FIGURE 3. Frequency distribution of the distances between consecutive nucleotide substitutions in known (a) and unknown (b) conserved blocks.
From the following article:
Numerous potentially functional but non-genic conserved sequences on human chromosome 21
Emmanouil T. Dermitzakis, Alexandre Reymond, Robert Lyle, Nathalie Scamuffa, Catherine Ucla, Samuel Deutsch, Brian J. Stevenson, Volker Flegel, Philipp Bucher, C. Victor Jongeneel and Stylianos E. Antonarakis
Nature 420, 578-582(5 December 2002)
doi:10.1038/nature01251
Note the periodicity (period = 3) in a due to the high frequency of synonymous substitutions at the third codon position, which is absent from b.