Abstract

Viruses have gained a notoriously bad reputation as infectious agents, but they are nonetheless fascinating for biologists, not least because they are the most numerous and fastest evolving organisms on the planet. Moreover, viruses have had a significant role in the evolution of all other organisms, from bacteria through vertebrates to humans. But the full extent of this role is only just emerging, and scientists are reappraising not just the relationship between viruses and their hosts, but also the main principle of evolution: that it operates largely on the basis of gene mutation coupled with selection. " …viruses have had a significant role in the evolution of all other organisms, from bacteria through vertebrates to humans In fact, infectivity is only the tip of the viral iceberg, and cohabitation is actually the norm—as is the case for bacteria. Similarly to bacteria and other microorganisms, most viruses and their hosts have co‐evolved either into commensal relationships where the parasite's impact is neutral, or into mutually beneficial symbiotic arrangements. Of course, for viruses there is also the additional genomic dimension: they insert themselves into the host's DNA and in doing so can generate novel proteins and RNA molecules that might be valuable to the host. Moreover, it also allows the virus to manipulate the host genomic structure itself. Although this ability could wreak potential genetic havoc, it might also trigger significant structural and functional changes in the host genome that could not occur through random mutation alone. Viruses have the unique ability to incorporate themselves permanently into the genomes of their hosts—a phenomenon known as endogenization—and thus lose their ability to infect other individuals. In return, they gain immortality, as they are passed down to successive generations of hosts. Yet, although no other microorganisms can invade the genome, endogenization has a parallel among …