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IB Biology 2025 · Theme A · Unity and diversity

A3.2 Classification and cladistics (HL). Practice questions with markscheme.

66 original IB-style questions on A3.2, written from the 2025 guide: 31 multiple-choice, 25 short-answer, 5 data-based, 2 extended-response parts, 2 drawing, 1 labelling. This subtopic is additional higher level in its entirety, so below is a 21-mark higher-level practice paper built from them. Hand one out as a class quiz or homework, or sit it yourself and mark against the scheme. Print it, project it, or build a fresh one on the same topic.

What the guide asks for

All 9 statements are additional higher level, so this subtopic is HL only.

  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

  • 31 multiple-choice
  • 25 short-answer
  • 5 data-based
  • 2 extended-response parts
  • 2 drawing
  • 1 labelling
  • 66 higher level only

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

Make your own

The practice paper

Additional higher level only · 21 marks
Take it online → Build a fresh paper Paper code BbB-EAFAAAAAABQAOice
Biology · topic quiz
Higher level · topic practice, not an exam format
30 minutes21 marks

Paper code: this paper was generated, so the code is its recipe. Enter it at biologybybradford.com/exam-maker to rebuild this exact paper and its markscheme.

Covers A3.2 Classification and cladistics · HL
Name:
1.

In the traditional hierarchy of taxa, which sequence runs from most inclusive to least inclusive?

[1]
  1. Kingdom → phylum → order → class → family → genus → species
  2. Kingdom → phylum → class → order → family → genus → species
  3. Kingdom → class → phylum → order → genus → family → species
  4. Species → genus → family → order → class → phylum → kingdom
2.

What is a clade?

[1]
  1. An ancestral species together with all of its descendants
  2. Any group of organisms given a scientific name
  3. A group of organisms that live in the same habitat
  4. A group of species with similar appearance
3.

Part of the same gene was sequenced in four species of frog, K, L, M and N, and in an outgroup: a species from a related group that is known to have separated from the ancestors of all four frogs before they diverged from each other. The table shows the bases at eight aligned positions, numbered 1 to 8.

Species / position12345678
outgroupACGTACGT
KACGTACGC
LATGCACGT
MATGTGCAT
NATATGCAT
(a)Outline how the outgroup is used in deducing a cladogram for the four frogs.[2]
(b)Identify the positions that provide evidence that M and N form a clade.[1]
(c)Deduce the branching order of K, L, M and N, using evidence from the table.[2]
4.

Five plant species, P to T, were scored for four characters. Species P has none of them. Species Q has vascular tissue only. Species R has vascular tissue and seeds. Species S has vascular tissue, seeds and flowers, with separate petals. Species T has vascular tissue, seeds and flowers, and its petals are fused. All five species are green and photosynthetic.

(a)Identify the species that shares the most recent common ancestor with T.[1]
(b)Deduce the order in which the four characters were gained during the evolution of this group.[2]
(c)Suggest why the character 'is green and photosynthetic' is of no use in constructing this cladogram.[2]
(d)Comment on the reliability of a cladogram built from only four morphological characters.[2]
5.

Explain why classifications of organisms are sometimes changed.

[2]
6.

The same mitochondrial gene was sequenced in four species, W, X, Y and Z. The table shows the number of base differences between each pair of species.

WXYZ
W–
X32–
Y930–
Z281227–
(a)Identify the two pairs of species that are most closely related.[2]
1.
2.
(b)Deduce the branching pattern (cladogram) that these data support, describing it in words or as a sketch.[1]
(c)Species X differs from W by 32 bases and from Y by 30 bases. Explain why these two numbers are so similar.[2]

Original practice questions © Biology by Bradford · CC BY-NC-SA 4.0 · Not affiliated with or endorsed by the International Baccalaureate Organization.
Rebuild or edit this exact paper (and its markscheme): biologybybradford.com/exam-maker?code=BbB-EAFAAAAAABQAOice

Show the markscheme

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. A3.2.2 [1]
  • B: (domain →) kingdom, phylum, class, order, family, genus, species;
2. A3.2.4 [1]
  • A: a clade is monophyletic: an ancestor plus all descendants (living and extinct);
3. A3.2.6
  • (a) [2 max]
    • the base found in the outgroup at a position is taken to be the base present in the common ancestor (the ancestral base);
    • a base that differs from the outgroup base is a change that arose within the group (a derived base);
    • only a derived base shared by two or more species is evidence that they form a clade / a base shared with the outgroup is not;
    • it shows which species branched off first, so the cladogram can be rooted, OWTTE;
  • (b) [1]
    • positions 5 and 7 (both required);
  • (c) [2 max]
    • K branches off first, because it has the ancestral C at position 2 while L, M and N share the derived T;
    • L branches off next, with M and N as the most closely related pair (sharing G at 5 and A at 7);
    • the bases at positions 3, 4 and 8 are each found in one species only, so they do not group any species / positions 1 and 6 do not vary, OWTTE;

    Accept a correct sketch of (K, (L, (M, N))) for the first two points, provided evidence is given. Do not credit grouping K with L from the pairwise counts alone.

4. A3.2.7
  • (a) [1]
    • S;
  • (b) [2 max]
    • vascular tissue was gained first, because it is present in Q, R, S and T but absent in P;
    • seeds were gained next, being present in R, S and T only;
    • flowers were gained after seeds, in S and T only;
    • fused petals were gained last, in T alone, OWTTE;

    Accept the order written as vascular tissue - seeds - flowers - fused petals. Award [2 max].

  • (c) [2]
    • it is present in all five species, so it does not separate them into groups;
    • it is an ancestral (shared primitive) character inherited from their common ancestor rather than a derived character, OWTTE;
  • (d) [2 max]
    • four characters is a very small sample, so misinterpreting a single character would change the whole tree;
    • morphological characters can be convergent (analogous), evolving independently in unrelated lineages, or can be lost secondarily;
    • reliability would be improved by adding many more characters and, above all, base or amino acid sequence data, OWTTE;
5. A3.2.8 [2 max]
  • classifications are hypotheses (about ancestry), not fixed facts;
  • new evidence, especially molecular/base-sequence data, may show a group is not a true clade / contains unrelated members;
  • (so) taxa are reorganized so that (each) group is monophyletic, OWTTE;
6. A3.2.5-A3.2.6
  • (a) [2]
    • W and Y (9 differences);
    • X and Z (12 differences);
  • (b) [1]
    • ((W,Y),(X,Z)): W pairs with Y and X pairs with Z, the two pairs joining at a deeper node;
  • (c) [2 max]
    • W and Y share a more recent common ancestor with each other than with X / X branched from the lineage leading to W and Y at one (deeper) node;
    • so the X lineage has been separate from W and from Y for the same length of time;
    • base differences accumulate at a (roughly) constant rate, so equal times give similar numbers of differences; the small gap (2 bases) reflects chance variation in the rate between the W and Y lineages, OWTTE;

    Content pivot.

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