A3.2 Classification and cladistics
Practice exam-style IB Biology questions for Classification and cladistics, aligned with the syllabus and grouped by topic.
Question 1
What is a main reason for classifying organisms?
A.
To show that morphology is always more reliable than molecular evidence
B.
To make the diversity of organisms searchable and useful for further study
C.
To ensure all organisms have the same number of taxonomic ranks
D.
To prevent revision of names when new evidence is found
Question 2
What is the correct order of traditional taxa from broadest to narrowest?
A.
Kingdom, class, phylum, order, genus, family, species
B.
Domain, kingdom, order, class, phylum, family, species
C.
Species, genus, family, order, class, phylum, kingdom
D.
Kingdom, phylum, class, order, family, genus, species
Question 3
What does a node represent in a cladogram?
A.
A hypothetical common ancestor where lineages diverged
B.
A living species at the end of a terminal branch
C.
A taxonomic rank between family and genus
D.
The total number of sequence differences in all taxa
Question 4
What are the three domains in the three-domain system?
A.
Prokaryota, Protista and Animalia
B.
Bacteria, Fungi and Plantae
C.
Archaea, Eubacteria and Protista
D.
Bacteria, Archaea and Eukaryota
Question 5
What feature of evolution causes difficulty when organisms are forced into fixed ranks such as family or order?
A.
Lineages diverge gradually rather than in evenly spaced taxonomic steps
B.
All mutations occur at a constant rate in every lineage
C.
Species within a genus cannot share a common ancestor
D.
Each species belongs to only one clade at any time
Question 6
What is a clade?
A.
A group of organisms that look similar in the same habitat
B.
A named group containing exactly one genus and all its species
C.
A common ancestor and all of its descendants
D.
A branch of a cladogram with no extinct organisms included
Question 7
What usually provides the most objective evidence for placing living organisms in the same clade?
A.
The common names used for the organisms
B.
The ecosystem in which the organisms are found
C.
Base sequences of genes or amino acid sequences of proteins
D.
The number of ranks assigned above the species level
Question 8
A cladogram has taxon P branching from the root first, then Q, with R and S sharing the most recent node. Which taxa are most closely related?
A.
P and R
B.
R and S
C.
Q and S
D.
P and Q
Question 9
What process can cause distantly related organisms to be wrongly grouped together when classification relies mainly on morphology?
A.
Convergent evolution
B.
Nested clade formation
C.
Ribosomal RNA conservation
D.
Genetic drift only in small populations
Question 10
Cladistic analysis showed that the traditional figwort family, Scrophulariaceae, did not correspond to one evolutionary branch. What conclusion follows?
A.
Morphological evidence can no longer be used for any plant classification
B.
The biological species concept must be used to define every family
C.
All species in Scrophulariaceae must have identical rRNA sequences
D.
Some plant species should be transferred so classification better matches phylogeny
Question 11
State one reason why classification is necessary in biology.
[1]
Outline one way classification facilitates further study of an unknown organism.
[1]
Question 12
Define clade.
[1]
State one type of evidence, other than DNA base sequences, that can be used to assign organisms to clades.
[1]
Question 13
What assumption is required when using a molecular clock to estimate divergence time?
A.
Every mutation changes the amino acid sequence
B.
Morphological traits are unaffected by natural selection
C.
Sequence differences accumulate at an approximately steady rate
D.
All organisms have identical generation times
Question 14
In parsimony analysis, what criterion is used to choose among possible cladograms?
A.
The cladogram that uses only morphological data
B.
The cladogram requiring the smallest number of evolutionary changes
C.
The cladogram with terminal branches arranged alphabetically
D.
The cladogram with the greatest number of named ranks
Question 15
In a cladogram, two terminal branches are rotated around a node so their left-to-right order changes. What effect does this have on the evolutionary relationships shown?
A.
It converts a clade into a traditional taxonomic rank
B.
It changes the relationships only if branch lengths are not drawn to scale
C.
It has no effect on the relationships shown by the branching pattern
D.
It changes the most recent common ancestor of the two taxa
Question 16
A named group contains a common ancestor and some, but not all, of its descendants. How would cladistics evaluate this group?
A.
It is a domain
B.
It is not a true clade
C.
It is confirmed by parsimony automatically
D.
It is more objective than a clade
Question 17
Why were rRNA base sequences useful in developing the three-domain system?
A.
rRNA is present in all cellular organisms and contains conserved and variable regions
B.
rRNA occurs only in eukaryotes and therefore identifies eukaryotic cells
C.
rRNA is a protein sequence that replaces the need for DNA evidence
D.
rRNA mutates so rapidly that closely related species always have identical sequences
Question 18
A cladogram does not state that branch lengths are proportional to time or sequence change. What conclusion is justified from a long branch drawn on the page?
A.
The lineage is unrelated to the taxa on shorter branches
B.
No conclusion about time or amount of change is justified from length alone
C.
The lineage must have accumulated the most mutations
D.
The lineage must have existed for the longest time
Question 19
Define taxon.
[1]
Distinguish between taxonomy and cladistics.
[1]
Question 20
State the sequence of the three narrowest taxa in the traditional hierarchy, from broadest to narrowest.
[1]
Explain why assigning fixed ranks can be arbitrary.
[1]
Question 21
A newly discovered mammal species is placed in a clade containing related mammals.
State one characteristic that could be predicted from its classification as a mammal.
[1]
Explain why classification based on evolutionary relationships allows such predictions.
[1]
Question 22
The diagram shows nested clades in a group of beetles.
Identify the smallest labelled clade that contains taxa B and C.
[1]
Outline what is meant by nested clades.
[1]
Question 23
The cladogram shows five taxa labelled A–E.
Identify the root.
[1]
Identify the pair of taxa that share the most recent common ancestor.
[1]
State why the horizontal order of terminal labels should not be used alone to infer relatedness.
[1]
Question 24
Name the three domains of life.
[1]
Explain why the three-domain system was a revolutionary reclassification.
[1]
Question 25
Compare the use of DNA base sequences and protein amino acid sequences in constructing cladograms.
[1]
State why sequences should be aligned before differences are counted.
[1]
Question 26
A cladogram includes labels for root, nodes and terminal branches.
Identify the feature that represents a taxon being compared.
[1]
Identify the feature that represents the common ancestor of all taxa shown.
[1]
Identify the feature that represents a hypothetical common ancestor where two lineages split.
[1]
Question 27
The table shows an aligned DNA sequence from the same gene in five lizard species.
| Species | Bases 1–9 | Bases 10–18 |
|---|---|---|
| A | ATGCTGTCA | ACGTTACAA |
| B | ATGCCGTTA | ACGTTGCAA |
| C | ACGCAGTCA | GTGCTGTAG |
| D | ATGCCGTTA | ACGTCGCAA |
| E | ATGCTGTCA | ATGTCACAA |
Identify the pair of species with the fewest base differences.
[1]
Deduce which pair probably diverged most recently.
[1]
Outline how the data could be used to construct a cladogram.
[1]
Question 28
The cladogram shows relationships among six bird taxa.
Identify the root of the cladogram.
[1]
Identify the sister taxon of taxon E.
[1]
Deduce whether taxa D, E and F form a clade.
[1]
Give a reason for your answer to part (c).
[1]
Question 29
The graph shows the proportion of species in several plant clades that produce a particular defensive chemical.
Identify the clade in which the chemical is most common.
[1]
Describe the pattern shown across related clades.
[1]
Suggest how classification could guide the search for another species producing this chemical.
[1]
Question 30
State what a molecular clock estimates.
[1]
Explain why a molecular clock gives only an estimate of divergence time.
[1]
Question 31
The same short DNA sequence was compared in four species. Species W and X differ by 1 base, W and Y by 6 bases, W and Z by 7 bases, and Y and Z by 2 bases.
Deduce the two pairs of species likely to share the most recent common ancestors.
[1]
State one limitation of using only this short sequence to construct a cladogram.
[1]
State the principle of parsimony in cladogram construction.
[1]
Question 32
A traditional group of aquatic vertebrates was based on streamlined body shape and fins. Molecular evidence shows that some members are more closely related to land vertebrates than to other members of the group.
State the evolutionary process that may explain the similar body shape.
[1]
Explain how cladistics could falsify the traditional classification.
[1]
State one consequence for taxonomy if the traditional group is not a clade.
[1]
Question 33
State one reason rRNA is suitable for comparing distantly related organisms.
[1]
Explain how rRNA sequence comparisons support classification into domains.
[1]
Question 34
Two clades of insects show many more DNA sequence differences than expected from fossil calibration.
Suggest one biological factor that could increase the rate at which sequence differences accumulate.
[1]
Explain how this factor could affect molecular clock estimates.
[1]
Question 35
State one advantage of using morphological traits in classification.
[1]
State one limitation of using morphological traits in classification.
[1]
Explain how molecular evidence can help resolve this limitation.
[1]
Question 36
The graph shows the relationship between amino acid sequence differences in a conserved protein and divergence times estimated from fossils for several mammal pairs.
Describe the relationship shown in the graph.
[1]
Identify the pair that appears to have accumulated sequence differences fastest relative to fossil age.
[1]
Explain how the graph supports the molecular clock concept.
[1]
Suggest one reason for an outlier in the graph.
[1]
Question 37
Two possible cladograms were produced for four orchid species using the same sequence data. The diagram shows the number of inferred base changes on each branch.
| Cladogram | Branch | Inferred base changes / number |
|---|---|---|
| I | root to AB node | 1 |
| I | AB node to species A | 2 |
| I | AB node to species B | 1 |
| I | root to CD node | 2 |
| I | CD node to species C | 3 |
| I | CD node to species D | 2 |
| II | root to AC node | 2 |
| II | AC node to species A | 3 |
| II | AC node to species C | 2 |
| II | root to BD node | 3 |
| II | BD node to species B | 1 |
| II | BD node to species D | 4 |
Calculate the total number of inferred base changes for cladogram I.
[1]
Calculate the total number of inferred base changes for cladogram II.
[1]
Deduce which cladogram is preferred by parsimony.
[1]
Evaluate whether parsimony proves that the preferred cladogram is the true evolutionary history.
[1]
Question 38
The table shows percentage similarity in small-subunit rRNA base sequences among three unknown microbes and representatives of Bacteria, Archaea and Eukaryota.
| Microbe | Bacteria / % | Archaea / % | Eukaryota / % |
|---|---|---|---|
| 1 | 91.8 | 76.4 | 74.9 |
| 2 | 75.8 | 92.6 | 77.1 |
| 3 | 73.5 | 78.2 | 90.4 |
Identify the domain to which microbe 1 should be assigned.
[1]
Identify the domain to which microbe 2 should be assigned.
[1]
Explain why rRNA data can be used for this classification.
[1]
Question 39
The table compares generation time and number of neutral DNA substitutions in three lineages over the same estimated time interval.
| Lineage | Generation time / years | Neutral substitutions / 10^6 bases |
|---|---|---|
| A | 0.8 | 36 |
| B | 4.5 | 17 |
| C | 13.0 | 6 |
Identify the lineage with the shortest generation time.
[1]
Describe the relationship between generation time and substitutions shown in the table.
[1]
Explain how this relationship could affect molecular clock estimates.
[1]
Question 40
A traditional plant family was defined using flower shape. The cladogram was produced from chloroplast DNA sequences; traditional family membership is shown beside each species.
Identify one traditional family that is not supported as a single clade.
[1]
Give evidence from the cladogram for your answer.
[1]
Suggest one reason flower shape may have misled the traditional classification.
[1]
Evaluate how the classification should be revised.
[1]
Question 41
The table shows presence (+) or absence (−) of five shared derived characteristics in five fossil mammals.
| Fossil | Single jaw bone | 3 ear bones | Diff. teeth | Cusp molars | Upright limbs |
|---|---|---|---|---|---|
| A | + | − | − | − | − |
| B | + | + | − | − | − |
| C | + | + | + | − | ? |
| D | + | + | + | + | + |
| E | + | + | + | + | + |
Identify the characteristic that is shared by all five fossils.
[1]
Identify the two fossils that share the greatest number of derived characteristics.
[1]
Deduce which two fossils are most likely to share the most recent common ancestor.
[1]
Explain one limitation of using only morphological traits from fossils to construct a cladogram.
[1]
Question 42
Outline two difficulties of classifying organisms using the traditional hierarchy of taxa.
[1]
Discuss why cladistics represents a paradigm shift from traditional ranked taxonomy.
[1]
Question 43
Define clade and shared derived characteristic.
[1]
Compare and contrast molecular and morphological evidence for assigning organisms to clades.
[1]
Question 44
State the evidence that led to the three-domain system and name the three domains.
[1]
Explain why the three-domain system changed biological classification.
[1]
Question 45
Two cladograms for the same group of reptiles are shown. Cladogram A is based on mitochondrial DNA and cladogram B is based on skull morphology.
Identify one relationship supported by both cladograms.
[1]
Identify one relationship that differs between the cladograms.
[1]
Suggest one reason why the two datasets may produce different hypotheses.
[1]
Evaluate how confidence in a cladogram could be increased.
[1]
Question 46
Describe how sequence differences are used in a molecular clock.
[1]
Evaluate the reliability of molecular clocks for estimating divergence times.
[1]
Question 47
Outline how molecular sequence data are prepared for constructing a cladogram.
[1]
Explain how base or amino acid sequences and parsimony can be used to choose a cladogram.
[1]
Question 48
Outline how convergent evolution can affect classification based on morphology.
[1]
Evaluate the use of cladistics to test whether traditional classifications correspond to evolutionary relationships.
[1]
Question 49
Outline why classification is useful after an organism has been identified.
[1]
Discuss the advantages and cautions of making predictions from a classification that reflects evolutionary relationships.
[1]
Question 50
Describe how to identify common ancestors and clades on a rooted cladogram.
[1]
Evaluate the interpretation of cladograms as hypotheses of evolutionary relationships.
[1]
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