With its segmented genome, influenza viruses can reassort with other influenza viruses to produce hybrid progeny. Think of it as being like shuffling a red and a blue deck of cards in a box, and picking out each member of the suite at random.
As part of my thesis work, I developed an phylogenetic heuristic algorithm to identify reassortant influenza viruses. Using this method, my colleagues and I were able to show that reassortment is over-represented (relative to a null model) when crossing between viral hosts; additionally, the more evolutionarily distant two viral hosts were, the more over-represented reassortment was.
This may generalize across domains of life, where reticulate evolution enables organisms to more easily switch between ecological niches.