IB Biology HL A3.2 Classification and cladistics questions

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Type
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Question 1

Paper 2Easy5 marks

A botanical team reviewed a classification in which flowering plant species A and B were grouped together because their flowers have a similar shape. The team compared the same 12-base region of DNA in the three species.

Species pairNumber of base differences
A and B12
A and C2
B and C10
(a)

Identify the pair with the most similar DNA sequences.

[1]
(b)

Does the DNA evidence support classifying A and B as the closest relatives among these three species? Justify your answer using one comparison from the table.

[2]
(c)

Suggest why classification based on flower shape could have been misleading.

[2]

Question 2

Multiple choiceHard1 mark

A museum’s catalogue covers many species and must organize them for comparative studies while reflecting inferred ancestry. The table gives the branching sequence, from earliest split to latest, and current class assignments. A clade contains a common ancestor and all its descendants. Which proposal meets both aims?

InformationData
Branching sequenceS separates from P, Q and R; then R separates from P and Q; then P separates from Q
Current classesM: P and Q; N: R and S
Show answer and explanation

Answer: B. A clade contains a common ancestor and all its descendants. The branching sequence gives nested clades P+Q and P+Q+R (and the full set); R+S is not a clade because their common ancestor also has P and Q as descendants. Classification can still organize extensive diversity for comparison, while unranked clades avoid implying that fixed taxonomic ranks measure evolutionary divergence.
A: It assumes both traditional classes are clades; class N excludes P and Q even though they descend from the same ancestor as R and S.
C: It treats the limitation of fixed ranks as a reason to discard classification, although classification facilitates comparative study.
D: It assigns rank by group size; the branching pattern does not require nested clades to have class and order ranks.
Remember: clades express common ancestry, whereas taxonomic ranks are fixed categories.

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