RNA viruses evolve rapidly under selection pressure as a result of the high error rates of viral RNA polymerase. 'Cooperation' between wild type and variant measles virus (MV) genomes through the heterooligomer formation of a viral protein has recently been shown to act as a mechanism of viral evolution. This type of cooperation between genomes producing a new phenotype may have implications for various aspects of evolution, including the expansion of viral tropism and host range, the emergence of segmented viral genomes, and the evolution of heteromultimeric molecules. It also lends support to the concept of the quasispecies acting as a unit of selection.