Biology · Unit 7: Natural Selection · 14 min read · Updated 2026-05-10
Natural Selection — AP Biology
AP Biology · Unit 7: Natural Selection · 14 min read
1. Core Principles of Natural Selection★☆☆☆☆⏱ 3 min
Natural selection is the primary mechanism of adaptive evolution, defined as differential survival and reproduction of individuals within a population due to differences in their heritable phenotypes. Co-developed independently by Charles Darwin and Alfred Russel Wallace, the theory rests on three core observations:
Populations have variation in heritable traits
More offspring are produced than can survive, leading to competition for limited resources
Traits that improve survival and reproduction will be passed on to more offspring, increasing their frequency in the population over generations
In the AP Biology CED, natural selection contributes 13–20% of the total Unit 7 exam score, with natural selection-specific questions making up 4–8% of the full exam. It appears in both multiple-choice (MCQ) and free-response (FRQ) questions.
2. Biological Fitness and Selective Pressures★★☆☆☆⏱ 3 min
Selective pressures are environmental factors that cause differential reproductive success. They can be abiotic (temperature, drought, toxin exposure) or biotic (predation, competition, parasitism), and act on existing phenotypic variation, not directly on genotypes.
Exam tip: If asked to explain what a fitness of 0 means, never say 'the individual dies young' — it means the individual produces zero surviving offspring, so the genotype cannot be passed to the next generation.
3. Modes of Natural Selection★★☆☆☆⏱ 3 min
Natural selection is categorized by how it changes the distribution of a continuous (polygenic) phenotype in a population. There are three core modes tested on the AP exam:
**Directional selection**: A single extreme phenotype is favored over all others, shifting the average phenotype of the population over time toward that extreme. Example: selection for smaller body size in marmots responding to climate warming.
**Stabilizing selection**: The intermediate phenotype is favored, and both extreme phenotypes are selected against. This reduces phenotypic variation and keeps the average phenotype near the original optimum. Example: human birth weight, where intermediate-weight babies have higher survival.
**Disruptive selection**: Both extreme phenotypes are favored, and the intermediate phenotype is selected against. This increases phenotypic variation and can lead to sympatric speciation. Example: beak size in finches when only large and small seeds are available.
Exam tip: When identifying a selection mode from a graph, remember: x-axis = phenotype value, y-axis = frequency. One shifted peak = directional; one narrow peak at the original center = stabilizing; two separate peaks at extremes = disruptive.
4. Modeling Selection with Hardy-Weinberg Equilibrium★★★☆☆⏱ 4 min
The Hardy-Weinberg principle states that allele and genotype frequencies remain constant from generation to generation only in the absence of evolutionary forces, including natural selection. When selection is acting, we use the Hardy-Weinberg framework to calculate how allele frequencies change between generations.
First, adjust each initial genotype frequency by its relative fitness, then normalize the adjusted frequencies by the mean fitness of the population ($\bar{w}$), which is the sum of all adjusted frequencies. The general formula for post-selection genotype frequencies is:
$p$ = initial frequency of the A allele, $q = 1-p$ = initial frequency of the a allele. After calculating post-selection genotype frequencies, the new allele frequency is:
p' = f(AA) + \frac{1}{2}f(Aa)
Exam tip: Always remember to normalize by mean fitness after multiplying by relative fitness. Skipping this step leaves you with total genotype frequencies that do not add up to 1, which will cost points on FRQs.
5. AP-Style Practice Questions★★★☆☆⏱ 4 min
Common Pitfalls
Why: Common language uses 'fitness' to mean physical condition, so students default to this incorrect definition.
Why: Students often anthropomorphize selection, thinking it acts toward a predetermined goal.
Why: Students focus on genetic variation when discussing evolution, so they misstate the level at which selection acts.
Why: Students remember directional selection favors an extreme, so they mix up the two when both extremes are favored.
Why: Students confuse the effect of stabilizing selection on variation vs. average phenotype.
Why: Students correctly multiply initial frequencies by relative fitness, but stop there, forgetting selection removes genotypes so the total must be renormalized.