Level 4 — ApplicationTaxonomy & Classification

Taxonomy & Classification

⏱ 60 minutes50 marksprintable — key stays hidden on paper

Level 4: Application (Novel Problems, No Hints)

Time Limit: 60 minutes Total Marks: 50

Instructions: Answer all questions. Apply your understanding to unfamiliar scenarios. Show reasoning where required.

Question 1 — Naming a New Species (10 marks)

A field team discovers a previously undescribed cave-dwelling salamander. They propose the name Proteus stygius for it.

(a) State which part of this name is the genus and which is the species (specific) epithet. (2)

(b) In a published journal, the name is written as Proteus stygius. Identify two formatting rules of binomial nomenclature that this printed form follows. (2)

(c) A second team, working independently, later finds the same animal and names it Hydrocavia nocturna. According to the rules of nomenclature, which name should be retained, and why? (2)

(d) A student writes the name in a handwritten notebook. Describe the correct convention they must use, and explain one reason a universal naming system like this is scientifically necessary. (4)

Question 2 — Placing an Unknown Organism (12 marks)

A biologist isolates an unknown microbe with the following features:

Single-celled, no membrane-bound nucleus
Lives in a boiling acidic hot spring (pH 1, 90 °C)
Cell wall contains no peptidoglycan
Ribosomal RNA sequence differs markedly from that of E. coli

(a) Assign this organism to one of the three domains. Justify your choice using two of the features above. (4)

(b) Explain why this organism could not be classified in the same domain as E. coli, referencing a specific molecular and a specific structural difference. (4)

(c) The biologist claims the organism is "more closely related to humans than to E. coli." Evaluate whether this claim is consistent with the three-domain system. (4)

Question 3 — Building a Dichotomous Key (12 marks)

Four leaves are collected with these traits:

Leaf	Margin	Venation	Apex
W	smooth	parallel	pointed
X	toothed	net	rounded
Y	smooth	net	pointed
Z	toothed	net	pointed

(a) Construct a paired-statement dichotomous key that correctly separates all four leaves. (8)

(b) State one property your first dividing character must have for the key to work efficiently, and explain why "leaf colour" would be a poor choice of character. (4)

Question 4 — Interpreting Molecular Phylogenetics (10 marks)

Two organisms, P and Q, look almost identical (same size, shape, habitat). Gene-sequence analysis shows their rRNA differs by 18%, while organism P differs from a third organism R (which looks very different) by only 3%.

(a) Based on molecular data, group these organisms by closest relationship. Justify. (3)

(b) Explain why the physical similarity of P and Q could mislead a scientist who classifies using morphology alone. Name the evolutionary process responsible. (4)

(c) State one advantage of molecular phylogenetics over traditional morphology-based classification. (3)

Question 5 — Kingdom Characterisation (6 marks)

For each organism below, name the kingdom and give one distinguishing feature justifying your choice:

(a) A multicellular organism with cell walls of chitin that absorbs nutrients from dead matter. (2) (b) A unicellular eukaryote that moves using a flagellum and does not fit plant, animal or fungal groups. (2) (c) A prokaryote with peptidoglycan cell walls living in the human gut. (2)

Answer keyMark scheme & solutions

Question 1 (10 marks)

(a) Proteus = genus (1); stygius = species/specific epithet (1). (2) Why: First word denotes genus, second the species within it.

(b) Any two (1 each): genus capitalised, species epithet lowercase; whole name italicised. (2)

(c) Proteus stygius should be retained (1) — Principle of Priority: the first validly published name takes precedence (1). (2)

(d) Convention: underline each word separately when handwritten (2) — because italics cannot be handwritten. Reason (any one, 2): a universal system avoids confusion from local/common names that vary by language or region, allowing scientists worldwide to refer unambiguously to the same species. (4)

Question 2 (12 marks)

(a) Domain Archaea (1). Justification — any two (1.5 each ≈ 3, cap 4):

Prokaryote (no nucleus) yet lacks peptidoglycan → not Bacteria.
Extreme environment (thermoacidophile) typical of archaeal extremophiles.
rRNA differs markedly from E. coli (a bacterium). (4)

(b) Molecular difference: divergent rRNA sequence places it in a separate lineage from E. coli (2). Structural difference: cell wall lacks peptidoglycan, unlike bacterial walls which contain it (2). (4)

(c) Consistent (2): in the three-domain tree, Archaea and Eukarya (humans) share a more recent common ancestor than either does with Bacteria (2). So genetically the archaeon may indeed be closer to humans than to E. coli, despite appearing microbial. (4)

Question 3 (12 marks)

(a) One valid key (8 — award for logical, dichotomous, all 4 keyed out):

1a. Venation parallel .......................... Leaf W
1b. Venation net ............................... go to 2
2a. Margin smooth ............................. Leaf Y
2b. Margin toothed ............................ go to 3
3a. Apex rounded ............................. Leaf X
3b. Apex pointed ............................. Leaf Z

Marks: correct pairing structure (2), each leaf correctly reached (4), characters mutually exclusive (2).

(b) First character must split the group (ideally into two, and be a fixed, unambiguous, contrasting trait) (2). Leaf colour is poor because it is variable/subjective — it changes with age, season, health, and light, so it is unreliable and non-discrete (2). (4)

Question 4 (10 marks)

(a) P and R are most closely related (rRNA differs only 3%); Q is more distant from both (18% from P) (2). Grouping: (P, R) together, Q separate (1). (3)

(b) Morphological classification would wrongly group P with Q because they look alike (2), but similarity arose independently — convergent evolution — not from shared ancestry (2). (4)

(c) Any one (3): reveals true evolutionary relationships from genetic data / objective and quantitative / unaffected by convergent morphology / can distinguish cryptic species and relate microbes lacking clear morphology. (3)

Question 5 (6 marks)

(a) Fungi (1); chitin cell wall + absorptive/saprotrophic nutrition (1). (2) (b) Protista (Protoctista) (1); unicellular eukaryote not fitting other kingdoms (1). (2) (c) Bacteria (Eubacteria) (1); peptidoglycan cell wall (1). (2)

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  {"claim":"Q1 marks sum to 10","code":"result = (2+2+2+4)==10"},
  {"claim":"Q2 marks sum to 12","code":"result = (4+4+4)==12"},
  {"claim":"Q3 dichotomous key uses 3 decision points for 4 taxa","code":"result = 3==(4-1)"},
  {"claim":"Paper total is 50","code":"result = (10+12+12+10+6)==50"},
  {"claim":"P-R closer than P-Q (3% < 18%)","code":"result = 3 < 18"}
]