A-LevelBiologySelection and evolution9700/42 · May/June 2020 · Q312 Marks
A subspecies is a genetically distinct population within a species that has some phenotypic differences from the rest of the species, but is not yet reproductively isolated. Nine subspecies of the tiger, Panthera tigris, have been identified. Six of these subspecies are found on mainland Asia. Three of the subspecies originate from the Sunda Islands. These islands include Bali, Java and the large island of Sumatra. [Figure 3.1] shows these three islands. The Bali tiger, Panthera tigris balica (P. t. balica), became extinct in the 20th Century. The Bali tiger was found only on the island of Bali. The Javan tiger, P. t. sondaica, became extinct in the 20th Century. The Javan tiger was found only on the island of Java. The Sumatran tiger, P. t. sumatrae, lives only on Sumatra and is the closest living relative of Bali and Javan tigers. 20000 years ago land bridges temporarily connected the Sunda Islands. A recent study carried out a genetic analysis of the nine subspecies of tiger. Specific sections of mitochondrial DNA (mtDNA) that are useful in studies of evolution were amplified using PCR and compared to assess their evolutionary history. The source of DNA for the extinct subspecies came from museum specimens. mtDNA was extracted and polymerase chain reaction (PCR) carried out using primers based on specific sections of tiger mtDNA. The mtDNA sections for the three island subspecies were genetically distinct from the other six mainland subspecies. The mtDNA sections for the three island subspecies were all found to be very similar.
- (a)Suggest and explain how the three subspecies of tiger on the Sunda Islands formed.[4]
- (b)Explain why specific primers were used for the tiger mtDNA sections.[2]
- (c)Describe and explain one characteristic of mtDNA that makes it more useful than using nuclear DNA to provide evidence of evolution.[2]
- (d)Suggest two reasons why P. t. balica and P. t. sondaica became extinct.[2]
- (e)Suggest why P. t. sumatrae is still considered to be a member of the species Panthera tigris.[2]
This question uses a diagram from the original paper, which isn't shown here. Open the full paper from the past papers archive to see it.
Mark scheme
Marking points from the official mark scheme (May/June 2020). The mark or mark type is shown in brackets.
- [4] any four from:
- 1 ref. to geographical isolation;
- 2 no, gene flow / breeding, between populations (on the different islands);
- 3 different, selection pressures / environmental conditions (on the different islands) ;
- 4 different mutations occur (on the different islands);
- 5 some mutations make individuals better adapted;
- 6 those individuals, survive / reproduce;
- 7 pass on advantageous alleles ;
- 8 ref. to many generations;
- 9 reproductive isolation;
- 10 allopatric speciation;
- [2] any two from:
- 1 bind to complementary base sequences in mtDNA ;
- 2 (so) only amplify specific mtDNA sections / AW;
- 3 mtDNA section, differences / similarities, used to assess how closely related the subspecies ;
- [2] any one pair from:
- 1 large quantity in the cell;
- 2 (so) easier to, extract/amplify, DNA for testing;
- or
- 3 small genome size;
- 4 (so) easier to locate specific section of DNA to test ;
- or
- 5 mtDNA is, a single copy of DNA / not paired alleles / haploid ;
- 6 (so) only mutation causes it to change;
- or
- 7 inherited maternally ;
- 8 (so) all mtDNA sections are shared between all members (of maternal) family;
- or
- 9 mutation rate is higher / no enzymes to repair mutations / faster molecular clock ;
- 10 (so)more choice of suitable sections of mtDNA to test / more accurate time estimate ;
- [2] any two from:
- 1 habitat loss;
- 2 more competition for, space / mates;
- 3 reduction in prey;
- 4 poaching / hunting / poisoning (by humans) ;
- 5 disease;
- [2] any two from:
- 1 morphological / physiological / behavioural / biochemical, similarity to mainland tigers / Panthera tigris ;
- 2 genetically similar to, mainland tigers / Panthera tigris ;
- 3 may be able to breed with, mainland tigers / Panthera tigris, to produce fertile offspring;
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