophila and Caribbean Anolis lizards, produces rapid speciation when new habitats become available.

- **Reproductive isolation**: The prevention of gene flow between populations, which is required for speciation to occur.
- **Allopatric speciation**: Geographic separation prevents gene flow; populations diverge independently and may become separate species.
- **Prezygotic isolation**: Barriers that prevent mating or fertilization, such as temporal isolation (different breeding seasons) or behavioral isolation (different courtship displays).
- **Postzygotic barriers**: Barriers acting after fertilization, such as hybrid inviability or hybrid sterility, that reduce hybrid fitness.
- **Punctuated equilibrium**: Long periods of stasis interrupted by rapid evolutionary change, often following environmental disruption or colonization of new habitats.

**Checkpoint:** Distinguish between allopatric and sympatric speciation, and give one example of a prezygotic and one postzygotic barrier.

Barrier type | When it acts | Example
--- | --- | ---
Habitat isolation | Before mating | Two species use different microhabitats in the same area
Temporal isolation | Before mating | Two species breed in different seasons
Behavioral isolation | Before mating | Different courtship songs or displays
Hybrid inviability | After fertilization | Hybrid embryo fails to develop normally
Hybrid sterility | After fertilization | Mule is sterile offspring of horse and donkey

### 7.11: Variations in Populations

Genetic diversity within a population determines how well it can respond to environmental change. Populations with high genetic diversity are more likely to contain individuals that can survive new selective pressures. Populations with low diversity, such as California condors, black-footed ferrets, and prairie chickens, face higher extinction risk because fewer individuals carry alleles suited to new conditions. An allele that is adaptive in one environment can be deleterious in another, which is why maintaining variation matters. Genetic bottlenecks and founder effects reduce diversity and can accelerate extinction risk.

- **Genetic diversity**: The range of alleles in a population's gene pool. Higher diversity increases the probability that some individuals will survive new environmental pressures.
- **Environmental pressure**: Any factor that causes differential survival and reproduction. Populations with low diversity may lack individuals capable of surviving a new pressure.
- **Extinction risk**: Populations with very low genetic diversity, such as those that have passed through a bottleneck, are more vulnerable to disease outbreaks and environmental change.

**Checkpoint:** Why are monoculture crops or populations that have passed through a bottleneck especially vulnerable to disease outbreaks?

### 7.12: Origins of Life on Earth

Earth formed approximately 4.6 billion years ago. Conditions were too hostile for life until about 3.9 billion years ago, and the earliest fossil evidence for life dates to approximately 3.5 billion years ago. The RNA world hypothesis proposes that RNA was the earliest genetic material because it can both store information and catalyze reactions. Three key assumptions of the RNA world hypothesis are: genetic continuity was first assured by RNA replication, base-pairing is necessary for replication, and genetically encoded proteins were not involved as early catalysts. Geological evidence, including stromatolites and isotopic data, supports the timeline for life's origin.

- **RNA world hypothesis**: Proposes that RNA served as the first genetic material, capable of self-replication and catalysis before DNA and proteins evolved.
- **Fossil evidence**: Stromatolites and microfossils dating to approximately 3.5 billion years ago provide the earliest physical evidence of life on Earth.
- **Geological timeline**: Earth formed 4.6 bya; hostile conditions prevented life until about 3.9 bya; earliest life evidence at 3.5 bya gives a plausible window for life's origin.

**Checkpoint:** What three assumptions does the RNA world hypothesis make, and what geological evidence supports the timeline for life's origin?

## Study Guides

- [7.11 Variations in Populations](/ap-bio/unit-7/extinction/study-guide/CpKuTxKrClQmBnYbY5tb)
- [7.5 Hardy-Weinberg Equilibrium](/ap-bio/unit-7/hardy-weinberg-equilibrium/study-guide/DQK5SLWcKmZatpBgPXmV)
- [7.10 Speciation](/ap-bio/unit-7/speciation/study-guide/EvkCBpDW4LggHrVIepHo)
- [7.7 Common Ancestry](/ap-bio/unit-7/common-ancestry/study-guide/FNiYICtpxNBjLu17IWjK)
- [7.2 Natural Selection](/ap-bio/unit-7/natural-selection/study-guide/Nc1t327OihZEnIVHHYtC)
- [7.6 Evidence of Evolution](/ap-bio/unit-7/evidence-evolution/study-guide/Vy9P6fJvRt1ZTEWg31KI)
- [7.4 Population Genetics](/ap-bio/unit-7/population-genetics/study-guide/W2p2XxaDmtKBRhLnXkYM)
- [7.3 Artificial Selection](/ap-bio/unit-7/artificial-selection/study-guide/YdhzRk9EPvFMpXZ8Cthc)
- [7.9 Phylogeny](/ap-bio/unit-7/phylogeny/study-guide/jpSuwEfOUXMb3aXNAeBk)
- [Origin of Life on Earth Review](/ap-bio/unit-7/origin-life-on-earth/study-guide/piE4nORgEvSV69tUm5nI)
- [7.1 Introduction to Natural Selection](/ap-bio/unit-7/intro-natural-selection/study-guide/v9Lf9qQpmpSXvd2ZUOqH)
- [7.12 Origins of Life on Earth](/ap-bio/unit-7/variations-population/study-guide/yLpYxsJWp5GUp1WgzReR)
- [7.8 Continuing Evolution](/ap-bio/unit-7/continuing-evolution/study-guide/fb67fTvqhnbBXkLYOazP)

## Practice Preview

### Multiple-choice practice

- **Stimulus-based practice question**: Science Practice 2 - Visual Representations | Which of the following best describes the pattern shown in the diagram?
- **Stimulus-based practice question**: Science Practice 1 - Concept Explanation | Which statement most accurately summarizes the process depicted in the diagram?
- **Stimulus-based practice question**: Science Practice 1 - Concept Explanation | Which statement best describes the island population?
- **Stimulus-based practice question**: Science Practice 1 - Concept Explanation | Which statement best describes the allele frequencies?
- **Stimulus-based practice question**: Science Practice 2 - Visual Representations | Which of the following best explains how this model demonstrates the founder effect?
- **Stimulus-based practice question**: Science Practice 2 - Visual Representations | Which of the following best explains how the figure illustrates a population bottleneck?

### FRQ practice

- **Pesticide resistance evolution in mosquito populations**: FRQ 1 – Interpreting and Evaluating Experimental Results (Long) | Pesticide resistance evolution in mosquito populations
- **Membrane protein structure and hydrophobic amino acids**: FRQ 2 – Interpreting and Evaluating Experimental Results with Graphing (Long) | Membrane protein structure and hydrophobic amino acids
- **Parasitoid-driven selection for silent crickets**: FRQ 4 – Conceptual Analysis (Short) | Parasitoid-driven selection for silent crickets

## Key Terms

- **Evolutionary fitness**: Measured by reproductive success: how many viable offspring an organism produces relative to others in the population. Not synonymous with physical strength or survival alone.
- **Selective Pressure**: Any biotic or abiotic environmental factor that causes differential survival and reproduction, driving changes in allele frequencies over generations.
- **Genetic Drift**: Random change in allele frequencies due to chance events, not selection. Most powerful in small populations and can fix or eliminate alleles regardless of their fitness effects.
- **Founder Effect**: A small group that colonizes a new area carries only a subset of the original gene pool, producing different allele frequencies in the new population.
- **Gene Flow**: Movement of alleles between populations through migration. Prevents divergence when populations exchange individuals; promotes divergence when it stops.
- **Allele Frequency**: The proportion of a specific allele among all alleles at a locus in a population, expressed as a decimal between 0 and 1. Changes in allele frequency across generations are evidence of evolution.
- **Heterozygote Advantage**: When heterozygotes have higher fitness than either homozygote. The HbS/HbA genotype in malaria-endemic regions is the AP Bio example: carriers have malaria resistance without severe sickle cell disease.
- **Reproductive Isolation**: Prevention of gene flow between populations through prezygotic or postzygotic barriers. Required for speciation to occur.
- **Allopatric Speciation**: Geographic separation of populations prevents gene flow, allowing independent divergence that can lead to new species over time.
- **Punctuated Equilibrium**: Long periods of species stability interrupted by brief periods of rapid evolutionary change, often following environmental disruption or colonization of new habitats.
- **Shared derived characteristic**: A trait inherited from a common ancestor and present in two or more lineages. Used to group organisms into clades on cladograms and phylogenetic trees.
- **RNA world hypothesis**: Proposes that RNA was the earliest genetic material, capable of both storing genetic information and catalyzing reactions, before DNA and proteins evolved.
- **Genetic Diversity**: The range of alleles in a population's gene pool. Higher diversity increases the probability that some individuals will survive new environmental pressures; low diversity increases extinction risk.
- **null hypothesis**: In Hardy-Weinberg analysis, the baseline expectation that a population is not evolving. Deviations from expected allele or genotype frequencies indicate that at least one evolutionary force is acting.

## Common Mistakes

- **Confusing fitness with physical strength**: Evolutionary fitness is measured only by reproductive success. An organism that is large and strong but leaves no offspring has zero fitness. Always define fitness in terms of passing alleles to the next generation.
- **Misreading Hardy-Weinberg: using q squared as q**: If a problem gives you the frequency of the homozygous recessive phenotype, that value is q squared, not q. Take the square root to find q, then calculate p = 1 minus q before finding genotype frequencies.
- **Treating cladogram tip proximity as relatedness**: Two species that appear next to each other on a cladogram are not necessarily most closely related. Relatedness is determined by the most recent shared node, not by how close the tips look on the page.
- **Saying populations evolve because they need to**: Evolution has no direction or goal. Natural selection acts on existing variation; it does not produce mutations in response to need. Antibiotic resistance alleles existed before antibiotics were used; antibiotics selected for them.
- **Confusing prezygotic and postzygotic barriers**: Prezygotic barriers prevent mating or fertilization from occurring at all. Postzygotic barriers act after fertilization and reduce hybrid viability or fertility. Hybrid sterility (like a mule) is postzygotic, not prezygotic.

## Exam Connections

- **Applying Hardy-Weinberg to population data**: AP Bio frequently presents allele or genotype frequency data and asks you to calculate missing values using p + q = 1 and p squared + 2pq + q squared = 1, then identify which evolutionary condition is being violated. Practice identifying whether the given value is q squared or q before solving.
- **Reading and interpreting phylogenetic diagrams**: Exam tasks often ask you to identify the most recent common ancestor of two lineages, determine which organisms are most closely related, or explain what a shared derived character indicates. Always trace back to the shared node rather than judging by tip position.
- **Constructing and evaluating evolutionary arguments**: Free-response tasks in AP Bio frequently ask you to explain a mechanism such as natural selection or speciation using specific evidence. Practice structuring answers around variation, selective pressure, differential reproduction, and change in allele frequency over generations, and connect evidence types (fossil, molecular, morphological) to the specific claims they support.

## Final Review Checklist

- **Explain the logic of natural selection**: State the three requirements: heritable phenotypic variation, competition for limited resources, and differential reproductive success. Apply this logic to examples like antibiotic resistance or sickle cell anemia.
- **Use Hardy-Weinberg equations correctly**: Practice solving for p, q, and genotype frequencies from given data. Identify which of the five conditions is violated when a population deviates from equilibrium.
- **Distinguish selection from random processes**: Know the difference between natural selection, genetic drift, the bottleneck effect, the founder effect, and gene flow. Be able to predict which force is acting based on population size and context.
- **Connect evidence types to evolutionary claims**: For each evidence type (fossil dating, morphological homology, vestigial structures, DNA sequences, shared eukaryotic features), state the specific claim it supports rather than just naming the evidence.
- **Read phylogenetic trees and cladograms accurately**: Identify the most recent common ancestor of two lineages by tracing back to the shared node. Distinguish cladograms from phylogenetic trees and explain what shared derived characters are used for.
- **Explain speciation mechanisms**: Compare allopatric and sympatric speciation. Give examples of at least two prezygotic and two postzygotic barriers. Distinguish gradualism from punctuated equilibrium.
- **Apply the RNA world hypothesis**: State the three assumptions of the RNA world hypothesis and connect the geological timeline (4.6 bya, 3.9 bya, 3.5 bya) to the evidence for life's origin.

## Study Plan

- **Start with natural selection and population genetics**: Review topics 7.1 through 7.4 together. For each evolutionary force (selection, drift, gene flow, mutation), write out what it does to allele frequencies and under what conditions it is strongest. Use the sickle cell and antibiotic resistance examples to practice applying the logic of selection.
- **Practice Hardy-Weinberg calculations**: Work through topic 7.5 by solving problems from given data. Practice identifying whether q squared, q, or p is given, and calculate all five values: p, q, p squared, 2pq, and q squared. Then practice identifying which condition is violated in a scenario.
- **Build your evidence and common ancestry review**: Review topics 7.6 and 7.7 by making a table of evidence types, what each shows, and a specific example. Make sure you can explain why shared eukaryotic features (membrane-bound organelles, linear chromosomes, introns) support common ancestry rather than just naming them.
- **Work through phylogeny and speciation together**: Review topics 7.9 and 7.10 as a pair. Practice reading cladograms by identifying nodes, outgroups, and shared derived characters. Then apply speciation concepts by classifying barriers as prezygotic or postzygotic and speciation as allopatric or sympatric.
- **Finish with continuing evolution, variation, and origins of life**: Review topics 7.8, 7.11, and 7.12. For 7.8, connect each resistance example to the natural selection mechanism. For 7.11, explain why low genetic diversity increases extinction risk using a specific example. For 7.12, state the RNA world hypothesis assumptions and match the geological timeline to the evidence.

## More Ways To Review

- [Topic study guides](/ap-bio/unit-7#topics)
- [FRQ practice](/ap-bio/frq-practice)
- [Cram archive videos](/cram-archives?subject=ap-biology&unit=unit-7)
- [Cheatsheets](/ap-bio/cheatsheets/unit-7)
- [Key terms](/ap-bio/key-terms)

## FAQs

### What topics are covered in AP Bio Unit 7?

AP Bio Unit 7 covers 12 topics on evolution and natural selection: Introduction to Natural Selection, Natural Selection, Artificial Selection, Population Genetics, Hardy-Weinberg Equilibrium, Evidence of Evolution, Common Ancestry, Continuing Evolution, Phylogeny, Speciation, Variations in Populations, and Origins of Life on Earth. Together they build a complete picture of how populations change over time. See the full topic list and study guides at [/ap-bio/unit-7](/ap-bio/unit-7).

### How much of the AP Bio exam is Unit 7?

AP Bio Unit 7 makes up 13-20% of the AP exam, making it one of the heavier-weighted units. It covers evolution, natural selection, Hardy-Weinberg equilibrium, speciation, phylogeny, and evidence for evolution. That range means you can expect roughly 8-12 multiple-choice questions tied to this unit on test day.

### What's on the AP Bio Unit 7 progress check (MCQ and FRQ)?

The AP Bio Unit 7 progress check includes MCQ and FRQ parts drawn from all 12 topics in the unit. MCQ questions test concepts like Hardy-Weinberg equilibrium calculations, genetic drift, natural selection mechanisms, and reading phylogenetic trees. The FRQ portion typically asks you to analyze evidence for evolution, interpret population genetics data, or explain speciation scenarios. Practicing with questions matched to these exact topics is the best way to prepare. You can find aligned practice at [/ap-bio/unit-7](/ap-bio/unit-7).

### How do I practice AP Bio Unit 7 FRQs?

AP Bio Unit 7 FRQs most often come from Hardy-Weinberg equilibrium calculations, natural selection and genetic drift scenarios, phylogenetic tree analysis, and evidence for evolution. Question types include data analysis, mathematical calculations of allele frequencies, and written explanations of speciation mechanisms. To practice, work through past College Board FRQs on these topics, write out full justifications for every claim, and check your answers against the scoring guidelines. You can find Unit 7 FRQ practice resources at [/ap-bio/unit-7](/ap-bio/unit-7).

### Where can I find AP Bio Unit 7 practice questions?

For AP Bio Unit 7 practice questions, including multiple-choice and practice test sets, head to [/ap-bio/unit-7](/ap-bio/unit-7). There you'll find MCQ practice covering natural selection, Hardy-Weinberg equilibrium, genetic drift, phylogenetic trees, speciation, and evidence for evolution, organized by topic so you can target the areas where you need the most work.

### How should I study AP Bio Unit 7?

Start AP Bio Unit 7 by building a strong foundation in evolution and natural selection before moving into the math-heavy Hardy-Weinberg equilibrium problems. Here's a solid approach: 1. **Learn the mechanisms first.** Understand natural selection, genetic drift, and artificial selection conceptually before tackling calculations.
2. **Practice Hardy-Weinberg math.** Work through allele frequency problems until the formulas feel automatic.
3. **Read phylogenetic trees.** Practice interpreting common ancestry and evolutionary relationships from tree diagrams.
4. **Connect evidence for evolution.** Know how geology, genetics, and molecular biology each support evolutionary theory.
5. **Review speciation types.** Be able to distinguish allopatric from sympatric speciation with real examples. Unit 7 is 13-20% of the exam, so it rewards consistent practice. Find topic guides and practice sets at [/ap-bio/unit-7](/ap-bio/unit-7).

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