With comparative genomics approaches, we evaluated the evolutionary characteristics of conservation of exons which are expressed abundantly, moderately or lowly in mammals. Using non-coding regions and pseudogenes as controls, sequence identity, phastCons and Ka/Ks analyses were carried out and our results showed that as the exons of high abundance are highly conserved, the minor and low exons also showed conservative characteristics in evolution. Our findings suggested that the exons with less abundance which constitute a large proportion of distinct species in transcriptome of organisms are under functional constraint and might play certain roles in enriching biological complexity in the evolution of organisms.
