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Diversity of Organisms · Paper 2 — Free Biology HL Practice Questions
1MasterySAQ-SDomain-based classification6 marksPaper 2~9 min
A phylogenetic tree was constructed using small subunit ribosomal RNA (SSU rRNA) sequences from three domains of life. The tree is rooted at a trifurcation point, with three branches of approximately equal length diverging simultaneously to Domain Bacteria (E. coli, 1542 nucleotides), Domain Archaea (Methanococcus, 1430 nucleotides), and Domain Eukarya (Homo sapiens, 1869 nucleotides). Within Domain Eukarya, the branch to Saccharomyces cerevisiae is longer than the branch to Homo sapiens.
(a)
State the domain that is the sister group to Domain Eukarya, based on this tree. [1 mark]
(b)
The SSU rRNA gene of E. coli contains 1542 nucleotides and that of Methanococcus contains 1430 nucleotides. Both genes are double-stranded. Calculate the total number of phosphodiester bonds present in both genes combined. [2 marks]
(c)
The branch lengths from the trifurcation point to each domain are approximately equal. Evaluate whether equal branch lengths alone are sufficient evidence to conclude that all three domains have evolved at the same rate since their divergence. [3 marks]
Solutions
2MasterySAQ-SEvolutionary relationships and cladistics5 marksPaper 2~8 min
A cladogram for five mammal species (platypus, kangaroo, bat, whale, and human) is constructed using three shared derived characters:
(1) mammary glands,
(2) placenta,
(3) echolocation. Presence is indicated by , absence by . Species — Mammary glands — Placenta — Echolocation Platypus — 1 — 0 Kangaroo — 1 — 0 Bat — 1 — 1 Whale — 1 — 0 Human — 1 — 0
(1) mammary glands,
(2) placenta,
(3) echolocation. Presence is indicated by , absence by . Species — Mammary glands — Placenta — Echolocation Platypus — 1 — 0 Kangaroo — 1 — 0 Bat — 1 — 1 Whale — 1 — 0 Human — 1 — 0
(a)
State why the character 'mammary glands' cannot be used to determine branching order among these five species. [1 mark]
(b)
A cladogram groups bat, whale, and human into a monophyletic clade, with platypus and kangaroo as successive outgroups. Assuming no character reversals, determine the minimum number of evolutionary changes required for each of the two characters — placenta and echolocation — and state the total number of changes. [3 marks]
(c)
Explain why echolocation being an autapomorphy of bat means it cannot be used to unite bat with any other species in this cladogram into a clade. [1 mark]
Solutions
3ChallengeLAQDomain-based classification8 marksPaper 2~12 min
Data
- DNA sequence comparisons of the small subunit ribosomal RNA (SSU rRNA) gene from the deep-sea hydrothermal vent archaeon Thermococcus celer show sequence similarity with Archaea and only similarity with Bacteria.
- A new, uncultured microbial lineage discovered in a subglacial Antarctic lake has membrane lipids composed of ester-linked fatty acids (characteristic of Bacteria and Eukarya), but its SSU rRNA gene sequences cluster phylogenetically with Archaea.
- The genome of Methanopyrus kandleri, an archaeon, contains a complete set of genes encoding histones, previously considered exclusive to Eukarya.
The three-domain system (Bacteria, Archaea, Eukarya) is based on fundamental molecular and cellular differences. However, discoveries of organisms with mixed characteristics challenge the assumption of monophyly — a group containing a common ancestor and all its descendants — for each domain.
(a)
(i) Analyse how the SSU rRNA sequence data for Thermococcus celer support the monophyly of a combined Archaea–Eukarya clade within the three-domain system. [2]
(ii) Analyse how the membrane lipid composition of the Antarctic microbe challenges the monophyly of the domain Archaea. [3 marks]
(ii) Analyse how the membrane lipid composition of the Antarctic microbe challenges the monophyly of the domain Archaea. [3 marks]
(b)
Evaluate the view that 'mix-and-match' organisms, such as the Antarctic microbe and Methanopyrus kandleri, require a fundamental revision of the domain-based classification system, referring to all three data sets provided. [5 marks]
Solutions
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