Biology  by Bradford
IB Biology 2025 · Theme A · Unity and diversity

A4.1 Evolution and speciation. Practice questions with markscheme.

46 original IB-style questions on A4.1, written from the 2025 guide: 24 multiple-choice, 12 short-answer, 6 data-based, 2 extended-response part, 2 drawing. Below is a 22-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

7 statements at SL and HL, 4 additional higher level.

  1. A4.1.1SL / HL Evolution as change in the heritable characteristics of a population
  2. A4.1.2SL / HL Evidence for evolution from base sequences in DNA or RNA and amino acid sequences in proteins
  3. A4.1.3SL / HL Evidence for evolution from selective breeding of domesticated animals and crop plants
  4. A4.1.4SL / HL Evidence for evolution from homologous structures
  5. A4.1.5SL / HL Convergent evolution as the origin of analogous structures
  6. A4.1.6SL / HL Speciation by splitting of pre-existing species
  7. A4.1.7SL / HL Roles of reproductive isolation and differential selection in speciation
  8. A4.1.8HL Differences and similarities between sympatric and allopatric speciation
  9. A4.1.9HL Adaptive radiation as a source of biodiversity
  10. A4.1.10HL Barriers to hybridization and sterility of interspecific hybrids as mechanisms for of preventing the mixing of alleles between species
  11. A4.1.11HL Abrupt speciation in plants by hybridization and polyploidy

In the bank for A4.1

  • 24 multiple-choice
  • 12 short-answer
  • 6 data-based
  • 2 extended-response part
  • 2 drawing
  • 19 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
35 minutes22 marks

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Covers A4.1 Evolution and speciation
Name:
1.

A river splits a population of frogs into two groups that experience different climates. Which sequence describes how speciation could occur?

[1]
  1. Reproductive isolation prevents gene flow, selection changes each group's inherited characteristics, and they eventually cannot interbreed
  2. Differential selection causes frogs in each group to mutate in the direction needed for their climate, after which the groups stop interbreeding
  3. Gene flow between the two groups increases the genetic differences between them until they are so different that they are separate species
  4. Both groups experience identical selection pressures, so they accumulate the same mutations and eventually merge into one new species
2.

The base sequence of the same mitochondrial gene was determined in four mammal species. The table shows the number of bases that differ between each species and species P. Which species shared the most recent common ancestor with species P?

[1]
Species compared with PNumber of base differences
Q62
R8
S31
  1. Species Q
  2. Species R
  3. Species S
  4. It cannot be determined from sequence data
3.

John Endler transferred guppies (Poecilia reticulata) between pools with different predator levels. Within a few generations, males in low-predation pools became more brightly coloured, while those with predators became drab. What did this demonstrate?

[1]
  1. That selection can measurably change a population within a few generations
  2. That coloration in guppies is determined mainly by genetic drift
  3. That sexual selection always overrides predation pressure
  4. That the transferred populations immediately became separate species
4.

Distinguish between convergent and divergent evolution.

[3]
5.

The diagram shows the bones of the forelimb of four mammals, drawn to different scales.

human armbat wingwhale flipperhorse foreleghumerusradius and ulnacarpalsmetacarpal and phalangesForelimb skeletons of four mammals, not drawn to the same scale
(a)Explain how the diagram provides evidence for evolution.[2]
(b)Distinguish between homologous and analogous structures, using one example of each.[2]
6.

Outline how the total number of species on Earth changes over time.

[3]
7.

Peter and Rosemary Grant studied the medium ground finch (Geospiza fortis) on the small Galápagos island of Daphne Major. The birds crack seeds with their beaks, and beak depth is a heritable characteristic. In 1977 a severe drought struck the island: plants produced few of the small, soft seeds the finches prefer, leaving mainly large, hard seeds. Over 80 % of the finches died during the drought. The researchers measured the beak depths of the population before the drought and of the offspring generation hatched after it.

GroupMean beak depth / mm ± standard error
Adults before the drought (1976)9.4 ± 0.1
Offspring generation after the drought (1978)9.9 ± 0.1
(a)Calculate the change in mean beak depth between the two groups.[1]
(b)Deduce, with a reason, whether the difference between the two means is likely to be statistically significant.[2]
(c)Explain how the drought caused the change in mean beak depth.[3]
(d)Justify the researchers' conclusion that natural selection occurred in this population.[2]
(e)State one further requirement for this population to give rise to a new species.[1]

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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Show the markscheme

Markscheme BbB-EACAAAAAABQAOite

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. A4.1.7 [1]
  • A: isolation stops gene flow; different selection pressures drive divergence; incompatibility follows;
2. A4.1.2 [1]
  • B: base-sequence differences accumulate over time since divergence, so the fewest differences (R, 8) indicate the most recent common ancestor with P;
3. A4.1.1 [1]
  • A: predation favours crypsis, female choice favours brightness; shifting the balance shifted the population, direct experimental evidence of rapid selection in the wild;
4. A4.1.4-A4.1.5 [3 max]
  • convergent: unrelated/distantly related lineages become more similar, whereas divergent: related lineages become more different;
  • convergent evolution is driven by similar selection pressures/environments, whereas divergent evolution occurs under different pressures (often after a lineage splits);
  • convergent evolution produces analogous structures, whereas divergent evolution produces homologous structures;
  • example credited: e.g. dolphin/shark body form (convergent) OR pentadactyl limbs used for flying/swimming/running (divergent);

Points must be explicitly comparative. Award converse statements.

5. A4.1.4, A4.1.5
  • (a) [2]
    • all four limbs have the same bones in the same arrangement (one humerus, radius and ulna, carpals, five digits) / are homologous;
    • this is best explained by inheritance from a common ancestor, with natural selection modifying the limb for different functions (adaptive radiation), OWTTE;
  • (b) [2]
    • homologous: same origin/ancestral structure but possibly different function, e.g. the pentadactyl limbs shown;
    • analogous: same function but different origin, arising by convergent evolution, e.g. wings of birds and insects / eyes of octopus and vertebrates;
6. A4.1.6 [3 max]
  • new species arise only by the splitting of pre-existing species / by speciation;
  • speciation increases the total number of species;
  • extinction decreases the total number of species;
  • the total changes according to the balance between the rates of speciation and extinction, OWTTE;
  • gradual evolutionary change within a species (without splitting) does not change the number of species / is not speciation;
7. A4.1.1, A4.1.6
  • (a) [1]
    • (+) 0.5 mm;
  • (b) [2]
    • likely to be significant;
    • because the ranges (mean ± standard error) do not overlap / 9.4 ± 0.1 and 9.9 ± 0.1 are clearly separated, OWTTE;
  • (c) [3 max]
    • during the drought mainly large/hard seeds were available;
    • finches with deeper/stronger beaks could crack them, so they survived (and reproduced) at higher rates / differential survival;
    • beak depth is heritable, so surviving parents passed deeper beaks to offspring;
    • (hence) the mean beak depth of the next generation increased;
  • (d) [2 max]
    • variation in a heritable characteristic (beak depth) existed in the population;
    • survival/reproduction was non-random with respect to that characteristic (deeper-beaked birds survived better);
    • (and) the characteristics of the population changed in the next generation, which together define natural selection, OWTTE;
  • (e) [1]
    • reproductive isolation (from other populations) / continued divergence until interbreeding (with fertile offspring) is impossible;

    Content pivot.

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