Average level of within-species sequence variation for R1 and R2. Sequence variation in the 400 bp at the 5' and 3' ends of the R1 and R2 elements from the 12 Drosophila species, the entire 28S rRNA gene and the internal transcribed spacer (ITS)1 region are shown (black bars). All values are calculated as the divergence from the consensus sequence for the species. Grey bars indicate the rates of nucleotide divergence (percent divergence per million years (myr)) of these same regions. Standard deviations are given for all values. The high standard deviation of the R1 5' end is a result of several species with no variation. The divergence data estimates were derived from comparison of the consensus sequences from D. simulans, D. sechellia, D. melanogaster and D. yakuba (divergence times: simulans versus sechellia, 0.25 myr; simulans or sechellia versus melanogaster, 3 myr; simulans, sechellia or melanogaster versus yakuba, 8 myr). Nucleotide variation data for 28S and ITS1 regions are derived from Supplemental Table 2 of .