Natural Selection

What this unit covers

The topics below follow the published Biology course framework for Unit 7. This unit is worth 13–20% of the exam, so budget your time against that rather than against how long the unit takes to teach.

Lessons in this unit

• Natural Selection & Fitness14 min · 3 objectivesExplain the four conditions Darwin identified that produce natural selection · Define fitness as reproductive success and separate it from survival alone · Distinguish directional, stabilizing, disruptive, and sexual selection
• Evidence for Evolution13 min · 3 objectivesInterpret the fossil record and transitional forms as a timeline of change · Distinguish homologous, analogous, and vestigial structures and what each implies about ancestry · Use molecular and biogeographical evidence to infer evolutionary relationships
• Hardy–Weinberg Equilibrium15 min · 3 objectivesState the five conditions required for Hardy–Weinberg equilibrium · Use p + q = 1 and p² + 2pq + q² = 1 to calculate allele and genotype frequencies · Interpret Hardy–Weinberg as a null model whose violation signals evolution
• Speciation & Reproductive Isolation13 min · 3 objectivesDefine a biological species in terms of reproductive isolation · Contrast allopatric and sympatric speciation by the role of geography · Classify prezygotic and postzygotic barriers to reproduction
• Phylogeny, Common Ancestry & Extinction12 min · 3 objectivesRead a phylogenetic tree or cladogram to infer evolutionary relationships · Use shared derived characters to identify clades and common ancestors · Explain how shared cellular features support common ancestry and how extinction reshapes biodiversity
• Hardy–Weinberg Problem-Solving15 min · 3 objectivesCompute allele frequencies from genotype counts (gene-counting) and from the recessive phenotype using q² · Predict next-generation genotype frequencies and test whether a population is in Hardy–Weinberg equilibrium · Adapt Hardy–Weinberg reasoning to X-linked and recessive-lethal cases
• Phylogenetics & Cladistics14 min · 3 objectivesIdentify synapomorphies, outgroups, and monophyletic clades on a cladogram · Build the most parsimonious tree from a character matrix · Interpret molecular-clock estimates and reconcile molecular with morphological data

Formulas in Unit 7

Every term in Unit 7

All 38 terms we publish for Natural Selection, with definitions. Reading them through is the fastest way to find the ones you cannot define — then drill those in cram mode until you can produce them without the prompt.

Natural selection

Heritable variation plus differential reproductive success. Individuals do not adapt; populations change in allele frequency across generations.

Fitness

Reproductive success relative to others in the population — not strength or longevity. An organism that survives long and leaves no offspring has fitness zero.

Directional selection

Favors one extreme, shifting the mean — as in antibiotic resistance or peppered moths during industrialization.

Stabilizing selection

Favors the intermediate and removes both extremes, reducing variation. Human birth weight is the standard example.

Disruptive selection

Favors both extremes over the middle, which can drive a population toward splitting into two.

Sexual selection

Selection on mating success rather than survival. Can produce traits that reduce survival, like a peacock's tail, if they raise mating success more.

Genetic drift

Random change in allele frequency between generations. Its effect is far stronger in small populations, where chance can fix or lose alleles outright.

Bottleneck effect

A sharp population crash leaves a small, unrepresentative sample of alleles, so diversity stays low even after numbers recover.

Founder effect

A few individuals colonize a new area, carrying an unrepresentative allele sample — why some rare conditions are common in isolated populations.

Gene flow

Movement of alleles between populations by migration. Tends to make populations more similar and opposes divergence.

Hardy-Weinberg equations

p + q = 1 and p² + 2pq + q² = 1. A null model: allele frequencies stay constant unless something acts on them.

Hardy-Weinberg conditions

No mutation, no selection, no gene flow, random mating, and a very large population. Real populations violate these — which is why the model is useful for detecting evolution.

Speciation

Formation of new species when gene flow between populations stops and they diverge until reproduction between them is no longer possible.

Allopatric vs sympatric speciation

Allopatric requires geographic separation; sympatric happens without it, through polyploidy, habitat shift or sexual selection.

Reproductive isolation

Prezygotic barriers prevent mating or fertilization (timing, habitat, behavior); postzygotic ones make hybrids inviable or sterile.

Convergent evolution

Unrelated lineages evolve similar traits under similar pressures — dolphin and shark body plans. Produces analogous, not homologous, structures.

Homologous vs analogous structures

Homologous share ancestry but may differ in function (a bat wing and a human arm); analogous share function but not ancestry (a bat wing and an insect wing).

Evidence for evolution

Fossil record, comparative anatomy, embryology, biogeography and molecular sequence similarity — independent lines converging on the same tree.

Phylogenetic tree

A hypothesis of evolutionary relationship. Nodes are common ancestors; the branch pattern, not the tip order, carries the meaning.

Endosymbiosis as evolutionary evidence

Mitochondrial and chloroplast DNA resembles bacterial DNA more than nuclear DNA, supporting descent from free-living prokaryotes.

Origin of life evidence

Miller-Urey showed organic monomers form under early-Earth conditions; the RNA world hypothesis proposes RNA carried information and catalysis before DNA and protein.

Variation as the raw material

Selection can only act on differences that already exist. Mutation and recombination generate variation; selection sorts it.

Mutation as the ultimate source of alleles

Only mutation creates genuinely new alleles. Recombination reshuffles existing ones, so both are needed for sustained adaptation.

Heritability requirement

A trait must be genetically transmitted to evolve by natural selection. Acquired characteristics do not qualify.

Artificial selection

Humans choosing breeders for desired traits. Darwin used it as evidence that selection can reshape a population quickly.

Antibiotic and pesticide resistance

Resistant individuals already present survive treatment and reproduce, so resistance spreads. Treatment selects; it does not create the resistance.

Heterozygote advantage

A heterozygote out-reproduces both homozygotes, keeping an otherwise harmful allele in the population — sickle-cell carriers in malarial regions.

Balancing selection

Selection that maintains two or more alleles rather than fixing one, through heterozygote advantage or frequency dependence.

Frequency-dependent selection

An allele's fitness depends on how common it is, so rare types are often favored and variation persists.

Coevolution

Reciprocal evolutionary change between interacting species — predator and prey, plant and pollinator, host and parasite.

Adaptive radiation

Rapid diversification from one ancestor into many niches, typically after colonizing new habitat or a mass extinction. Darwin's finches.

Punctuated equilibrium vs gradualism

Long stasis broken by rapid change, versus slow steady change. The fossil record shows examples of both.

Molecular clock

Using the roughly constant rate of neutral mutation to estimate when two lineages diverged; calibrated against dated fossils.

Cladogram and shared derived characters

Branching diagram grouped by traits shared through common ancestry. A derived trait shared by a group defines a clade.

Out-group in a cladogram

A more distantly related species used as a reference point to establish which character states are ancestral.

Mass extinction and its effects

Five major events removed most species and each was followed by adaptive radiation of survivors into vacated niches.

Lab: Artificial Selection

Selecting an extreme phenotype in a fast-generating organism such as Wisconsin Fast Plants and measuring the shift in mean over generations.

Lab: Hardy-Weinberg modeling

Simulating allele frequencies over generations with and without selection, showing that frequencies stay constant only when every condition holds.

What examiners penalize here

• On free-response questions, do not just name the type of selection — justify it by pointing to which part of the trait distribution is favored (one extreme = directional, the middle = stabilizing, both extremes = disruptive) and tie the advantage to reproductive success in that specific environment.
• The AP exam treats evidence for evolution as convergent lines pointing to one conclusion: fossils, homologies, molecular sequences, and biogeography independently agree on the same tree of life. In free response, cite more than one line of evidence and state what each independently demonstrates.
• On the AP exam, always begin Hardy–Weinberg problems from the recessive phenotype: set it equal to q², square-root to get q, then p = 1 − q. Show every step. Graders award points for the correct setup (q² = recessive frequency) even if arithmetic slips later.
• For any isolation scenario on the exam, ask two questions in order: (1) Did a hybrid zygote form? No → prezygotic; Yes → postzygotic. (2) Then name the specific mechanism. Being able to justify the pre-/post- split earns more than just labeling it.
• For cladogram questions, translate "most closely related" into "shares the most recent common ancestor," and read shared derived characters from the branch points. When asked to justify relatedness on free response, cite the specific shared derived character and the node it defines.
• On the exam, state your setup explicitly before calculating: write "recessive phenotype = q²" (or "affected males = q" for X-linked), then show p = 1 − q, then p², 2pq, q². When asked whether a population is at equilibrium, compute allele frequencies by gene-counting and compare the observed genotypes to the predicted p²/2pq/q² — a mismatch is the evidence that the population is evolving.
• For tree questions, work in this order: use the outgroup to decide which states are derived, group taxa by shared *derived* characters (synapomorphies), and pick the tree with the fewest changes (parsimony). Treat a morphology-versus-molecule conflict as a homoplasy problem — name convergent evolution explicitly and explain why the character misleads.

Practice Biology

Our practice bank is drawn from across the whole course rather than filtered to one unit, which is closer to how the exam asks anyway — it will not tell you which unit a question is testing.

Questions about this unit

All 8 units of AP Biology

1. Unit 1 · Chemistry of Life
2. Unit 2 · Cell Structure & Function
3. Unit 3 · Cellular Energetics
4. Unit 4 · Cell Communication & Cycle
5. Unit 5 · Heredity
6. Unit 6 · Gene Expression & Regulation
7. Unit 7 · Natural Selection
8. Unit 8 · Ecology