Average lengths of RIs compared with lengths of constitutive introns and introns adjacent to CEs. RI length, which is constrained by ID, is fairly constant across all organisms. In protists and fungi, average RI length is close to that of constitutive introns, because ID is the primary mode of splice site recognition for both groups. In animals, constitutive intron length differs substantially from RI length because most constitutive introns are recognized by ED and are not subject to the same length constraints as RIs, which are recognized by ID. Plants fall between unicellular organisms and animals. Data are shown only for organisms with at least five RIs. For introns next to CEs, data are shown only for organisms with at least eight CEs with unambiguous adjacent intron lengths on both sides. Introns next to CEs are usually longer than constitutive introns, because these introns are recognized by ED and are free from ID length constraints.