Fig 2. Reassortant viruses are often less fit than parental strains.

The evolutionary success of reassortant progeny viruses depends on the compatibility of the reassortant genes and the selection conditions of the host environment. Thus, even when reassortment occurs efficiently (A), the prevalence of reassortant viruses may be limited by inherently low fitness and/or competition with parental viruses present in the same host or host population (B).