Tajima's D

Three ways to assess the nucleotide diversity (heterozygosity).
The first is mean pairwise difference ∏, the average number of differences between pairs of sequences.
The second is number of segregating sites S. A segregating site is a site that is polymorphic in the data. the number of such sites is usually denoted by S (sometimes using K).
Last one is number of alleles Na.

Mean pairwise difference per nucleotide pi =  ∏/L, where L is the length of sequences.

Tajima's D test the neutral mutation hypothesis θ=4Neμ=θT=θW. D=[(θT–θW)/Var(θT–θW)] , where θw=S/a, a=[1+1/2+1/3+...+1/(n–1)] (Watterson 1975), θt= ∏ (Tajima 1983)

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