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D4.1.1—Natural selection as mechanism

Natural selection is the mechanism by which heritable variation leads to differential survival and reproduction in changing environments in evolving populations.

Syllabus
First assessment 2025
Objective
D4.1.1
Level
HL

Exam analysis

Chance of appearing11%of analysed past papers
Latest appearanceMay 2023
Most common paperPaper2
Typical marks2–7

Common command terms

  • Explain
  • Identify
  • Evaluate
  • Outline
  • State
  • Describe
  • Discuss

Recent exam appearances

May 2023Paper3 ["HL"] · TZ28(b)[ 3 ]D4.1.1—Natural selection as mechanism
May 2023Paper3 ["HL"] · TZ17(d)[ 2 ]D4.1.1—Natural selection as mechanism
November 2019Paper2 ["HL"] · TZ03(b)[ 3 ]D4.1.1—Natural selection as mechanism
November 2018Paper2 ["HL"] · TZ07(c)[ 7 ]D4.1.1—Natural selection as mechanism
November 2016Paper2 ["HL"] · TZ04(b)[ 3 ]D4.1.1—Natural selection as mechanism

Coverage 2013–2023 · Updated 16 Jul 2026

Natural Selection Changes Populations Through Unequal Reproductive Success

Natural selection is differential survival and reproduction caused by heritable differences among individuals. Across generations, alleles associated with greater reproductive contribution can become more frequent.

  1. Individuals in a population vary.
  2. Some variation is heritable.
  3. The environment creates selection pressures.
  4. Heritable variants cause differences in survival or mating success.
  5. Individuals leave different numbers of surviving offspring.
  6. Allele frequencies change across generations.

Individuals are selected; populations evolve. An individual does not genetically adapt because it needs to—the frequency of inherited variants changes in descendants.

Peppered-Moth Frequencies Changed When Camouflage Changed Survival

  1. The population already contained pale and melanic heritable forms.
  2. Soot darkened resting surfaces and reduced pale lichen cover in industrial areas.
  3. Birds more often detected the less-camouflaged form.
  4. Better-camouflaged moths survived and reproduced at a higher rate.
  5. The allele associated with that form increased locally across generations.

When air pollution later declined and lichens returned, the direction of the camouflage advantage could reverse. Fitness belongs to a phenotype–environment relationship, not permanently to one colour.

Pale and melanic peppered moth forms are shown side by side, with pale associated with non-polluted habitats and melanic with industrially polluted habitats.

Natural Selection Connects Variation, Environment and Reproduction Across Generations

mutation creates alleles + sexual reproduction reshuffles them → offspring vary → limited resources and abiotic or biotic pressures produce unequal survival and mating → heritable variants reach offspring unequally → population trait and allele frequencies change

Claim to test Evidence needed
adaptation occurred by selection heritable variation plus unequal reproductive contribution across generations
sexual selection acted trait differences predict mating success
environmental pressure caused change controlled manipulation or strong comparative evidence links pressure to fitness
population evolved heritable trait or allele frequency changed, not merely one individual

Avoid purpose-driven language. Populations do not create useful mutations on demand; selection filters inherited variation already produced by mutation and recombination.

Natural selection as mechanism

[ 8 ]

Explain how evolution may happen in response to environmental change with evidence from examples.

Concept essentials

  • Natural selection acts on individuals but changes populations across generations.
  • Advantageous traits must be heritable to increase in frequency.
  • Selection depends on environmental context and reproductive success.