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
- SL
Exam analysis
Chance of appearing1%of analysed past papers
Latest appearanceMay 2025
Most common paperPaper1A
Typical marks1
Common command terms
- Describe
- Explain
Recent exam appearances
May 2025Paper1A ["SL"] · TZ129[ 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 marksHow 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
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 X | Predator Y | Male guppy colouration | |
|---|---|---|---|
| Pond 1 | 12 | 0 | Large colourful spots |
| Pond 2 | 15 | 0 | Medium colourful spots |
| Pond 3 | 5 | 17 | None/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.