A-LevelBiologySelection and evolution9700/42 · May/June 2018 · Q410 Marks

Adult Drosophila fruit flies feed on yeasts on the surface of ripe fruits. Female flies lay their eggs inside the fruit and the larvae feed inside the fruit before developing into adults. The tree Morinda citrifolia produces fruits called noni fruits. Noni fruits contain a toxin and are avoided by nearly all Drosophila species, including Drosophila yakuba in mainland Africa. One population of D. yakuba lives on the island of Mayotte off the coast of Africa. The population of D. yakuba on Mayotte has specialised to feed on noni fruit. Flies of this population produce enzymes that give them resistance to the toxin in noni fruits. Adults are able to feed on yeasts on the fruit surface and their eggs and larvae can develop and grow inside the fruit. Scientists have used bioinformatics to estimate that the unique D. yakuba population on Mayotte originated in the last 30000 years.

(a)Describe how a population of Drosophila can develop resistance to noni fruit toxin.[4]
(b)Explain why living on an island increases the likelihood of the noni toxin-resistant population of D. yakuba on Mayotte becoming a separate species to the mainland population of D. yakuba.[3]
(c)Suggest how scientists can use bioinformatics to estimate the length of time that the population of D. yakuba has been living on Mayotte.[3]

Mark scheme

Marking points from the official mark scheme (May/June 2018). The mark or mark type is shown in brackets.

[4] any 4 of: 1 spontaneous / random, mutation ; 2 produces an allele coding for enzyme (that gives resistance) ; 3 toxin is selection pressure ; 4 flies or larvae, with, allele / enzyme / mutation, have selective advantage; 5 (flies with, allele / enzyme / mutation) survive / reproduce ; ora 6 pass on this allele (to offspring); 7 this allele increases in frequency;
[3] any 3 of: 1 separated by a geographical barrier / description ; 2 different, environmental conditions / selection pressures ; 3 allopatric speciation; 4 reproductive isolation / no breeding with mainland flies / no gene flow (between two populations) ; A pre-zygotic isolation 5 new allele not, lost / diluted in frequency; 6 founder effect / genetic drift ; 7 (more pronounced) in small population;
[3] any 3 of: 1 obtain, DNA / genome / gene / base / nucleotide, sequences (from database); 2 obtain, amino acid / protein, sequences (from database); 3 compare sequences (of two populations); 4 calculate percentage of, bases / amino acids, changed; 5 mitochondrial DNA ; 6 molecular clock / mutation rate, (to estimate time);

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