In nature, every species can eventually mutate into a new species. It just takes an enormous amount of time and offspring. For mammals the timeframe to mutate into some genetically different species is exceptionally long in normal circumstances, unless you think, like I do, that species also can mutate quickly via quantavolution. However species that grow and divide rapidly, like bacteria and viruses, and might evolve much quicker.
Influenza is feared for its potential to evolve. There are two different mechanisms by which influenza viruses evolve:
 Because the RNA genome of influenza viruses is segmented, new strains can suddenly be produced by reassortment creating new pandemic strains. This process is called antigenic shift;
 New viruses evolve relatively slowly by stepwise mutation and selection. This process is called antigenic drift.
Assuming that the Influenza B and C viruses originated from avian viruses, they may be viewed as evolutionary relicts of previously widespread avian virus strains. To account for their high degree of divergence from the A viruses and their relative evolutionary equilibrium, they probably diverged many hundreds to thousands of years ago from their avian influenza virus ancestors.
A very interesting theory was put forward by Hoyle and Wickramasinghe in their book 'Diseases from Space' (1981). They claimed that viruses and bacteria responsible for the infectious diseases of plants and animals arrive at the Earth from space. Although not very probable the book still offers a fascinating read.