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

A3.2 Classification and cladistics. Practice questions with markscheme.

45 original IB-style questions on A3.2, written from the 2025 guide: 19 multiple-choice, 17 short-answer, 5 data-based, 2 extended-response part, 2 drawing. Below is a 20-mark higher-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

0 statements at SL and HL, 9 additional higher level.

  1. A3.2.1HL Need for classification of organisms
  2. A3.2.2HL Difficulties classifying organisms into the traditional hierarchy of taxa
  3. A3.2.3HL Advantages of classification corresponding to evolutionary relationships
  4. A3.2.4HL Clades as groups of organisms with common ancestry and shared characteristics
  5. A3.2.5HL Gradual accumulation of sequence differences as the basis for estimates of when clades diverged from a common ancestor
  6. A3.2.6HL Base sequences of genes or amino acid sequences of proteins as the basis for constructing cladograms
  7. A3.2.7HL Analysing cladograms
  8. A3.2.8HL Using cladistics to investigate whether the classification of groups corresponds to evolutionary relationships
  9. A3.2.9HL Classification of all organisms into three domains using evidence from rRNA base sequences

In the bank for A3.2

  • 19 multiple-choice
  • 17 short-answer
  • 5 data-based
  • 2 extended-response part
  • 2 drawing
  • 45 higher level only

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

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

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Covers A3.2 Classification and cladistics
Name:
1.

Humans share a more recent common ancestor with chimpanzees than either does with gorillas. Which grouping is therefore a true clade?

[1]
  1. Chimpanzees and gorillas, excluding humans
  2. Humans, chimpanzees and gorillas together
  3. Gorillas and humans, excluding chimpanzees
  4. None of these groups can be a clade
2.

Two orders of insects diverged from each other about 300 million years ago, whereas two orders of mammals diverged about 60 million years ago. What does this show about the traditional hierarchy of taxa?

[1]
  1. A rank such as 'order' does not correspond to a fixed amount of divergence, so the ranking is arbitrary
  2. The insect orders should be promoted to classes, because they are much older than the mammal orders
  3. The mammal orders are not genuine taxa, because they diverged too recently to differ in their morphology
  4. Evolution has been slower in insects than in mammals, which is why insect orders contain fewer species
3.

The cladogram was constructed from base sequences of five plant genera. Dashed boxes show the families to which the genera were traditionally assigned. Which conclusion is supported by the cladogram?

[1]
genus Agenus Bgenus Dgenus Cgenus Etraditional family 1traditional family 2time →
  1. Genus A is the most recently evolved genus.
  2. Genera B and D are more closely related to C and E than to A.
  3. Traditional family 2 does not correspond to a clade.
  4. Traditional family 1 is a clade because all its members share an ancestor.
4.

Comparison of ribosomal RNA base sequences led in 1977 to a reclassification of all living organisms. Which statement describes this change?

[1]
  1. Kingdoms were replaced by three domains, so the rank of kingdom is no longer used in classification
  2. A new rank, the domain, was added above kingdoms, and the prokaryotes were divided between two domains
  3. All prokaryotes were placed in one domain and all eukaryotes in a second domain, giving two domains in total
  4. Eukaryotes were divided into three domains, corresponding to the plants, the animals and the fungi
5.

About two million species have been named, and millions more remain to be discovered. Which is the main reason why classifying these species into groups is necessary?

[1]
  1. It allows the immense diversity of species to be organized so that they can be identified and studied
  2. It removes the need to describe each species separately, because all members of a group are identical
  3. It allows the genome of each species to be predicted exactly from the group to which it belongs
  4. It provides a permanent classification that will not need to change when new evidence is found
6.

In one group of songbirds, differences between the base sequences of a mitochondrial gene accumulate at an average rate of 2.0 % per million years since two lineages diverged.

(a)Two of these species differ at 6.5 % of the bases in this gene. Estimate the time since they diverged from a common ancestor.[2]
(b)Suggest two reasons why this estimate should be quoted as a range of dates rather than as a single value.[2]
(c)Identify two kinds of independent evidence that could be used to check this estimate.[2]
1.
2.
7.

Two populations of a freshwater snail live in separate lakes. They look almost identical but differ slightly in the shape of the shell. A researcher wants to know whether they should be classified as one species or two.

(a)Design a molecular investigation to determine how closely the two populations are related.[4]
(b)Suggest one result that would support classifying the populations as two separate species.[1]
8.

Explain how evidence from cladistics can lead to the reclassification of a group of organisms, using at least one example.

[4]

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-EAFAAAAAABQAOice

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. A3.2.4 [1]
  • B: any complete branch is a clade; {human, chimp, gorilla} with their ancestor is monophyletic, whereas the pairings that exclude the closest relative are not;
2. A3.2.2 [1]
  • A: the same rank can represent very different amounts of evolutionary divergence, so fixed ranks do not reflect the gradation of variation (NOS); B fails because changing the rank would not remove the arbitrariness; C fails because divergence time does not decide whether a taxon is genuine; D fails because the data say nothing about rate of evolution or species number;
3. A3.2.8 [1]
  • B: genera B and D share a more recent common ancestor with C and E than with A, so traditional family 1 groups genera that do not form a clade, whereas family 2 (C and E) is a clade;
4. A3.2.9 [1]
  • B: the three-domain system added a level above kingdoms and split the prokaryotes into Bacteria and Archaea, with all eukaryotes in Eukarya; A fails because kingdoms remain within domains; C fails because rRNA showed Archaea to be as distinct from Bacteria as from eukaryotes; D fails because plants, animals and fungi are kingdoms within one domain;
5. A3.2.1 [1]
  • A: classification is needed because of the immense number of species; organizing them into groups makes identification and further study possible; B fails because members of a group are similar, not identical; C fails because only shared characteristics can be predicted, not a whole genome; D fails because classifications are revised as evidence (e.g. sequence data) accumulates;
6. A3.2.5
  • (a) [2]
    • 6.5 / 2.0;
    • 3.25 (million years) OR about 3.3 million years ago;

    Award [2] for the correct answer with no working. Accept 3.2-3.3 million years. ECF.

  • (b) [2 max]
    • the mutation rate is an average and does not stay exactly constant over time or between lineages;
    • the same base position may mutate more than once (back mutation/saturation), so differences underestimate the true number of changes;
    • the calibration of the clock (from fossils) is itself uncertain, so the error is carried into the estimate, OWTTE;
  • (c) [2]
    • the fossil record (dated radiometrically) for the group;
    • sequences of a different gene / of nuclear rather than mitochondrial DNA;
    • amino acid sequences of a protein shared by the two species;
    • dated geological events that separated the populations (e.g. formation of an island or mountain range);

    Mark the first two only.

7. A3.2.6
  • (a) [4 max]
    • collect a sample of several individuals from each lake (and one individual of a related species as an outgroup);
    • extract DNA and sequence the same gene/region (e.g. a mitochondrial gene) from every individual;
    • align the sequences and count the number of base differences between every pair of individuals;
    • compare the mean number of differences within each population with the mean number between the two populations;
    • use the differences to construct a cladogram, and repeat with a second, independent gene to check the result, OWTTE;
  • (b) [1]
    • differences between the populations are much greater than differences within either population / each population forms its own clade (separate branch), OWTTE;
8. A3.2.7-A3.2.8 [4 max]
  • cladograms are built from (shared) base/amino-acid sequence differences;
  • the pattern of branching shows which species share the most recent common ancestors;
  • if a traditional group is found not to be a clade (paraphyletic/polyphyletic), it is reclassified;
  • example: plants placed in the figwort family (Scrophulariaceae) were moved to other families when sequences showed closer kinship elsewhere;
  • (or) "reptiles" are not a clade unless birds are included, since crocodiles are closer to birds than to lizards;
  • (reclassification restores) groups whose members all descend from one ancestor, OWTTE;

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