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Classification and phylogeny: sorting by ancestry, not by looks questions
The taxonomic hierarchy from domain to species and the rules of binomial naming; the biological species concept and where it fails; phenetic against phylogenetic classification; homologous and analogous structures and convergent evolution; the three domains and the ribosomal RNA comparison that produced them; DNA and amino acid sequence comparison, immunological evidence and molecular clocks; and how to read a cladogram without reading things into it.
6 original questions · 18 marks · the classification and phylogeny: sorting by ancestry, not by looks notes · Classification, biodiversity and conservation
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Explain why grouping organisms by how similar they look places a dolphin with a shark, and explain how homologous structures avoid that error.
Mark scheme
- B1 a phenetic classification counts observable resemblances without asking where the resemblance came from
- B1 unrelated lineages exposed to the same selective pressure end up with similar features, so a dolphin and a shark are both streamlined with fins because both hunt in open water; that is convergent evolution and the structures are analogous
- B1 a dolphin's flipper contains the pentadactyl limb: one upper arm bone, two forearm bones, wrist bones and five digits. That is the same plan as a bat's wing and a human hand, while a shark's fin has nothing of the kind
- B1 a shared underlying plan is homologous and can only be explained by inheritance from a common ancestor, so a phylogenetic classification uses homology as evidence of ancestry and discards analogy
Two bird species are known from the fossil record to have diverged 12 million years ago, and a protein 150 amino acids long differs at 4 positions between them. The same protein differs at 7 positions between two other bird species. Calculate the rate of change and estimate when the second pair diverged. The counts are idealised.
Mark scheme
- M1 calibrate first: rate = number of amino acid differences divided by the time since divergence, using the pair whose date is known
- A1 4 ÷ 12 = 0.33 amino acid differences per million years
- M1 time since divergence = number of differences divided by that rate, so 7 ÷ 0.33
- A1 21 million years
Describe the evidence from ribosomal RNA that led to the prokaryotes being separated into two domains.
Mark scheme
- B1 the base sequence of the RNA in the small subunit of the ribosome was compared across as many organisms as could be obtained
- B1 the prokaryotes fell into two groups whose sequences differed from each other about as much as either differed from those of eukaryotes
- B1 that supported a rank above kingdom, the domain, with Bacteria, Archaea and Eukarya, and with Archaea sharing a more recent common ancestor with Eukarya than with Bacteria
Suggest why ribosomal RNA rather than haemoglobin was chosen as the molecule for comparing organisms drawn from all three domains.
State the eight taxonomic ranks in order, beginning with the largest, and state the conventions for writing a binomial.
Give two situations in which the biological species concept cannot be applied, and give the reason in each case.
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