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D4.1.12 (HL)—Types of selection

Directional, stabilizing and disruptive selection differ in which phenotypes are favoured and how variation changes in evolving populations in evolving populations.

Syllabus
First assessment 2025
Objective
D4.1.12
Level
HL

Exam analysis

Chance of appearing10%of analysed past papers
Latest appearanceNovember 2025
Most common paperPaper1
Typical marks1

Common command terms

  • Explain
  • Identify

Recent exam appearances

November 2025Paper1A ["HL"] · TZ138[ 1 ]D4.1.12 (HL)—Types of selection
May 2025Paper1B ["HL"] · TZ14(c)[ 2 ]D4.1.12 (HL)—Types of selection
May 2023Paper1 ["HL"] · TZ135[ 1 ]D4.1.12 (HL)—Types of selection
May 2022Paper1 ["HL"] · TZ236[ 1 ]D4.1.12 (HL)—Types of selection
November 2021Paper1 ["HL"] · TZ036[ 1 ]D4.1.12 (HL)—Types of selection

Coverage 2016–2025 · Updated 16 Jul 2026

Three Selection Modes Differ in Which Phenotypes Have Highest Fitness

HL only
Mode Highest fitness Distribution after selection
directional one extreme mean shifts toward that extreme
stabilizing intermediate variance narrows around the existing mean
disruptive both extremes intermediates decline; distribution can become bimodal

The graphs describe phenotype distributions, but evolutionary change ultimately requires associated heritable alleles or allele combinations to change frequency.

Before-and-after phenotype distributions show directional selection favoring one extreme, stabilizing selection favoring intermediates and disruptive selection favoring both extremes.

Antibiotics Cause Directional Selection of Resistant Bacteria

HL only
  1. Resistance variation exists before or arises independently of treatment.
  2. The antibiotic kills susceptible bacteria more effectively.
  3. Resistant survivors reproduce and contribute a larger fraction of the next population.
  4. Resistance alleles rise in frequency; plasmid transfer can spread them between cells.

The antibiotic does not teach individual bacteria to become resistant. It changes the survival difference between pre-existing or newly mutated variants.

A mixed bacterial population is exposed to antibiotic; susceptible cells die, resistant survivors reproduce, and resistance can also move by plasmid transfer.

Stabilizing Selection Removes Both Extremes Around an Intermediate Optimum

HL only
Birth-weight region Selective disadvantage can arise from
very low immature organs, low reserves and greater heat loss
intermediate lowest mortality under the measured historical conditions
very high delivery complications and associated maternal or fetal risks

When both extremes leave fewer surviving offspring, intermediate phenotypes contribute more. The mean may remain similar while the distribution narrows.

Human birth weight is affected by genes and environment. Historical hospital data show selection on the phenotype, but do not make every difference genetic or define one universal optimum.

Disruptive Selection Favors Both Extremes over Intermediates

HL only

environment contains two contrasting opportunities → individuals near either phenotypic extreme exploit one successfully → intermediate phenotype fits neither well → both extremes contribute more offspring → a bimodal distribution can develop

Yearling male plumage Territorial outcome proposed in the study
dull brown may be tolerated as non-threatening and gain territory
bright blue may be avoided as a strong rival and gain territory
intermediate attacked more often and gains poorer territory

Disruptive selection can increase divergence, but two peaks do not by themselves prove speciation. Reproductive isolation and continued genetic divergence are additional requirements.

Population Genetics Measures How Selection and Other Forces Change Gene Pools

HL only

count allele copies in the gene pool → establish p and q → use Hardy–Weinberg as a no-change expectation → compare observed genotype or allele frequencies across generations → use phenotype, fitness and population evidence to identify selection or another evolutionary force

Pattern or action First model to examine
mean shifts toward one extreme directional selection
variance narrows around intermediate values stabilizing selection
both extremes rise and intermediates fall disruptive selection
observed genotypes differ from p²:2pq:q² one or more equilibrium assumptions may be violated
humans repeatedly choose parents artificial selection

Keep description, calculation and cause separate. A changed distribution describes an outcome; allele counts quantify evolution; causal claims require evidence about inheritance, survival, mating, migration, drift or deliberate breeding.

Types of selection

HL only

[ 2 ]

Phenotypic variation allows natural selection within populations. Compare and contrast directional and disruptive selection.

Concept essentials

  • Directional selection shifts the population toward one extreme.
  • Stabilizing selection favours intermediate phenotypes.
  • Disruptive selection favours both extremes over intermediate forms.