Question 1
Paper 1A style
HL only (what this means)
HL only: additional Higher Level content, only for HL students. SL students can skip it. What the labels meanWhat does a node on a cladogram represent?
- A species alive today
- A hypothetical common ancestor
- A mutation in a single base
- The time since a species became extinct
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Answer: B [1]
Question 2
Paper 1A style
HL only (what this means)
HL only: additional Higher Level content, only for HL students. SL students can skip it. What the labels meanWhich evidence led to the classification of all organisms into three domains?
- Comparisons of cell wall structure
- Base sequences of ribosomal RNA
- Numbers of chromosomes
- Fossils of the earliest cells
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Answer: B [1]
Question 3
Paper 1A style
HL only (what this means)
HL only: additional Higher Level content, only for HL students. SL students can skip it. What the labels meanWhy can the molecular clock only give an estimate of when two clades diverged?
- Sequence differences stop accumulating once two clades have diverged.
- Only amino acid sequences, not base sequences, can be used.
- Mutation rates are affected by generation time, population size and selective pressure.
- The number of sequence differences decreases as time passes.
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Answer: C [1]
Question 4
Paper 2A style
HL only (what this means)
HL only: additional Higher Level content, only for HL students. SL students can skip it. What the labels mean(a) Outline the need for classification of organisms. [2] (b) Explain one advantage of classifying organisms according to evolutionary relationships. [2]
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- (a) immense diversity of species / very large number of species [1]
- (a) classification makes diversity manageable / allows identification [1]
- (a) after classification, further study is facilitated / information about related species can be found [1]
- (b) all members of the group have evolved from a common ancestor [1]
- (b) characteristics of members can be predicted [1]
- (b) because characteristics are shared within a clade [1]
- (b) e.g. a species related to one that produces a useful drug may produce it too [1]
- max 4
Question 5
Paper 1B style
HL only (what this means)
HL only: additional Higher Level content, only for HL students. SL students can skip it. What the labels meanThe table shows the number of amino acid differences in a protein between four species, P, Q, R and S (practice data). (a) Identify the two most closely related species. [1] (b) Assuming differences accumulate at 1 difference per 10 million years, estimate when P and S diverged. [1] (c) Deduce the sequence in which the species diverged, and describe the most probable cladogram. [2] (d) Suggest why the estimate in (b) may be inaccurate. [1]
| P | Q | R | S | |
|---|---|---|---|---|
| P | – | 9 | 10 | 12 |
| Q | 9 | – | 3 | 8 |
| R | 10 | 3 | – | 7 |
| S | 12 | 8 | 7 | – |
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- (a) Q and R [1]
- (b) 12 × 10 = 120 million years ago [1]
- (c) P diverged first / P is the outgroup [1]
- (c) then S; Q and R share the most recent common ancestor [1]
- (c) cladogram: P branches from the root, then S, with Q and R as sister species at the last node [1]
- (d) mutation rate may not be constant / varies with generation time / population size / selective pressure [1] Accept: P–Q differences (9) give a different date (90 million years) for the same divergence, so rates differ between lineages
- max 5
Question 6
Paper 2A style
HL only (what this means)
HL only: additional Higher Level content, only for HL students. SL students can skip it. What the labels meanA cladogram shows a root leading to node 1, which splits into species A and node 2. Node 2 splits into species B and node 3. Node 3 splits into species C and species D. (a) State which node represents the most recent common ancestor of B and D. [1] (b) Deduce how many clades contain species B. [1] (c) Identify the species most closely related to C. [1]
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- (a) node 2 [1]
- (b) two (the clade from node 1 and the clade from node 2) [1] Accept three if B alone is counted as a clade
- (c) D [1]
Question 7
Paper 2B style
HL only (what this means)
HL only: additional Higher Level content, only for HL students. SL students can skip it. What the labels meanDiscuss how cladistics has changed the classification of organisms.
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- traditional hierarchy of kingdom, phylum, class, order, family, genus, species [1]
- fixed ranks are arbitrary / do not reflect gradation of variation / do not always match patterns of divergence [1]
- cladistics uses unranked clades: an ancestor and all its descendants [1]
- evidence from base sequences of genes / amino acid sequences of proteins [1]
- sequence evidence is more objective than morphology [1]
- morphology can mislead because of convergent evolution [1]
- cladograms built using parsimony / fewest sequence changes [1]
- traditional groups found not to be clades were reclassified [1]
- e.g. figwort family / Scrophulariaceae: many species moved to other families [1]
- e.g. birds lie within the reptiles, so the class Reptilia is not a clade [1]
- rRNA base sequences led to three domains: Bacteria, Archaea, Eukarya (1977) [1]
- a new level above kingdoms / prokaryotes split into two domains [1]
- paradigm shift / earlier classifications falsified [1]
- but cladograms are hypotheses; different criteria / data can give different trees [1]
- max 7