There are living examples of this. Just google "ring species" and you will see groups with increasing differences around a barrier, such as a mountain or other major landform, until the groups at the opposite ends cannot interbreed--the definition of a species. The interesting thing is, each adjacent group can interbreed, but the two endpoints cannot.
Ring species provide evidence for speciation (macro-evolution), and you can even see all of the intermediate steps!
Very fascinating, I didn't know that!