The same length of one gene was sequenced in four species, W to Z. The evolutionary tree shows how scientists think the species are related, and the table g...
Question 1 Report
The same length of one gene was sequenced in four species, W to Z. The evolutionary tree shows how scientists think the species are related, and the table gives the number of base differences in the gene compared with species W.
| species | X | Y | Z |
|---|---|---|---|
| base differences per 100 bases, compared with W | 8 | 45 | 60 |
(a) State the feature of DNA that is compared when classifying organisms in this way. [1]
(b) Using the tree, state which species is most closely related to W. [1]
(c) Calculate how many more base differences species Y has than species X, compared with W. [1]
(d) Explain how the number of base differences can be used to judge how closely related two species are. [3]
(e) Suggest two reasons why DNA data can give more reliable evidence of relationships than external body features. [2]
(f) State the complementary base-pairing rules in DNA. [2]
(g) Using the tree, explain how you can tell that all four species share a common ancestor. [2]
Answer Details
(a) The feature of DNA compared [1]. The base (nucleotide) sequence, that is the sequence of bases in the gene [1].
(b) Species most closely related to W [1]. Species X [1]: it branches from W most recently on the tree and has the fewest base differences (only 8).
(c) How many more base differences Y has than X, compared with W [1].
\[45-8=37\]Species Y has 37 more base differences than X [1].
(d) How base differences show how closely species are related [3]. Fewer base differences mean the species are more closely related and share a more recent common ancestor [1]. Base differences arise from mutations in the DNA [1]; mutations build up gradually over time, so a larger number of differences means a longer time has passed since the two species shared an ancestor [1].
(e) Two reasons DNA data are more reliable than body features [2]. Any two of: external features can look alike without shared ancestry (convergent evolution); features can be changed by the environment; DNA is directly inherited and reflects genetic relatedness rather than appearance [2].
(f) The complementary base-pairing rules [2]. Adenine pairs with thymine [1], and cytosine pairs with guanine [1].
(g) How the tree shows a common ancestor [2]. All of the branches join up and can be traced back to a single point at the base of the tree [1]; this branch point represents an ancestor common to all four species [1].
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