Insertion-deletion (indel) direction, relative to the major allele, as a function of intron size. For the 10 rare indels of minor allele frequency f < 0.06 (blue), the resultant size changes drive the introns back to their optimal size of 92 bp. The two common indels with f > 0.35 (red) are the only exceptions to this rule, presumably because they arose from a different population dynamics.