Why this unit matters
Evolution by natural selection is a central framework that unifies biological diversity and adaptation.
What you will learn
- Distinguish evolution as population change from acclimation in an individual organism.
- Explain how mutation, gene flow, genetic drift, and selection can shift allele frequencies.
- Evaluate evolutionary claims using data from fossils, comparative anatomy, molecular evidence, and population studies.
Understand the core ideas
Natural selection explains adaptation when heritable variation affects reproductive success under specific environmental conditions. Individuals do not evolve during their lifetimes, but populations change when allele frequencies shift across generations. Mutation introduces new variation, recombination reshuffles existing variation, and selection can increase or decrease allele frequency depending on trait effects in context. Fitness in AP Biology means relative reproductive contribution to the next generation, not physical strength or health in a generic sense.
Population genetics provides quantitative tools for evaluating evolutionary claims. Hardy Weinberg equilibrium sets null expectations under assumptions including random mating, no selection, no migration, large population size, and no mutation. Deviations from these assumptions can signal mechanism candidates. Genetic drift causes random allele frequency changes, especially in small populations, while gene flow can homogenize differences between populations. Selection is non random with respect to phenotype, though variation arises without direction toward future needs.
Evidence for evolution includes fossil sequences, homologous structures, shared molecular patterns, and direct observations in contemporary populations. AP level reasoning requires matching claim strength to data quality and considering alternatives. For example, a time series of trait distributions before and after environmental change can support directional selection if changes are heritable and associated with differential survival or reproduction. Explanations should avoid teleological wording and should describe mechanism based on inherited variation and population level outcomes.
Key terms
- allele frequency
- The proportion of all copies of a gene in a population that are a particular allele.
- genetic drift
- Random change in allele frequencies due to chance sampling effects, strongest in small populations.
- fitness
- Relative reproductive success of a genotype or phenotype compared with others in a population.
- Hardy-Weinberg equilibrium
- A null model in which allele and genotype frequencies remain constant across generations under specific assumptions.
Data reasoning: trait shift after drought
A bird population has mean beak depth 8.2 mm before drought and 9.0 mm after drought. Larger seeds were more common during the drought year.
- Identify directional phenotypic change by comparing mean beak depth before and after the event.
- Connect environmental change to resource distribution, with harder larger seeds becoming more available.
- Infer that birds with deeper beaks likely had higher survival or reproductive success in that period.
- Predict allele frequencies associated with deeper beaks increased in the post drought generation.
A common misconception
Claim: Organisms evolve traits because they need them when conditions change.
Correction: Selection acts on existing heritable variation; populations change when variants differ in reproductive success.
Lessons in this unit
- Population genetics foundationsDefine allele frequency and conditions for Hardy-Weinberg equilibrium.
- Mechanisms of evolutionary changeCompare directional effects of selection with stochastic effects of drift.
- Evidence for common ancestryIntegrate anatomical, molecular, and fossil evidence into coherent evolutionary explanations.
- Speciation and macroevolution patternsConnect reproductive isolation and divergence to new species formation.
Study task
Unit checkpoint
Why is natural selection described as population-level change?
Selection acts on individual phenotypes, but evolution is measured as changes in allele frequencies across generations within a population.