Asked by: John Awbery, Reading
Viruses are a huge source of selective pressure in the evolution of a species. In fact, a leading hypothesis for the origin of sexual reproduction is that it may have evolved as a way of allowing large organisms to shuffle their genes fast enough to keep up with the mutation rates of disease organisms, including viruses. Viruses can also affect the evolution of a species in a more direct way. A provirus is a virus that inserts its DNA directly into the chromosomes of the host cell. Rather than just having a separate strand of DNA or RNA floating around in the cell, the provirus adds itself to the host genome and gets replicated with the other genes. HIV is an example of a disease virus that does this, but there are other viruses, such as the adeno-associated viruses that insert themselves into the human genome without causing any obvious symptoms. This leads to the possibility of horizontal gene transfer where genes from one species can be transferred to another by means of a virus vector. This has been shown in bacteria, fungi, plants and some arthropods (animals and insects with jointed limbs). It is likely that viruses have played an important role in shuffling the genes of the higher vertebrates as well.