Orc Breeds.

Perhaps the orcs originating from Mordor (those that Sauron didn't recruit from other parts of Middle Earth) would be better described as a tribe, although I beleive places like Mordor and the Misty Mountains would have been the home of many different orc tribes.
However,the Uruk-hai and the Trackers are definetly different breeds, having unique characteristics not seen in most orcs.
I think that in the case of Mordor and the mountains the particular circumstances of those regions have shaped different breeds. The mountain orcs were, for a long time free of Sauron and able to raid and pillage as they pleased.
The orcs originating from Mordor, which were a bit tougher than their mountain cousins (Grishnakh and the other Mordor orcs were able to keep pace with the Uruks) would have almost always been enslaved and had to worry about the Ring Wraiths and other such dangers of living in Sauron's home. This, in my opinion, would have made for a harder, more warlike life and thus a breed tougher than the orcs of the mountains.