Because evolution is an observable fact. Like I said, Google the Bird Flu virus.
I haven't made it clear in this post but I do not buy into all of the teachings of evolution. Why? Because, as it stands now, it does not do a very good job at predicting all of the facts.
So there are different varieties of the virus? Will it, given enough mutations, become something other than a virus?
Mutations and gene swapping have been happening in virus’ for hundred of millions (or billions and billions) of generations yet has any virus become more than a virus? So what fact of evolution am I to observe here?