Tajima'’s D

Tajima's test, or D test statistic (Tajima, 1989) tests the neutral theory of molecular evolution (Kimura, 1983). That is, the vast majority of molecular differences that arise through spontaneous mutation does not influence the fitness of the individual. A corollary to this theory is then that genomes evolve primarily through the process of genetic drift. Tajima's D statistic compares the difference between two estimates of the amount of nucleotide variation, one being simply the number of segregating sites (Watterson, 1975) and the other one being the average number of pairwise differences (Nei and Li, 1979; Tajima, 1983). In a constant-sized population experiencing only genetic drift, both estimates should give equal values. Dissimilar values suggest that some form of selection could be acting on this sequence. A positive value of Tajima's D indicates that there has been 'balancing selection' and the data will show a few divergent haploypes, whereas a negative value suggests that 'purifying selection' may have occurred and the data will reveal an excess of singletons.