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D4.1.8—Modelling selection

Models of selection simplify populations so changes in survival, reproduction or allele frequency can be tested and visualized in evolving populations.

Syllabus
First assessment 2025
Objective
D4.1.8
Level
HL

Exam analysis

Chance of appearing2%of analysed past papers
Latest appearanceNovember 2025
Most common paperPaper1B
Typical marks1–4

Common command terms

  • Describe
  • Explain

Recent exam appearances

November 2025Paper1B ["HL"] · TZ14(d)[ 4 ]D4.1.8—Modelling selection
November 2025Paper1B ["HL"] · TZ14(c)[ 2 ]D4.1.8—Modelling selection
May 2025Paper1A ["HL"] · TZ337[ 1 ]D4.1.8—Modelling selection

Coverage 2025–2025 · Updated 16 Jul 2026

A Selection Experiment Must Manipulate Pressure and Measure Reproductive Consequences

Design element Selection experiment question
independent variable which pressure is deliberately changed?
dependent variable which phenotype, survival or mating outcome is measured?
controls which habitat, starting population and observation conditions stay comparable?
replication are multiple populations exposed to each treatment?
time are enough generations observed for frequency change?

Random assignment and controlled treatments make a causal claim stronger; repeated measurements reveal whether a pattern persists rather than reflecting one generation of chance.

A simulation or enclosure simplifies nature. Its value depends on whether the manipulated pressure and measured fitness components represent the biological system.

Endler Separated Predation Strength from Gravel Background

Treatment Predator pressure Prediction for male spots
K none female choice can favour conspicuous males
R weak, Rivulus hartii sexual selection remains influential
C dangerous, Crenicichla alta conspicuous males suffer stronger predation

Fine and coarse gravel were crossed with predator treatments. Under predation, spot size matching the substrate can improve camouflage; without strong predation, contrast can improve visibility to females.

Guppies were allowed to breed, predator treatments were imposed, and spot number and size were measured over later generations in replicated ponds.

Endler’s Guppies Reveal a Trade-off Between Predation and Mate Choice

Evidence Interpretation
spots increased before strong predators were added female choice favored conspicuous males
mean spot number fell with C. alta dangerous predators selected against conspicuous males
weak-predator field transplants became more colorful over 15 generations reduced predation allowed sexual selection to dominate
spot size tracked gravel under predation background matching improved camouflage

Natural and sexual selection acted on the same heritable color traits in opposing directions. The observed phenotype reflected the balance of those pressures, not one universal best color.

Guppy treatments K, R and C show spot number rising with no or weak predation and falling under dangerous C. alta predation.

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.

Modelling selection

[ 1 ]

John Endler experimented on populations of guppies (Poecilia reticulata) with different colouration. A male guppy fish is shown with large spots, which makes the fish more attractive to females, but more visible to predators.

The table shows the male colouration of guppy fish and number of predators in three different ponds.

Predator XPredator YMale guppy colouration
Pond 1120Large colourful spots
Pond 2150Medium colourful spots
Pond 3517None/very small drab spots

What can be concluded from the data?

Concept essentials

  • Models can show how selection changes variant frequencies over generations.
  • A model must define what its symbols or organisms represent.
  • Model conclusions are limited by simplifications and controlled assumptions.