The genomes of some vertebrates are much smaller than those of others

Question

For example, the genome of the pufferfish Fugu is much smaller than the human genome, and even much smaller than genomes of other fish, primarily because of the small size of its introns.
A. Describe a mechanism that might drive evolution toward small introns or loss of introns and could therefore account for the evolutionary loss of introns according to the "introns early" hypothesis.
B. Describe a mechanism that might drive evolution toward more or larger introns and could thereby account for the evolutionary appearance of introns according to the "introns late" hypothesis.

Answer 1

A. Spontaneous deletions or selection pressure to decrease the time or cost of DNA replication may cause a loss of introns.
B. Acquisition of intron sequences provides a selective advantage for those organisms that experience transposon insertions. According to this idea, introns became sinks for transposon and virus insertion to protect the rest of the genome. Alternatively, introns may provide another advantage to the host genome: by providing ample sites for crossing-over, larger introns could facilitate exon shuffling and thus the generation of new genes with novel functions.