Evolution of genic regions over time
The comparison of genomes of various species has shown that regions of the genome can evolve at different rates. Exons change more slowly than introns. This observation has some important implications for genomic studies.
Immediately after speciation, the two genomic regions are 100% identical. Over time, regions under little or no selective pressure, such as introns, are changed by mutations, whereas the sequences under positive selection, such as most exons, retain their identity.
Image from: Miller W, Makova KD, Nekrutenko A, Hardison RC (2004) Comparative genomics. Annu Rev Genomics Hum Genet 5:15-56