Evolution can be estimated by reconstructing the pathways by phylogenetic analysis of aligned sequences.
Genetic recombination is a major source of variation.
How recombinants fare under competition from non-recombinants is an issue of fundamental and practical interest. The success of a virus in a host plant may be greatly affected by the presence of another virus in the same plant. One virus may assist the other by, for example, facilitating the spread of infection of the second virus. Such help can be given by genes that encode movement proteins , proteins required for movement of the infection from one cell to another. The interaction of two viruses in a plant may exacerbate the symptoms of disease.