1. 1
   Traditionally, the figwort family (Scrophulariaceae) contained about 275 genera, including foxgloves (Digitalis), snapdragons (Antirrhinum) and speedwells (Veronica). They were grouped together mainly because of similarities in flower structure, such as tubular, two-lipped flowers. When the base sequences of several chloroplast genes were compared, many of these genera turned out to be more closely related to species in other families than to the true figworts (Scrophularia). More than half of the genera, including foxgloves, snapdragons and speedwells, were transferred to other families such as the plantain family (Plantaginaceae), leaving a much smaller figwort family.

   (a)

   Why did the traditional classification wrongly group these genera in one family?

   [1 mark]

   • AChloroplast genes mutate too rapidly to show relationships
   • BFlowers of the genera were never examined closely
   • CTheir similar flower structures arose by convergent evolution rather than from a recent common ancestor
   • DThe genera all grow in the same habitats

   (b)

   Why are base sequences of genes considered the most objective evidence for placing organisms in the same clade?

   [1 mark]

   • ASequence differences can be counted directly and accumulate gradually, so they are less affected by judgement and by convergent similarity in appearance
   • BBase sequences never change over time
   • CMorphological traits are not inherited
   • DOnly plants have chloroplast genes, so the method is specific to plants

   (c)

   Outline what this reclassification shows about the nature of scientific knowledge.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   In 1977 Carl Woese and George Fox compared the base sequences of ribosomal RNA from the small subunit of the ribosome in many prokaryotes and eukaryotes. Methane-producing prokaryotes (methanogens) had until then been classified as bacteria, because like bacteria they are single cells without a nucleus. However, their ribosomal RNA sequences were as different from those of typical bacteria as either group was from eukaryotes.

   (a)

   What classification did Woese propose from these results?

   [1 mark]

   • AFive kingdoms, with methanogens in the kingdom of bacteria
   • BTwo domains, prokaryotes and eukaryotes
   • CA new genus of methanogens within the bacteria
   • DThree domains, Archaea, Bacteria and Eukarya, as a new level above kingdoms

   (b)

   Why are ribosomal RNA base sequences suitable for classifying all organisms?

   [1 mark]

   • ARibosomal RNA is identical in all organisms
   • BRibosomal RNA is found in all organisms and its sequence changes slowly
   • CRibosomal RNA is found only in prokaryotes
   • DRibosomal RNA codes for the amino acid sequence of proteins

   (c)

   Explain why the methanogens were placed in a separate domain rather than kept with the bacteria.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   The bases at eight variable positions of one gene were compared in an outgroup species O and four related species P, Q, R and S. Reading from position 1 to position 8, the sequences are: O, ACGTACGT; P, GTATATGT; Q, GTATACGT; R, GCGCGCGA; S, GCGCGCAT. Across the whole 1,500-base gene, R and S differ at 36 bases, P and Q differ at 24 bases, and P and R differ at 90 bases. Fossil evidence shows that R and S diverged from their common ancestor 12 million years ago.

   (a)

   Using the principle of parsimony, deduce the most probable relationships between species P, Q, R and S, and justify your answer.

   [3 marks]

   (b)

   Estimate how long ago P and Q diverged from their common ancestor, and explain why this can only be an estimate.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   In the traditional hierarchy of taxa, crocodiles, lizards, snakes and turtles are placed together in the class Reptilia, and birds in a separate class, Aves. Comparisons of the base sequences of many genes, together with shared features such as a four-chambered heart and a muscular gizzard, show that crocodiles share a more recent common ancestor with birds than with lizards and snakes. A cladogram built from these data has its root at the common ancestor of all these animals, and the terminal branches leading to crocodiles and birds meet at the most recent node.

   (a)

   Explain why the class Reptilia is not a clade, and outline the advantages of a classification that corresponds to evolutionary relationships.

   [6 marks]

   (b)

   Discuss whether biologists should replace the traditional hierarchy of ranked taxa with classification by cladistics.

   [6 marks]

   Total for question 4: 12 marks

End of questions