This is a very interesting article from "New Scientist" magazine:

www.newscientist.com/article...ife.html

It discusses the fact that mutation and natural selection are not the only processes driving evolutionary change. Horizontal gene transfer (HGT) is quite important for one-celled organisms which has been well known for a while now. The article discusses new evidence this also "has been a major driving force in animal evolution." They say, "The most likely agents of this genetic shuffling are viruses, which constantly cut and paste DNA from one genome into another, often across great taxonomic distances. In fact, by some reckonings, 40 to 50 per cent of the human genome consists of DNA imported horizontally by viruses, some of which has taken on vital biological functions (New Scientist, 27 August 2008, p 38)." It also discusses endosymbiosis and hybridisation.