Classification and phylogenyEdexcel A-Level Biology A: Subtopic test

1. 1
   A taxonomist is studying two populations of lizards that live in separate valleys. The lizards look very similar and she is unsure whether they are one species or two. She has tissue samples from both populations.

   (a)

   Which of these is the best definition of a species?

   [1 mark]

   • AA group of organisms that can interbreed to produce fertile offspring
   • BA group of organisms that look identical
   • CAll the organisms that live in the same habitat
   • DAll the organisms that share a common ancestor

   (b)

   What is a limitation of using the ability to interbreed to define a species?

   [1 mark]

   • AIt cannot be used for organisms that reproduce sexually
   • BIt can be used only for plants
   • CIt needs the DNA of the organisms to be sequenced
   • DIt cannot be applied to organisms that reproduce asexually or to extinct species known only from fossils

   (c)

   Describe how DNA could be used to help decide whether the two populations of lizard are the same species.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   Cytochrome c is a protein used in respiration and is found in all aerobic organisms. A biologist compares the amino acid sequence of human cytochrome c with that of other species. The numbers of differences from the human sequence are: rhesus monkey 1, horse 12 and tuna 21.

   (a)

   Which of these species is the most closely related to humans?

   [1 mark]

   • AHorse
   • BRhesus monkey
   • CTuna
   • DAll three are equally related

   (b)

   Why is cytochrome c suitable for comparing species that are distantly related?

   [1 mark]

   • AIt is found only in animals
   • BIt never mutates
   • CIt is found in all aerobic organisms, so it can be compared between them
   • DIt is identical in every species

   (c)

   Explain how the data suggest that humans are more closely related to horses than to tuna.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   Until the 1970s living things were classified into five kingdoms, and all prokaryotes were placed in one kingdom because of their similar cell structure. In 1977 Carl Woese and colleagues compared the base sequences of ribosomal RNA from many organisms. They found that some prokaryotes, later called Archaea, had ribosomal RNA sequences very different from those of both bacteria and eukaryotes. Later, other scientists found that archaeal cell membranes contain different lipids from bacterial membranes. In 1990 a classification with three domains, Bacteria, Archaea and Eukaryota, was proposed.

   (a)

   Explain how the comparison of ribosomal RNA sequences led to a new classification of living organisms.

   [3 marks]

   (b)

   Explain why it was important that other scientists critically evaluated Woese's data before the new classification was accepted.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   Researchers sample a boiling hot spring and find a single-celled organism with no nucleus. Before the 1970s all prokaryotes were placed in a single kingdom. The researchers report that the organism's cell wall contains no peptidoglycan, that its membrane lipids differ from those of bacteria, and that the base sequence of its ribosomal RNA is more similar to that of a known archaeon than to that of any bacterium.

   (a)

   Explain how phenotypic and genotypic evidence could be used to classify the organism, and why genotypic evidence is regarded as more reliable.

   [6 marks]

   (b)

   The researchers propose that their organism should be placed in the domain Archaea. Discuss the importance of the scientific community critically evaluating this claim, and how new data have changed classification.

   [6 marks]

   Total for question 4: 12 marks

End of questions