Molecular evolution of plant viruses occurs through a combination of point mutations, recombination and reassortment of viral genome components. These processes produce a high degree of variation that undergoes selection to generate a subset of viral variants well adapted to their plant hosts and efficiently transmitted by their insect vectors. In complex pathosystems, the evolutionary outcomes are influenced by many ecological factors including agricultural practices, different vector populations, interactions between crops and reservoir plants, and climate. Existing genetic variation allows viruses to rapidly adapt to changes in their hosts, their vectors and the environment. This evolvability (adaptive potential) confounds efforts to combat viral diseases in crops and to generate durable resistance.