What you are talking about, darwin referred to as diversification. We see this in cats, dogs, all kinds of species will adapt to conform with their natural environment.
The same phenomenon is responsible for drug resistance. However, this is different then say a virii transforming into a bacillus, which is what evolution insists we ought to see.
Darwin assumes that diversification eventually leads to the transformation of species, but cannot prove his assertion.
http://www.talkorigins.org/faqs/faq-speciation.html
Scroll down to 5.0 Observed Instances of Speciation.