The proportion of each type of A↔G and C↔T substitution (the ancestral allele vs. the recent allele of 7,239 iSNPs and 2,374 FFD SNPs. (a) The proportion of each type of substitution corrected by nucleotide compositions. (b) The FFD SNP distribution corrected by FFD codon compositions. The non-CpG site sSNP distribution is corrected by four types of FFD CpG codons, i.e. NDA, NNT|H, NDG, and NNC|H (D represents A or G or T, and H represents A or C or T). FFD CpG site SNP distribution is corrected by four types of FFD CpG codon compositons, i.e., NCA, NNT|G, NCG, and NNC|G.