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D4.1.4—Abiotic factors as selection pressures

Abiotic factors such as temperature, water availability, salinity, light and pH can act as selection pressures in evolving populations in evolving populations.

Syllabus
First assessment 2025
Objective
D4.1.4
Level
HL

Abiotic Conditions Select Existing Variants Without Regard to Population Density

Abiotic factor Variant that could be favoured in one context
drought lower water loss or deeper roots
salinity ion exclusion or salt tolerance
temperature enzymes and membranes that function at local extremes
light leaf form or pigment investment suited to the light regime
pH proteins and transport systems stable at local pH

A density-independent pressure can affect survival regardless of how crowded the population is, although its total impact may still differ among places and times.

The environment does not cause organisms to develop the needed inherited trait. It changes which already varying individuals contribute most to the next generation.

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.

Abiotic factors as selection pressures

[ 2 ]

Explain how natural selection is influenced by changes in the environment.

Concept essentials

  • Abiotic factors are non-living parts of the environment.
  • Abiotic selection pressures can change survival and reproductive success.
  • Heritable tolerance traits can become more common under persistent abiotic pressure.