and the basis for the paper:
Tomita M,, Shimizu, N., Brutlag, D. L. 1996. Introns and reading frames: correlation between splicing sites and their codon positions. Molecular Biology and Evolution 13(9): 1219-1223.
During the addition of new, adaptive genes to the genome, if certain sequences are better donors in creating new genes than others, then a kind of selection process results that can improve the ability of the genome to generate new, useful genes. When applied to the evolution of exons, this idea predicts that there should be a predominance, among translated exons, of exons and pairs of exons that are multiples of 3 bases in length. It also predicts that more introns should fall between codons rather than splitting them. Empirical verification of these predictions is described.