AP Biology Unit 7 Natural Selection: Evolution, Hardy-Weinberg, Speciation

AP Biology Unit 7 Key Takeaways

Individuals do not evolve; populations evolve.
Natural selection acts on heritable variation.
Evolution is measured as changes in allele frequencies.
Fitness means reproductive success.
Hardy-Weinberg helps test whether a population is evolving.
Evidence, phylogeny, and speciation explain evolutionary relationships.

Learning journey

AP Biology Unit 7 Learning Journey

Work through every Unit 7 topic below. Each card links to a full study guide or practice page so you can move from natural selection basics to Hardy-Weinberg math, evidence, phylogeny, speciation, and exam-style practice.

Natural SelectionLearn how selection pressures favor heritable traits that increase reproductive success.Study guide Artificial SelectionCompare human-directed selection with natural selection.Study guide Evolutionary FitnessUnderstand why fitness means reproductive success, not strength.Study guide Types of Natural SelectionMaster directional, stabilizing, and disruptive selection graphs.Study guide Population GeneticsTrack gene pools, allele frequencies, and genotype frequencies.Study guide Hardy-Weinberg EquilibriumUse p, q, p², 2pq, and q² to test population change.Study guide Hardy-Weinberg PracticePractice allele frequency, carrier frequency, and genotype calculations.Practice Evidence of EvolutionConnect fossils, homologous structures, molecular data, and biogeography.Study guide Common AncestryExplain how shared traits and DNA similarities support common ancestry.Study guide Phylogenetic Trees and CladogramsRead nodes, clades, outgroups, and shared derived traits.Study guide SpeciationLearn how populations become separate species.Study guide Reproductive IsolationCompare prezygotic and postzygotic barriers.Study guide Practice QuestionsTest Unit 7 concepts with AP-style MCQs.Practice FRQ PracticePractice writing evidence-based Unit 7 FRQ explanations.Practice

Suggested study path: Suggested path: natural selection → fitness → selection types → population genetics → Hardy-Weinberg → evidence → common ancestry → phylogenetic trees → speciation → reproductive isolation → practice questions → FRQ practice.

AP Biology Unit 7 Natural Selection Review Slides

Use this slide deck for a fast Unit 7 review of natural selection, Hardy-Weinberg, evidence of evolution, and speciation before you open the topic guides below.

Unit 7 Reasoning Ladder

Use this chain on MCQs and FRQs to explain population-level evolutionary change.

Identify variation.What traits differ among individuals in the population?

Identify whether variation is heritable.Can the trait be passed genetically to offspring?

Identify selection pressure or evolutionary mechanism.Is the change from selection, drift, gene flow, mutation, or mating patterns?

Predict allele frequency change.Which alleles should increase, decrease, or stay the same over generations?

Connect to fitness or reproductive success.Who survives and reproduces more in this environment?

Support the claim with evidence.Use data from fossils, DNA, trees, tables, or graphs in the prompt.

AP exam clue: AP exam clue: Strong Unit 7 answers explain population-level change, not individual effort or need.

Unit 7 Big Ideas

Concept	What it means	AP exam clue
Evolution	Change in allele frequencies in a population over time.	Define population change, not individual change.
Natural selection	Nonrandom survival and reproduction based on heritable phenotypes.	Name variation, pressure, and reproductive success.
Fitness	Reproductive success in a specific environment.	Avoid equating fitness with strength.
Adaptation	Heritable trait that improves fitness in an environment.	Environment-dependent, not universally best.
Hardy-Weinberg	No-evolution baseline for comparing expected and observed genotype frequencies.	Deviation suggests an assumption is violated.
Common ancestry	Shared evolutionary origin supported by homologous traits and molecular similarity.	Connect evidence to relationship, not progress.
Phylogeny	Hypothesis of evolutionary relationships among species.	Read most recent common ancestor, not tip order.
Speciation	Formation of new species when populations diverge and become reproductively isolated.	Reduced gene flow is the starting clue.
Reproductive isolation	Barriers that prevent successful interbreeding between populations.	Prezygotic vs postzygotic barriers.

Natural Selection Overview

Natural selection changes populations when heritable variation affects reproductive success in a specific environment. Natural selection acts on individuals, but populations evolve.

Individuals in a population differ in traits. Some differences are genetic and can be passed to offspring. The environment makes some phenotypes more successful than others, so alleles linked to higher reproductive success may become more common across generations.

For the full reasoning chain, selection types, and practice, use the natural selection AP Biology guide.

AP Biology Unit 7 evolution mechanisms showing natural selection genetic drift gene flow mutation and allele frequency change — Evolution mechanisms change allele frequencies through selection, drift, gene flow, mutation, or nonrandom mating.

Mechanism	Random or nonrandom?	Biggest clue	Effect
Natural selection	Nonrandom	Trait affects survival/reproduction	Adaptive alleles may increase
Genetic drift	Random	Small population, chance event	Alleles change by chance
Gene flow	Often nonrandom movement	Migration between populations	Populations become more similar or new alleles enter
Mutation	Random source of variation	DNA sequence change	New alleles can appear
Nonrandom mating	Nonrandom	Mate choice or inbreeding	Genotype frequencies shift

Fitness and Adaptation

In AP Biology, fitness means how well an organism passes its genes to the next generation. Fitness is measured by reproductive success, not by being the strongest, fastest, or longest-lived. A trait can be helpful in one environment and harmful in another.

For examples, graphs, and practice questions, open the evolutionary fitness AP Biology guide.

Common mistake: Fitness is environment-dependent. There is no universally best trait.

Population Genetics and Hardy-Weinberg

Evolution occurs when allele frequencies change. Hardy-Weinberg equilibrium predicts genotype frequencies when a population is not evolving at a gene: p + q = 1 and p² + 2pq + q² = 1.

Hardy-Weinberg equilibrium AP Biology showing p q allele frequencies and genotype frequencies — Hardy-Weinberg equilibrium is a no-evolution baseline for comparing allele and genotype frequencies.

Track gene pools and allele frequencies in the population genetics guide, learn the baseline logic in Hardy-Weinberg equilibrium, and drill calculations in Hardy-Weinberg practice.

Bottleneck effect

A random event sharply reduces population size. Survivors may not represent the original gene pool.

Founder effect

A small group starts a new population. Allele frequencies reflect the founders, not the source population.

Common mistake: q is an allele frequency. q² is a genotype frequency.

Hardy-Weinberg Assumptions

Hardy-Weinberg is a null model, not a claim that real populations never evolve. If observed data differ from expected values, at least one assumption may be violated: no mutation, no migration, no natural selection, very large population size, or random mating.

Evidence of Evolution and Common Ancestry

Evidence for evolution supports common ancestry and change over time. Strong AP answers connect evidence to a mechanism or relationship.

Fossils

Show changes in organisms over time and extinct forms.

Homologous structures

Similar structures from common ancestry, even if functions differ.

Vestigial structures

Reduced structures inherited from ancestors.

Molecular evidence

DNA, RNA, and protein similarities show evolutionary relationships.

Biogeography

Species distribution patterns reflect geography and evolutionary history.

Embryology

Developmental similarities can support common ancestry.

Deepen each evidence type in the evidence of evolution guide and the common ancestry AP Biology guide.

Phylogenetic Trees and Cladograms

Phylogenetic trees show hypotheses about evolutionary relationships. Closest relatives are identified by the most recent common ancestor, not by which tips appear next to each other visually.

For nodes, clades, outgroups, and shared derived traits, use the phylogenetic trees and cladograms guide.

AP exam clue: Read common ancestors and shared derived traits, not visual ladder position.

Speciation and Reproductive Isolation

Speciation occurs when populations become reproductively isolated and diverge over time. Geographic, behavioral, temporal, or genetic barriers can reduce gene flow.

Prezygotic barriers

Prevent fertilization before a zygote forms.

Postzygotic barriers

Reduce hybrid survival or fertility after fertilization.

Study full speciation pathways in the speciation AP Biology guide and reproductive isolation guide.

AP Biology Unit 7 Data Patterns

Trait frequency over generations Data pattern: A graph shows one phenotype becoming more common across generations. Consider: Connect the trend to selection type and heritable variation affecting reproductive success.

Hardy-Weinberg genotype counts Data pattern: Observed genotype frequencies differ from p² + 2pq + q² expectations. Consider: Identify which Hardy-Weinberg assumption may be violated.

Fossil or DNA similarity table Data pattern: Species share percent DNA similarity or fossil sequence patterns. Consider: Explain how similarity supports common ancestry and evolutionary relationships.

Phylogenetic tree Data pattern: Branch points connect species with shared ancestors. Consider: Identify closest relatives using most recent common ancestor, not tip position.

Isolation/speciation scenario Data pattern: Two populations stop interbreeding after geographic or behavioral separation. Consider: Name the isolation barrier and explain reduced gene flow over time.

Small population random change Data pattern: Allele frequencies shift after a random event in a small population. Consider: This pattern fits genetic drift, not adaptive natural selection.

Diagnostic

Unit 7 Diagnostic Quiz

10 questions covering natural selection, fitness, Hardy-Weinberg, drift, gene flow, evidence, phylogenies, and speciation.

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Flashcards

AP Biology Unit 7 Flashcards

60 flashcards covering natural selection, Hardy-Weinberg, evidence, phylogeny, and speciation vocabulary.

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Practice

AP Biology Unit 7 Practice Questions

50 MCQs with explanations across natural selection, population genetics, Hardy-Weinberg, evidence, phylogenies, and speciation.

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FRQ strategy

AP Biology Unit 7 FRQ Strategy

Unit 7 FRQs score evidence-based population reasoning. A strong answer names the mechanism, cites the evidence, and predicts the allele-frequency effect across generations.

FRQ answer template

The population has variation in ____. The selection pressure/mechanism is ____. Individuals with ____ have higher reproductive success because ____. Therefore, allele frequency ____ over generations. The evidence is ____.

Mini FRQ scenarios with strong answers

Natural selection

Strong answer: Brown beetles have higher fitness in this environment because they are less visible to predators and are more likely to survive and reproduce. If color is heritable, alleles associated with brown coloration may increase in frequency over generations.

Antibiotic resistance

Strong answer: The antibiotic acts as a selection pressure. Bacteria with resistance alleles survive and reproduce more than susceptible bacteria, so resistance alleles become more common in the population.

Genetic drift

Strong answer: This is genetic drift because allele frequencies changed by chance after a random event. The effect is stronger in small populations, and the surviving gene pool may not represent the original population.

Gene flow

Strong answer: Gene flow can introduce new alleles or change existing allele frequencies in the receiving population. It can also make the two populations more genetically similar if migration continues.

Hardy-Weinberg deviation

Strong answer: One Hardy-Weinberg assumption may be violated, such as natural selection, migration, mutation, genetic drift, or nonrandom mating. For example, if one genotype has higher survival, selection can shift genotype and allele frequencies away from expected values.

Phylogenetic tree

Strong answer: Species A and B are more closely related to each other because they share a more recent common ancestor. This does not mean A evolved from B; it means they share a common ancestral lineage.

Speciation

Strong answer: Geographic isolation reduces gene flow between the populations. Over time, mutation, selection, and genetic drift can cause the populations to diverge. If reproductive isolation evolves, the populations may become separate species.

Open Unit 7 FRQ practice →

Common Unit 7 Mistakes That Cost Points

Animals evolved because they needed to survive.

AP Bio wording: Heritable variation already existed; individuals with favorable traits reproduced more.

Individuals evolve.

AP Bio wording: Individuals are selected; populations evolve.

Fitness means strongest.

AP Bio wording: Fitness means reproductive success.

Natural selection creates mutations.

AP Bio wording: Mutations create variation; selection acts on phenotypes produced by variation.

Genetic drift helps organisms adapt.

AP Bio wording: Drift is random and can increase, decrease, or eliminate alleles regardless of benefit.

Hardy-Weinberg means evolution is happening.

AP Bio wording: Hardy-Weinberg is the no-evolution baseline.

Phylogenetic trees show which species is more advanced.

AP Bio wording: Trees show common ancestry, not progress.

Unit 7 Must-Know Terms

Term	Student-friendly meaning	AP exam use
Evolution	Allele-frequency change over time.	Define population change.
Population	Same species in one area.	Unit of evolution.
Gene pool	All alleles in a population.	Track variation.
Allele frequency	How common an allele is.	Measure evolution.
Genotype frequency	How common a genotype is.	Hardy-Weinberg comparisons.
Natural selection	Nonrandom reproductive success.	Explain adaptive change.
Selection pressure	Environmental factor favoring traits.	Identify cause.
Fitness	Reproductive success.	Avoid strength trap.
Reproductive success	Passing genes to offspring.	Fitness evidence.
Adaptation	Heritable trait that improves fitness.	Explain helpful traits.
Heritable variation	Genetic differences passed on.	Selection requirement.
Mutation	DNA change creating alleles.	Variation source.
Genetic drift	Random frequency change.	Small population clue.
Bottleneck effect	Drift after population crash.	Random survivor clue.
Founder effect	Drift in new small population.	Island/colony clue.
Gene flow	Allele movement between populations.	Migration clue.
Nonrandom mating	Mate choice or inbreeding.	Genotype frequency shifts.
Hardy-Weinberg equilibrium	No-evolution baseline.	Expected frequencies.
p and q	Allele frequencies.	p + q = 1.
p² + 2pq + q²	Genotype-frequency equation.	Find expected genotypes.
Directional selection	One extreme favored.	Shifted distribution.
Stabilizing selection	Average favored.	Narrower distribution.
Disruptive selection	Both extremes favored.	Two peaks clue.
Artificial selection	Human-chosen breeding.	Domestication examples.
Sexual selection	Mating success affects fitness.	Mate choice traits.
Common ancestry	Shared evolutionary origin.	Tree reasoning.
Homologous structures	Similar structure from ancestry.	Evidence of evolution.
Vestigial structures	Reduced ancestral structures.	Evidence clue.
Molecular evidence	DNA/protein similarity.	Relationship evidence.
Phylogenetic tree	Relationship diagram.	Common ancestor reading.
Cladogram	Branching relationship diagram.	Shared trait reasoning.
Clade	Ancestor and descendants.	Group identification.
Shared derived trait	New trait shared by a clade.	Branch evidence.
Outgroup	Reference lineage outside group.	Root trees.
Speciation	Formation of new species.	Isolation reasoning.
Reproductive isolation	No successful interbreeding.	Speciation requirement.
Prezygotic barrier	Prevents fertilization.	Before-zygote clue.
Postzygotic barrier	Weak or sterile hybrids.	After-zygote clue.
Extinction	Loss of a species.	Environmental change reasoning.

Practice and FRQ Hub

Unit 7 Practice QuestionsFull MCQ set with topic filters and explanations.Practice hub Unit 7 FRQ PracticeEvidence-based FRQ prompts with scoring checklists.FRQ hub

AP Biology Unit 7 FAQ

What is AP Biology Unit 7 about?

AP Biology Unit 7 Natural Selection explains how populations evolve over generations. Students study heritable variation, natural selection, fitness, allele frequency change, Hardy-Weinberg equilibrium, evidence of evolution, phylogenetic trees, speciation, and reproductive isolation.

Is Unit 7 Natural Selection hard?

Unit 7 is moderate to challenging because questions combine vocabulary, data interpretation, and population-level reasoning. Tracing variation, mechanism, and allele-frequency change step by step usually improves MCQ and FRQ scores.

What is natural selection in AP Biology?

Natural selection is a mechanism of evolution where individuals with heritable traits that improve survival or reproductive success in a specific environment tend to leave more offspring, so those alleles can become more common across generations.

What does fitness mean in AP Biology?

Fitness means reproductive success. An organism with higher fitness passes more genes to the next generation. Fitness depends on the environment and does not simply mean strongest, fastest, or longest-lived.

What is Hardy-Weinberg equilibrium?

Hardy-Weinberg equilibrium is a no-evolution baseline for a gene in a population. If observed genotype frequencies differ from expected values using p + q = 1 and p² + 2pq + q² = 1, at least one assumption may be violated.

What evidence supports evolution?

Evidence includes fossils, homologous and vestigial structures, molecular data, biogeography, and embryology. Strong AP answers connect the evidence to common ancestry or evolutionary change over time.

How do phylogenetic trees show common ancestry?

Phylogenetic trees show evolutionary relationships through branch points. Species that share a more recent common ancestor are more closely related. The tree does not show that one modern species evolved from another modern species.

What is speciation?

Speciation occurs when populations become reproductively isolated and diverge over time until they are separate species. Geographic, behavioral, temporal, or genetic barriers can reduce gene flow and allow divergence.

What is reproductive isolation?

Reproductive isolation prevents successful interbreeding between populations. Prezygotic barriers block fertilization; postzygotic barriers reduce hybrid survival or fertility after fertilization.

How should I study AP Biology Unit 7?

Start with the learning journey on this page, run the diagnostic, drill flashcards, practice Hardy-Weinberg problems, interpret phylogenetic trees, and write short FRQ chains that connect mechanism, evidence, and allele-frequency change.

What are common Unit 7 FRQ mistakes?

Common mistakes include saying organisms evolved because they needed to, claiming individuals evolve, confusing fitness with strength, treating genetic drift as adaptive, and reading phylogenetic trees as ladders of progress.

Continue learning

Next: AP Biology Unit 8 Ecology

Selection pressures connect to environments, populations, communities, and ecosystems. Continue into Unit 8 to link evolution with ecology.

Start Unit 8 → AP Biology course hub