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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
Practice this objective

Coverage 2025–2025 · Updated 16 Jul 2026

Endler Controlled Predation and Background in Guppy Selection

John Endler modelled natural and sexual selection in Trinidadian guppies by experimentally controlling predation pressure and gravel background.

Controlled factor Comparison Selection prediction
Predation No predator, weak predator, dangerous predator Strong predation favours less conspicuous males; low predation allows female choice to favour conspicuous males
Gravel background Coarse versus fine gravel Under predation, spot size that better matches the background improves camouflage

After dangerous predators were introduced, mean spot number decreased; with no or weak predation it continued to increase. Coarse gravel favoured larger spots and fine gravel smaller spots when predators were present.

A field transfer from a dangerous-predator site to a weak-predator site produced more colourful males over 15 generations, consistent with sexual selection becoming stronger relative to predation.

The experiment shows a trade-off: conspicuous colour can improve mating success yet reduce survival. Correlation alone is weaker evidence than Endler's controlled manipulation of selection pressures.

Modelling selection

Assessment in practice

1–4 marks
How it is assessed

This objective is assessed through multiple choice, commonly using Describe / Explain.

Command terms

Describe / Explain

What earns marks

Build the answer around this relationship: Models can show how selection changes variant frequencies over generations.

Representative question

Question 1

[Maximum number: 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?

A

There is a positive correlation between numbers of predator X and size of spots.

B

Predator Y has least influence on colouration.

C

There is a negative correlation between number of predators and size of spots.

D

There is no sexual selection.

Retrieve the Core Natural Selection Route

Core D4.1 examples follow the same causal route: heritable variation exists, a selection pressure acts, individuals differ in fitness, and alleles linked to higher reproduction become more common. Endler’s guppies and sexual selection are evidence versions of the same chain.

  • mutation creates alleles; meiosis and fertilization reshuffle combinations
  • overproduction, limited resources, and abiotic factors filter variants
  • passing alleles to offspring in a particular environment
  • Endler controlled predation pressure and guppy colour patterns changed

Core Natural Selection

Core natural-selection exam answers should never stop at “the best adapted survive.” They need the chain: heritable variation exists, a named pressure acts, some individuals have higher fitness, and their alleles become more common over generations. Use this for abiotic pressure, overproduction, sexual selection, and Endler-style data.

  • Explain natural selection using heritable variation, selection pressure, differential survival/reproduction, and population change.
  • Distinguish mutation/recombination as sources of variation from selection as the filtering process.
  • Use examples such as abiotic pressure, sexual selection, or Endler guppy data to support the chain.

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.