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Diversity of Organisms · Paper 2 — Free Biology SL Practice Questions
1MasterySAQ-SEvolutionary relationships and cladistics6 marksPaper 2~9 min
The cladogram below shows proposed evolutionary relationships among five tetrapod groups: amphibians, reptiles, birds, marsupials, and placental mammals. Four shared derived characters (synapomorphies) are mapped onto the tree.
Group — Amniotic egg — Endothermy — Mammary glands — Hair/fur
Amphibians — — — —
Reptiles — ✓ — — — —
Birds — ✓ — —
Marsupials — ✓ — — — ✓
Placental mammals — ✓ — — — ✓
(a)
State the number of tetrapod groups in the table that possess the amniotic egg. [1 mark]
(b)
The cladogram places birds as the sister group to reptiles, and marsupials as the sister group to placental mammals, with both mammal groups together forming the outgroup to reptiles + birds.
Using the parsimony principle, calculate the minimum number of evolutionary origins (steps) required to explain the distribution of all four characters across the five groups. Assume each character, once gained, is never lost. Show your reasoning for each character. [3 marks]
(c)
A student proposes that endothermy evolved independently in birds and in the mammal lineage, rather than once in a common ancestor of all four non-amphibian groups.
Using the data in the table, explain why the cladogram supports the student's proposal. [2 marks]
Solutions
2MasterySAQ-SEvolutionary relationships and cladistics5 marksPaper 2~8 min
Researchers studying six insect species (A–F) have identified three shared derived characters: (i) folded wings, (ii) social behaviour, (iii) modified mouthparts for piercing. The presence (+) or absence (–) of each character is shown below.
Species — (i) folded wings — (ii) social behaviour — (iii) piercing mouthparts
A — – — –
B — + — –
C — + — –
D — + — +
E — – — +
F — – — +
The outgroup (ancestral state) for all three characters is absent (–). Characters evolve only in the direction absent → present (no reversals).
(a)
State the principle of parsimony as applied to cladogram construction. [1 mark]
(b)
A cladogram is constructed in which species B, C, and D form a monophyletic clade, with A as the outgroup to all others, and E and F forming a separate clade sister to the BCD clade. Calculate the total number of character-state changes required to map all three characters onto this cladogram. [3 marks]
(c)
The distribution of character (iii) across the six species is described as an example of homoplasy. Evaluate this claim by referring to the cladogram topology and the distribution of character (iii). [1 mark]
Solutions
3ChallengeSAQ-LEvolutionary relationships and cladistics8 marksPaper 2~12 min
Data
Parsimony — the cladogram requiring the fewest independent evolutionary changes is preferred.
A research team investigates evolutionary relationships among four species of flowering plants (Silene: species P, Q, R, S) and one outgroup species T. A cladogram is constructed from three shared derived characters (synapomorphies). The table below summarises character presence (+) or absence (−).
Species — Character 1: Red flowers — Character 2: Fused petals — Character 3: Nectar spur
P — + — −
Q — + — −
R — + — +
S — − — −
T (outgroup) — − — −
(a)
Determine the most parsimonious cladogram for species P, Q, R, S, and T. Describe the branching order and state which synapomorphy defines each internal node. [3 marks]
(b)
Explain how the principle of parsimony is applied when choosing between two alternative cladograms in this analysis. [2 marks]
(c)
The team also sequences the chloroplast gene rbcL from all five species. Evaluate the use of morphological characters compared with molecular sequence data for determining evolutionary relationships in Silene, referring to the data above. [3 marks]
Solutions
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