Abstract
Viruses are very diverse and they infect organisms from all domains of life and across all ecosystems, but related viruses often infect very different types of organisms, pointing to their very ancient origins. Three commonly proposed mechanisms for the origins of viruses are: Viruses descended from primitive precellular life forms; viruses are escaped cellular genetic elements; viruses devolved from more complex intracellular parasites. None of these theories easily explains the origins of all viruses and it is widely accepted that all viruses did not share a single common ancestor. Instead, distinct lineages of viruses probably evolved by different mechanism. There is also good evidence that viruses have shaped the evolution of their hosts, for at least hundreds of millions of years. While some virologists consider the ancient origins of viruses, others examine the forces that drive virus evolution today. Examples of ongoing virus evolution include: Cross-species jumps; decreased or increased virulence; emergence of drug resistance; escape from immune responses at the level of individuals and populations. Virus evolution is the outcome of two independent events. The first is genome mutation and the second is selection. Viruses with RNA genomes tend to have high mutation rates, but some appear to be evolutionarily stable nonetheless. Viruses have distinct phenotypes. For examples, some influenza viruses are highly virulent but others are much less so; influenza viruses also differ tremendously in their ability to be transmitted. Modern studies of virus genetics seek to correlate diverse phenotypes with specific genes and sequences. The ability to do reverse genetics is key to understanding viral genes. Reverse genetics involves cloning viral genomes, designing specific mutation, and introducing the mutated genomes back into cells to produce infectious particles whose phenotypes can then be measured.