Evolution of Ki within short introns (49 to 1029 nucleotides). The last bar of the histogram represents the cumulative data for all long introns. Data are presented for first and nonfirst introns separately, and are pooled in categories of increasing size class of 100 nucleotides for visual clarity. Nonfirst introns reach a plateau of mean Ki around 300 nucleotides, whereas this pattern is not as clearly discernable in first introns. nt, nucleotides.