A-LevelBiologySelection and evolution9700/42 · May/June 2019 · Q610 Marks

(a) State the general theory of evolution. (b) Different types of organism have evolved different structures containing light receptors. Eyes are organs containing light receptors. Fig. 6.1 describes the light receptors of several types of organism. [Figure 6.1] Using the information in Fig. 6.1, suggest how a complex eye such as that of mammals could have evolved in successive stages. (c) Octopuses are molluscs that have eyes very similar to those of mammals. Octopuses and mammals are not closely related. Octopuses and mammals have lenses that can change shape to focus on objects at different distances. Suggest reasons why octopuses and mammals have evolved similar eye structures. (d) Molecular evidence is used to investigate evolution. One study involved a marine worm, Platynereis dumerilii, that still has characteristics similar to its ancestors from 600 million years ago. Researchers sequenced all the proteins in light receptor cells of P. dumerilii and humans. The results showed that there are many similarities between the protein sequences of P. dumerilii and humans, particularly in the light-detecting protein opsin.

(a)State the general theory of evolution.[1]
(b)Using the information in Fig. 6.1, suggest how a complex eye such as that of mammals could have evolved in successive stages.[4]
(c)Octopuses are molluscs that have eyes very similar to those of mammals. Octopuses and mammals are not closely related. Octopuses and mammals have lenses that can change shape to focus on objects at different distances. Suggest reasons why octopuses and mammals have evolved similar eye structures.[2]
(d)(i)State what this molecular evidence indicates about the evolutionary origins of P. dumerilii and humans.[1]
(d)(ii)Explain how amino acid sequences indicate how close the relationship is between two species.[2]

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Mark scheme

Marking points from the official mark scheme (May/June 2019). The mark or mark type is shown in brackets.

[1] idea that organisms have changed over time;
[4] any four from:
1 ref. to mutation;
2 (mutation) results in a change in eye structure ;
3 ref. to natural selection / selective advantage;
4 light receptor (cells) develop ;
5 light receptor layer becomes cup-shaped;
6 idea that entry for light becomes narrower ;
7 lens forms;
8 fixed lens / cornea, (forms);
[2] any two from: must refer to both octopus and mammal
1 had similar, selection pressures / light conditions ;
2 allows organisms to escape predators ;
3 allows organisms to find food;
4 convergent evolution / described ;
[1] shared origin / common ancestor / same ancestor ;
[2] any two from:
1 compare (amino acid sequences of two species);
2 more similar (amino acid sequence) more closely related the species are ; ora
3 more similar (amino acid sequence) more recent common ancestor / less time has elapsed (since a common ancestor); ora
4 ref. to molecular clock;

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