Directional, stabilizing and disruptive selection differ in which phenotypes are favoured and how variation changes in evolving populations in evolving populations.
Coverage 2016–2025 · Updated 16 Jul 2026
Directional, stabilizing and disruptive selection differ in which phenotypes have highest fitness; all can change allele frequencies.
| Mode | Favoured phenotype(s) | Distribution outcome |
|---|---|---|
| Directional | One extreme | Mean shifts toward that extreme |
| Stabilizing | Intermediate | Variation narrows around the existing mean |
| Disruptive | Both extremes | Intermediate values decline and the distribution may become bimodal |
Antibiotic exposure can cause directional selection toward resistance; selection against very low and very high human birth weights illustrates stabilizing selection.
These labels describe fitness patterns, not the number of genes involved. Disruptive selection does not automatically produce new species.
This objective is assessed through structured response, commonly using Explain / Identify.
Explain / Identify
Build the answer around this relationship: Directional selection shifts the population toward one extreme.
Representative question
Phenotypic variation allows natural selection within populations. Compare and contrast directional and disruptive selection.
HL D4.1 turns selection into measurable population genetics. A gene pool changes when allele frequencies shift. Hardy-Weinberg gives a no-evolution baseline; selection graphs, isolated populations, artificial selection, and resistance show how forces move populations away from that baseline.
HL population-genetics questions ask students to quantify or model evolution. The answer starts with the gene pool and allele frequencies, then uses the model or selection graph to decide whether the population is at equilibrium or being shifted by selection, mutation, migration, drift, artificial selection, or isolation.
