Question 1

Paper 2Hard16 marks

Two bacterial lineages, A and B, reproduce by binary fission. They can be distinguished by an inherited feature unrelated to gene marker M. At the start of an investigation, M was detected in all sampled A colonies and none of the B colonies. A mixed culture contained both lineages; a control culture contained B only. At each time point, 100 colonies of each lineage were sampled and tested for M.

01020300102030405060708090100Generations after start of cultureColonies carrying M (%)Percentage of sampled colonies carrying gene marker M○ Lineage A, mixed culture△ Lineage B, mixed culture□ Lineage B, B-only control
(a)

State two features of the biological species concept used to decide whether populations belong to the same species.

[2]
(b)

Describe the change in the percentage of lineage B colonies carrying M in the mixed culture over the 30 generations.

[2]
(c)

Suggest the most likely explanation for the appearance of M in lineage B, using all three series in Figure 1.

[3]
(d)

Two populations, P and Q, of another organism reproduce only asexually. Explain why the biological species concept cannot establish whether P and Q are the same or different species.

[3]
(e)

A student concludes that A and B must be one biological species because they share M after mixing, and that P and Q must be different species because they cannot interbreed. Assess these two conclusions, using Figure 1 and the biological species concept.

[6]

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