The nucleotide divergence ( d ) between the L and M opsin genes in five species of gibbons, H. agilis , H. lar , H. pileatus , N. leucogenys and S. syndactylus. The d values of the exons are represented by black, the introns by white, the synonymous sites by light gray, and the non-synonymous sites by dark gray bars. The error bars indicate the estimated standard error of the d values based on the 1000 × bootstrap resampling. The asterisks indicate that the d values are significantly higher than the d value of the combined sequence of the introns 3 and 4 in each species. The single and double asterisks represent the statistical significance at 0.05 and 0.01 levels, respectively, based on the one-tailed Z test. Ex3, exon 3; Ex4, exon 4; Ex5, exon 5, Int3, intron 3; Int4, intron 4; Int, the introns 3 and 4 combined; S, synonymous sites in the exons 3, 4 and 5; N, non-synonymous sites in the exons 3, 4 and 5.