Someone asked about examples of intron loss. Try looking at:
J. Kwiatowski, D. Skarecky, and F.J. Ayala. 1992. Structure and
sequence of the Cu,Zn Sod gene in the Mediterranean Fruit Fly,
Ceratitis capitata: intron insertion/deletion and evolution of the gene.
Mol. Phylogenetics and Evol. 1(1), 72.
An intron that is present in many animals, including the Med fly, is
absent in the Drosophila subgroup. Probably, it was lost >30 MYA
but < 80 MYA. The above paper contains one of the most intelligent
discussions available on the problems associated with making
inferences about loss & gain of introns.