AP Biology Unit 7, Natural Selection, is about how populations change genetically over time through evolution, with natural selection as the central mechanism that drives adaptation to the environment. The single biggest idea is that variation plus differential reproduction equals evolution: individuals with traits that boost survival and reproduction leave more offspring, so those traits become more common. This unit is 13-20% of the AP exam, and it ties together the genetics from Units 5 and 6 with the ecology coming in Unit 8, while bringing in Hardy-Weinberg math, speciation, evidence for evolution, and phylogenetic trees.
What this unit covers
How natural selection works and what drives it
- Natural selection is a major mechanism of evolution. Competition for limited resources means not everyone survives, so individuals with more favorable phenotypes survive and reproduce more, passing those traits on.
- Fitness is measured by reproductive success, not strength or size. A trait only counts as "fit" if it helps you leave more offspring in your current environment.
- Selection acts on phenotypic variation, the visible differences among individuals. No variation means nothing for selection to act on.
- Environments shift, so selective pressures shift too. Both biotic (living) and abiotic (nonliving) factors fluctuate, which can speed up, slow down, or reverse the direction of evolution. Flowering time shifting with global climate change is a classic example.
- Real cases to know: sickle cell anemia (the allele persists because it protects against malaria) and DDT resistance in insects (resistant bugs survive spraying and reproduce).
- Artificial selection is the same logic, but humans pick who reproduces. Breeding dogs or crops shows selection can reshape a population fast when the selecting pressure is intentional.
Random forces and population genetics
- Evolution is not only natural selection. Random processes also change allele frequencies.
- Mutation is the original source of new genetic variation. It is random and provides the raw material selection acts on.
- Genetic drift is a change in allele frequencies from chance, and it matters most in small populations.
- Bottleneck effect is drift after a population crashes to a small size for at least one generation, which wipes out variation by luck.
- Founder effect is drift when a few individuals start a new, isolated population, carrying only a slice of the original gene pool.
- Gene flow is the movement of alleles between populations. It keeps populations similar and prevents them from diverging into separate species.
- Changes in allele frequencies over time are direct evidence that evolution is happening.
Hardy-Weinberg equilibrium
- Hardy-Weinberg is a model that predicts allele and genotype frequencies in a population that is NOT evolving. It is your null hypothesis, the "what if nothing changed" baseline.
- Five conditions must hold: a large population, no migration, no new mutations, random mating, and no natural selection.
- These conditions are never all met in real life. That is the point. When real frequencies don't match the predicted ones, evolution is occurring.
Speciation, variation, and continuing evolution
- Speciation happens when two populations become reproductively isolated so they can no longer exchange genes.
- The biological species concept defines a species as a group that can interbreed and produce viable, fertile offspring.
- Allopatric speciation needs geographic separation; sympatric speciation happens even when populations overlap.
- Pre-zygotic barriers stop mating or fertilization from happening; post-zygotic barriers make hybrids inviable or infertile.
- Gradualism is slow change over hundreds of thousands to millions of years. Punctuated equilibrium is long stasis broken by rapid bursts of change.
- Divergent evolution and adaptive radiation produce many forms fast when new habitats open up. Convergent evolution gives unrelated species similar traits under similar pressures. Know Hawaiian Drosophila, Caribbean Anolis lizards, and the apple maggot Rhagoletis.
- Genetic diversity determines resilience. Populations with little variation are at high risk of decline or extinction; diverse populations are more likely to contain individuals that survive a new pressure.
- Evolution is ongoing: antibiotic and pesticide resistance, chemotherapy-resistant cancer, and emerging pathogens are all evolution happening right now.
Evidence, common ancestry, and phylogeny
- Evidence for evolution comes from many fields: geographical, geological, physical, biochemical, and mathematical data.
- Fossils can be dated by the rock layers they're in, by isotope decay (like carbon-14), and by geographic data.
- Morphological homologies, including vestigial structures, point to common ancestry. DNA and amino acid sequence comparisons do too, and more similarity means closer relatedness.
- All eukaryotes share features that signal common ancestry: membrane-bound organelles, linear chromosomes, and genes with introns.
- Origins of life: Earth formed about 4.6 billion years ago, was too hostile for life until about 3.9 bya, and the earliest fossils date to about 3.5 bya. The RNA world hypothesis proposes RNA could store information AND catalyze reactions before DNA and proteins took over.
- Phylogenetic trees and cladograms are testable hypotheses about evolutionary relationships. Trees show time (calibrated by fossils or a molecular clock); cladograms do not. Nodes mark the most recent common ancestor, and an out-group anchors the comparison.
Unit 7, Natural Selection at a glance
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| Natural selection | Differential survival based on phenotype | Selective pressure on variation | Sickle cell, DDT resistance |
| Genetic drift | Random change in allele frequency | Chance, small population | Bottleneck, founder effect |
| Gene flow | Alleles move between populations | Migration | Prevents divergence |
| Hardy-Weinberg | Null model for a non-evolving population | 5 conditions hold | Mismatch = evolution |
| Speciation | New species from reproductive isolation | Allopatric or sympatric isolation | Hawaiian Drosophila, Anolis |
| Phylogeny | Hypothesis of evolutionary relationships | Shared/derived traits, sequences | Trees show time, cladograms don't |
Why Unit 7, Natural Selection matters in AP Bio
This is the unit that explains the "why" behind all of biology. Every adaptation you study elsewhere, from enzyme function to cell signaling, exists because it gave organisms a reproductive edge over time. Unit 7 turns biology from a list of parts into a story of how those parts came to be.
- It anchors the big idea of evolution, the thread that connects all living things through common ancestry.
- It is where biology gets quantitative, using Hardy-Weinberg to model and predict allele frequencies and test whether a population is evolving.
- It builds the skill of reading evidence across disciplines, combining fossils, anatomy, molecular data, and math into one argument.
- It connects mechanism to outcome: random mutation and selection at the molecular level scale up to new species and whole patterns of biodiversity.
How this unit connects across the course
- Heredity (Unit 5) gives you the alleles and genotypes that Hardy-Weinberg tracks. Mendelian inheritance is the engine; population genetics is that engine running across a whole population over generations.
- Gene Expression and Regulation (Unit 6) supplies the mutations and the phenotypic variation that selection acts on. A change in gene regulation is exactly the kind of raw material that can become an adaptation here.
- Ecology (Unit 8) pays off the selective pressures introduced here. Competition, predation, and changing environments are the forces that decide which phenotypes win, and population dynamics build directly on genetic diversity from this unit.
- Cells (Unit 2) comes back through common ancestry. The shared organelles, linear chromosomes, and introns of all eukaryotes are structural evidence that ties cell biology to evolutionary history.
Key equations and processes
- p + q = 1: allele frequencies for a gene with two alleles always sum to 1, where p is the dominant allele frequency and q is the recessive.
- p² + 2pq + q² = 1: genotype frequencies sum to 1, where p² is homozygous dominant, 2pq is heterozygous, and q² is homozygous recessive. Start from q² (the recessive phenotype) when solving for frequencies.
- Natural selection process: variation exists, overproduction creates competition, individuals with favorable phenotypes survive and reproduce more, and advantageous alleles increase in frequency over generations.
- Genetic drift process: random sampling of alleles changes frequencies, with the largest effect in small populations (bottleneck and founder effects).
- Speciation process: gene flow stops because of reproductive isolation, populations diverge under different pressures, and they eventually can no longer interbreed.
- Building/reading a phylogeny: use shared derived traits or sequence similarities, place an out-group to root it, and read nodes as most recent common ancestors.
Unit 7, Natural Selection on the AP exam
This unit is 13-20% of the exam, one of the heavier weights in the course, so expect it in both multiple-choice and free-response. Hardy-Weinberg shows up as grid-in and calculation questions where you solve for allele or genotype frequencies, often starting from the recessive phenotype, and then explain whether a population is in equilibrium. Phylogenetic trees and cladograms appear as stimulus questions where you read relationships, identify common ancestors, and use the out-group. Evidence for evolution questions ask you to analyze data (molecular sequences, fossil dating, homologies) and justify a claim about relatedness or common ancestry. On free-response, you'll commonly explain how a mechanism works (selection, drift, speciation), predict how a population's genetics change under a given pressure, and design or interpret an experiment that tests an evolutionary hypothesis. Connecting molecular variation to fitness and tracing change over time are the recurring skills.
Essential questions
- How does variation in a population get turned into adaptation over generations?
- Why is Hardy-Weinberg useful if its conditions are never actually met in nature?
- What makes two populations become separate species, and what keeps them apart?
- How do scientists use evidence from completely different fields to reconstruct the history of life?
Key terms to know
- Fitness: an organism's reproductive success in a specific environment, the real measure of how "favorable" a trait is.
- Phenotypic variation: the visible differences among individuals that natural selection acts on.
- Genetic drift: a change in allele frequencies caused by random chance, strongest in small populations.
- Bottleneck effect: drift caused by a sharp, temporary crash in population size that wipes out variation.
- Founder effect: drift caused when a small group starts a new isolated population with limited genetic diversity.
- Gene flow: the transfer of alleles between populations, which keeps them genetically similar.
- Hardy-Weinberg equilibrium: a model predicting allele and genotype frequencies in a population that is not evolving.
- Reproductive isolation: the inability of two populations to interbreed, the requirement for speciation.
- Allopatric speciation: speciation that requires geographic separation between populations.
- Sympatric speciation: speciation that occurs without geographic separation, in overlapping populations.
- Adaptive radiation: rapid speciation into many forms as new habitats and niches open up.
- Convergent evolution: unrelated species independently evolving similar traits under similar selective pressures.
- Homologous structures: features shared because of common ancestry, even if their functions now differ.
- Molecular clock: using the steady rate of sequence change to estimate when lineages diverged.
Common mix-ups
- Natural selection vs. genetic drift: selection is non-random (phenotype determines survival), while drift is pure chance. Drift matters most in small populations; selection can act in any size.
- Homologous vs. analogous structures: homologous means similar from shared ancestry (whale flipper and bat wing); analogous means similar function from convergent evolution (bird wing and insect wing), not common ancestry.
- Phylogenetic tree vs. cladogram: trees show an actual time scale or amount of change; cladograms only show branching order, not time.
- Fitness misread as strength: fitness is only about reproductive success. A fast, strong organism with no surviving offspring has zero fitness.
