Classification and the species conceptEdexcel A-Level Biology B: Subtopic test

1. 1
   Researchers collected several specimens of an unfamiliar beetle from the canopy of a rainforest. All the specimens are female, no males have ever been found, and the beetles are thought to reproduce without mating.

   (a)

   Which sequence lists the taxonomic ranks in order from the highest to the lowest?

   [1 mark]

   • ADomain, kingdom, phylum, class, order, family, genus, species
   • BKingdom, domain, phylum, class, family, order, genus, species
   • CDomain, kingdom, class, phylum, order, family, species, genus
   • DSpecies, genus, family, order, class, phylum, kingdom, domain

   (b)

   Why is it difficult to apply the biological species definition to this beetle?

   [1 mark]

   • AThe beetles cannot produce fertile offspring
   • BBeetles that reproduce without mating do not interbreed, so the definition cannot be tested
   • CThe definition applies only to plants
   • DAll the specimens are female and so must belong to different species

   (c)

   Suggest how the researchers could use DNA to decide whether the beetle is a new species.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   Gel electrophoresis was used to compare DNA from four related species of finch, A to D. DNA from each species was cut with the same restriction enzyme and the fragments were separated on an agarose gel. Species A and B shared five of their six bands, species A and C shared three bands, and species A and D shared only one band.

   (a)

   Why do the DNA fragments move through the gel?

   [1 mark]

   • ADNA is positively charged, so the fragments move towards the negative electrode
   • BDNA is uncharged, so the fragments move by diffusion
   • CDNA is negatively charged because of its phosphate groups, so the fragments move towards the positive electrode
   • DDNA is negatively charged, so the fragments move towards the negative electrode

   (b)

   Which conclusion about the four species is best supported by the results?

   [1 mark]

   • ASpecies A and D are the most closely related
   • BSpecies A, B and C belong to the same species because they share bands
   • CThe bands show that species A and B have identical DNA base sequences
   • DSpecies A and B are the most closely related, and species D is the least closely related to species A

   (c)

   Explain why the smaller DNA fragments are found further from the wells at the end of the run.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   Until the 1970s, living organisms were classified into five kingdoms: Prokaryotae, Protoctista, Fungi, Plantae and Animalia. In 1977 Carl Woese compared the base sequences of ribosomal RNA from many organisms. He found that the prokaryotes fell into two groups, bacteria and archaea, which differed from each other as much as each differs from the eukaryotes. Archaea also differ from bacteria in the lipids of their membranes and in their RNA polymerase.

   (a)

   Explain how Woese's evidence supports the three-domain model rather than the five-kingdom model.

   [3 marks]

   (b)

   Explain how the scientific community would have helped to validate Woese's claim before it was accepted.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   The species is the basic unit of the classification hierarchy. Biologists describe thousands of new species each year, including organisms from deep oceans, fossils and microbial samples, and they have a growing range of techniques for comparing them.

   (a)

   Evaluate the limitations of defining a species as a group of organisms that can interbreed to produce fertile offspring.

   [6 marks]

   (b)

   Evaluate gel electrophoresis and DNA sequencing with bioinformatics as methods for distinguishing species and determining evolutionary relationships.

   [6 marks]

   Total for question 4: 12 marks

End of questions