Biology  by Bradford
IB Biology 2025 · Theme D · Continuity and change

D4.1 Natural selection. Practice questions with markscheme.

46 original IB-style questions on D4.1, written from the 2025 guide: 21 multiple-choice, 17 short-answer, 5 data-based, 2 extended-response part, 1 drawing. Below is a 20-mark standard-level practice paper built from them, ready to hand out as a class quiz or homework, or to sit yourself and mark against the scheme. Print it, project it, or build a fresh one on the same topic.

What the guide asks for

8 statements at SL and HL, 7 additional higher level.

  1. D4.1.1SL / HL Natural selection as the mechanism driving evolutionary change
  2. D4.1.2SL / HL Roles of mutation and sexual reproduction in generating the variation on which natural selection acts
  3. D4.1.3SL / HL Overproduction of offspring and competition for resources as factors that promote natural selection
  4. D4.1.4SL / HL Abiotic factors as selection pressures
  5. D4.1.5SL / HL Differences between individuals in adaptation, survival and reproduction as the basis for natural selection
  6. D4.1.6SL / HL Requirement that traits are heritable for evolutionary change to occur
  7. D4.1.7SL / HL Sexual selection as a selection pressure in animal species
  8. D4.1.8SL / HL Modelling of sexual and natural selection based on experimental control of selection pressures
  9. D4.1.9HL Concept of the gene pool
  10. D4.1.10HL Allele frequencies of geographically isolated populations
  11. D4.1.11HL Changes in allele frequency in the gene pool as a consequence of natural selection between individuals according to differences in their heritable traits
  12. D4.1.12HL Differences between directional, disruptive and stabilizing selection
  13. D4.1.13HL Hardy–Weinberg equation and calculations of allele or genotype frequencies
  14. D4.1.14HL Hardy–Weinberg conditions that must be maintained for a population to be in genetic equilibrium
  15. D4.1.15HL Artificial selection by deliberate choice of traits

In the bank for D4.1

  • 21 multiple-choice
  • 17 short-answer
  • 5 data-based
  • 2 extended-response part
  • 1 drawing
  • 20 higher level only

Every question is original and tagged to a guide statement. See the whole bank →

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The practice paper

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Biology · topic quiz
Standard level · topic practice, not an exam format
30 minutes20 marks

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Covers D4.1 Natural selection
Name:
1.

Which of these differences between individuals could natural selection act on?

[1]
  1. Beak depth differing between alleles
  2. Fur dyed a different colour
  3. A scar acquired in a fight
  4. Muscle size gained through exercise
2.

In many bird species, females choose mates with elaborate plumage. What is this an example of?

[1]
  1. Sexual selection for mating success
  2. Natural selection for better camouflage
  3. Selective breeding by humans
  4. Genetic drift in a small population
3.

In a population of shore snails, shell thickness varies, and thick-shelled individuals are less often killed by crabs. Which sequence correctly shows how this leads to natural selection?

[1]
  1. Crabs cause mutations for thicker shells
  2. All snails thicken their shells together
  3. Shells thicken because the snails need them
  4. Thick-shelled snails survive and reproduce more
4.

A single female Atlantic cod (Gadus morhua) can release several million eggs in one spawning season, yet the size of a cod population stays roughly constant from year to year. Which statement explains this?

[1]
  1. Most offspring die because food and other resources are limited, so only a few survive competition to reproduce
  2. Most of the eggs are never fertilized, because males release far fewer sperm than the number of eggs produced
  3. The population is limited by the number of females, which is fixed by an equal sex ratio among the hatched fry
  4. Adult cod die soon after spawning, so on average each female is replaced by only one of her offspring
5.

Trees growing on an exposed, windy ridge are short and twisted. Seeds collected from them and grown in a sheltered valley produce tall, straight trees. What do these observations show?

[1]
  1. Wind causes mutations inherited by the seeds
  2. The ridge population is evolving to be shorter
  3. The stunting is acquired, so not heritable
  4. Tree height is not influenced by genes
6.

A population of bacteria is repeatedly exposed to an antibiotic. Over time, resistant strains dominate. Which explanation is correct?

[1]
  1. The antibiotic caused the resistance mutations
  2. All bacteria gradually became slightly resistant
  3. Rare pre-existing resistant variants survived
  4. Bacteria learned tolerance and taught others
7.

John Endler placed guppies from the same original population into artificial ponds. Some ponds contained no predators and others contained a predatory fish. After about ten generations, males in the predator-free ponds had more and larger brightly coloured spots than males in the ponds with predators. Explain these results in terms of sexual selection and natural selection.

[4]
8.

Outline the sources of the variation on which natural selection acts.

[3]
9.

Insect pests have repeatedly evolved resistance to pesticides. Explain how this happens, and why it happens faster when pesticides are used heavily.

[4]
10.

In a dry grassland, plants of one grass species vary in the length of their roots. Explain how differences in adaptation among these plants can lead to natural selection.

[3]

Original practice questions © Biology by Bradford · CC BY-NC-SA 4.0 · Not affiliated with or endorsed by the International Baccalaureate Organization.
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Markscheme BbB-EAAAAAAAIBQAO9CB

One mark per point; / separates alternative wording within a point, OR separates alternative answers, words in brackets are not required, underlined words are essential. OWTTE = or words to that effect.

1. D4.1.1 [1]
  • A: only heritable variation is raw material for selection; acquired characteristics are not transmitted;
2. D4.1.7 [1]
  • A: sexual selection favours traits that win mates (display, weaponry), explaining showy characteristics that natural (survival) selection alone would not;
3. D4.1.5 [1]
  • D: differences in adaptation lead to differential survival, then differential reproduction, so the trait becomes more common in the population; need or use does not direct variation;
4. D4.1.3 [1]
  • A: overproduction of offspring in an environment with limited resources (food, space, shelter) means most die before reproducing; the survivors are those best able to compete, which is the basis of natural selection; sperm are not the limiting factor (B), sex ratio does not fix population size (C), and cod spawn in several seasons (D);
5. D4.1.6 [1]
  • C: the difference disappears when the plants share an environment, so it was not genetic; only heritable variation can respond to selection;
6. D4.1.4 [1]
  • C: mutation is random and prior; the antibiotic changes the selection pressure, so pre-existing resistant variants are favoured;
7. D4.1.8 [4 max]
  • female guppies prefer to mate with brightly coloured males, so bright males have more offspring (sexual selection);
  • in ponds without predators, this preference alone acts, so alleles for brighter / larger spots increase in frequency over the generations;
  • in ponds with predators, bright males are more visible / more easily seen and eaten, so they leave fewer offspring (natural selection against bright colour);
  • the two selection pressures act in opposite directions, and the outcome depends on which is stronger in each pond;
  • spot colour and size are heritable (genetic variation was present in the original population), so the change is evolutionary rather than a change within individuals;
  • because Endler controlled the selection pressure (presence or absence of predators) while keeping other conditions the same, the difference between ponds can be attributed to predation, OWTTE;
8. D4.1.2 [3 max]
  • mutation, the (original) source of new alleles;
  • meiosis: crossing over and random orientation create new allele combinations in gametes;
  • random fertilization / sexual reproduction combines alleles from two parents;
  • (in some contexts) gene transfer between organisms, OWTTE;
9. D4.1.4 [4 max]
  • (random) mutation produces rare resistant variants (before/independently of spraying);
  • spraying kills susceptible insects, so resistant ones survive and reproduce / differential survival;
  • resistance alleles are inherited, so their frequency rises over generations;
  • heavy use = stronger/more constant selection pressure, susceptible genotypes are removed faster;
  • insects' short generation times and large numbers speed the process, OWTTE;
10. D4.1.5 [3 max]
  • long-rooted plants are better adapted: they reach deeper water and survive drought better;
  • (so) the better-adapted individuals live longer / grow more and produce more offspring/seeds (differential survival and reproduction);
  • if root length is heritable, the offspring (tend to) inherit longer roots;
  • (so) alleles for longer roots increase in frequency over generations, and the population becomes better adapted to drought, OWTTE;

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