The rapid ability of many viruses to emerge in novel hosts, adapt to new environmental conditions, develop resistance to antiviral drugs and break virus-resistant plant cultivars stems from their ability to generate and maintain high genetic diversity in their populations. This diversity is driven by high mutation rates aided by frequent recombination in some viruses, including begomoviruses. Ultimately, the persistence of potentially beneficial mutations and recombinants is due to the population size of viruses within and among infected hosts, as the number of viral genomes encoding these variants determines the amount of variation available upon which natural selection can act. The genetic variation within a viral population is therefore a direct measure of its evolvability.