Q3 - K2P Genetic Divergence Across Regions

Theoretical 1 Real exam question - full text reproduced under IBO's CC BY-NC-SA 4.0 license

Taxonomy has traditionally been based on morphology. DNA Barcoding is a new approach that aims to allow accurate and relatively simple species identification based on the nucleotide sequence of a 650-bp fragment of the mitochondrial COI gene. The Kimura-2 parameter (K2P) distance is a widely accepted index that reflects the divergence between two DNA sequences. Selected results relating to 16 species from five barcoding studies from different regions of the world are presented below. Three types of divergence values were calculated, global, intraregional, and interregional distances. Global divergence which is commonly used was the average of all pairwise comparisons of sequences belonging to the same species regardless of location of origin. Intraregional divergences were calculated by averaging the distances of all sequences belonging to the same species from the same location. Finally, interregional distances were calculated by averaging all distance values obtained from comparing each of the sequences from one location with all sequences of the same species in a second location. Results of three calculations (i, ii, and iii) are given below: i. The average standard deviation (SD) of 17 intraregional comparisons pertaining to 10 species was 0.11% (minimum: 0%; maximum: 0.3%). The SD of divergences of an 18th intraregional comparison pertaining to one of these species was 1.26%. ii. The intraregional divergence of Argyrops spinifer specimens from India was 0.20%, and the interregional divergence of specimens from India and South Africa was 0.13%. iii. Global divergence for Platycephalus indicus was 9.46%. Interregional divergences for this species were as follows: India/China 15.78%, China/Australia 12.05%, India/Australia 10.61%, China/S Africa 16.05%, India/S Africa 4.05%, Australia/S Africa 10.95%.

Using the information and data, determine which of the statements are true or which are false.

A. The SD value of the 18th comparison in calculation (i) above is consistent with the suggestion that the fish compared in fact do not all belong to the same species.

B. The K2P divergence values reported in calculation (ii) is consistent with the proposal that A. spinifer populations of India and South Africa arose from a common source population, but that there is greater variation in the ecological niches of India as compared to South Africa.

C. The divergence values in calculation (iii) show that, as compared to interregional divergence values, global divergence values are more informative of extent of divergence that exists for P. indicus in the world.

D. With reference to calculation (iii), the difference between global divergence value (9.46%) and average of interregional divergence values (11.58) can be explained by unequal number of specimens from different regions.

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