Classification and the three-domain systemEdexcel International A Level Biology: Subtopic test

1. 1
   Two populations of small beetles live on different islands. They look very similar, but the males of one population make a different courtship sound. Biologists want to decide whether the two populations are the same species or different species.

   (a)

   Which statement defines a species?

   [1 mark]

   • AA group of organisms that look the same
   • BA group of organisms with similar characteristics that can interbreed to produce fertile offspring
   • CA group of organisms that live in the same habitat
   • DA group of organisms with the same number of chromosomes

   (b)

   Which is a limitation of the biological species concept?

   [1 mark]

   • AIt cannot be applied to organisms that only reproduce asexually, or to fossils
   • BIt can be applied to bacteria that reproduce by binary fission
   • CIt does not rely on information about breeding
   • DIt states that hybrids such as mules are fertile

   (c)

   Beetles from the two islands are brought together, and they mate and produce offspring, but all the offspring are sterile. Explain what this suggests about whether the beetles are the same species.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   Biologists have discovered three new single-celled organisms, P, Q and R, in a hot spring. None of them has a nucleus. The biologists compare the base sequences of the genes for ribosomal RNA (rRNA) in the three organisms. The sequences of P and Q are similar to each other, but both differ greatly from the sequence of R.

   (a)

   Which feature is found in bacteria but not in archaea?

   [1 mark]

   • ARibosomes in the cytoplasm
   • BCircular DNA
   • CNo membrane-bound nucleus
   • DPeptidoglycan in the cell wall

   (b)

   Why is the sequence of ribosomal RNA useful for classifying organisms?

   [1 mark]

   • AIt is found only in prokaryotes
   • BIt is different in every individual of a species
   • CIt is found in all living organisms and changes slowly, so the number of differences shows how closely related they are
   • DIt shows the shape and size of the organism

   (c)

   Suggest why using molecular evidence is more reliable than using observable features alone for classifying organisms such as P, Q and R.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   Before 1977 living organisms were classified into kingdoms using observable features. In 1977 Carl Woese and his colleagues compared the sequences of ribosomal RNA from many organisms, and the results led to a new system in which all living organisms are placed in one of three domains.

   (a)

   State the three domains, and give two features of cells in the domain Eukarya that distinguish them from cells in the other two domains.

   [3 marks]

   (b)

   Explain how the molecular evidence led to the three-domain system, and why scientists did not accept the new system until the evidence had been evaluated by others.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A marine biologist isolates a single-celled organism, U, from a deep-sea hydrothermal vent where the water temperature is 95 °C. U has no nucleus, its cell wall does not contain peptidoglycan, and it reproduces by binary fission. The biologist sequences the genes for ribosomal RNA in U and compares them with the sequences from known organisms.

   (a)

   Explain how phenotypic and genotypic evidence could be used to decide to which domain U belongs.

   [6 marks]

   (b)

   Explain why the biological species concept cannot be used to classify U, and discuss how molecular evidence overcomes this limitation.

   [6 marks]

   Total for question 4: 12 marks

End of questions