How do different modes of natural selection shape phenotype distributions?
Topic 7.2 Natural Selection: explain how directional, stabilizing and disruptive selection change the distribution of phenotypes in a population.
A focused answer to AP Biology Topic 7.2, covering directional, stabilizing and disruptive selection, sexual selection, and how each changes a phenotype distribution, with a worked interpretation of selection on a trait.
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What this topic is asking
The College Board (Topic 7.2) wants you to explain how the modes of natural selection (directional, stabilizing and disruptive) change the distribution of phenotypes, and to recognize sexual selection. You should interpret graphs of trait distributions.
The three modes of selection
Sexual selection
Reading a distribution graph
When a graph shows a phenotype distribution before and after selection, the shift in the curve identifies the mode: a sideways shift means directional; a narrowing around the mean means stabilizing; a splitting into two peaks means disruptive. A useful check is to ask which phenotypes were favored (reproduced more) and which were selected against, then see whether the new distribution matches.
All three modes act on the same underlying process, differential reproduction of heritable variants, but they reshape the population differently depending on which phenotypes the environment favors. The same population can experience different modes at different times: a trait under stabilizing selection in a constant environment can come under directional selection when the environment changes, as the finch beak example shows during a drought. This is why describing the selective pressure (what in the environment favors certain phenotypes) is essential in a full answer.
Try this
Q1. State which mode of selection narrows variation around the mean. [1 point]
- Cue. Stabilizing selection.
Q2. Explain how disruptive selection can lead toward two distinct groups. [2 points]
- Cue. It favors both extremes and selects against the intermediate, so the two extreme types reproduce more; over time the population can split into two groups, a possible first step toward speciation.
Exam-style practice questions
Practice questions written in the style of College Board exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
AP 2020 (style)4 marksSection II (long FRQ excerpt, graph). A graph shows the distribution of beak depth in a finch population before and after a drought that left only large, hard seeds. (a) Identify the type of selection shown and justify your answer. (b) Explain how this change in the population occurred.Show worked answer →
A 4-point identify-justify-explain FRQ on modes of selection.
(a) Identify and justify (2 points): (1 point) directional selection; (1 point) the distribution shifted toward greater beak depth (one extreme), rather than narrowing around the mean or splitting.
(b) Explain (2 points): (1 point) only large, hard seeds remained, so birds with deeper, stronger beaks could crack them, survived better and reproduced more; (1 point) they passed on the alleles for deep beaks, so the average beak depth in the next generation increased.
Markers reward identifying directional selection from the shifted distribution and explaining the survival-and-reproduction mechanism.
AP 2018 (style)1 marksSection I (multiple choice). Selection that favors the average phenotype and acts against both extremes is called: (A) directional selection. (B) disruptive selection. (C) stabilizing selection. (D) sexual selection.Show worked answer →
The correct answer is (C).
Stabilizing selection favors intermediate phenotypes and selects against both extremes, reducing variation around the mean (human birth weight is a classic example). Directional selection (A) favors one extreme; disruptive selection (B) favors both extremes; sexual selection (D) is based on mating success.
Related dot points
- Topic 7.1 Introduction to Natural Selection: explain the conditions required for natural selection and how it leads to changes in a population.
A focused answer to AP Biology Topic 7.1, covering Darwin's reasoning, the conditions for natural selection (variation, heritability, overproduction, differential reproduction), fitness, and how selection changes allele frequencies, with a worked example.
- Topic 7.4 Population Genetics: explain how natural selection, mutation, gene flow, genetic drift and non-random mating change allele frequencies.
A focused answer to AP Biology Topic 7.4, covering the gene pool, allele frequencies, and the five mechanisms of microevolution (selection, mutation, gene flow, genetic drift, non-random mating), including bottleneck and founder effects, with a worked allele-frequency calculation.
- Topic 7.5 Hardy-Weinberg Equilibrium: use the Hardy-Weinberg equations to calculate allele and genotype frequencies and test whether a population is evolving.
A focused answer to AP Biology Topic 7.5, covering the Hardy-Weinberg conditions, the equations p + q = 1 and p squared plus 2pq plus q squared = 1, and how to calculate and interpret allele and genotype frequencies, with worked calculations.
- Topic 7.10 Speciation: explain how reproductive isolation leads to speciation, including allopatric and sympatric speciation.
A focused answer to AP Biology Topic 7.10, covering the biological species concept, reproductive isolation (prezygotic and postzygotic barriers), allopatric and sympatric speciation, and rates of speciation, with a worked example.
- Topic 7.12 Variations in Populations: explain why genetic variation within a population is important for survival and the response to environmental change.
A focused answer to AP Biology Topic 7.12, covering the sources and importance of genetic diversity, how variation buffers populations against change, the risks of low diversity, and the role of variation in evolution, with a worked example.
Sources & how we know this
- AP Biology Course and Exam Description — College Board (2020)