Intuitively, transposable elements (TEs) are harmful as they may cause genome instability. However, some people argue that TEs are also sources of variations, especially regulatory sequences.
If TEs are harmful, then their proportions may be low in most species. And if they are beneficial, then their proportions should be high. Either case leads to the conclusion that their proportions should stay close to each other. However, the fact is that the proportion varies greatly across different species:
- Maize: 85% 
- Human: 45% 
- Arabidopsis: 10% 
- Plasmodium: absent 
Why is that? Is there other factors that influences the proportion of TE, such as evolutionary pressure?
 Rita R, et al. 2012. Transposable elements: an abundunt and natural source of regulatory sequences for host genes. Annu. Rev. Genet 46:21-42
 Schnable PS, et al. 2009. The B73 maize genome: complexity, diversity, and dynamics. Science 326:1112-15
 International Human Genome Sequencing Consortium. 2001. Initial sequencing and analysis of the human genome. Nature 409:860-921
 Arabidopsis Genome Initiative. 2000. Analysis of the genome sequence of the flowering plant Arabidopsis thaliana. Nature 408:796-815
 Biemont C, Vieira C. 2006. Genetics: junk DNA as an evolutionary force. Nature 443:521-24