[Man-of-the-Wold]

Well, just to be clear, I consider Uruks to be the result of selective breeding among orcs, which Sauron probably intiated long before they are acknowledged by the Free Peoples and the Wise as a breed apart.

Whether in the process Sauron and later Saruman used other powers, or even some admixture with Men "genes" to give them greater height or straightness, is possible. But any genetic influence from other peoples to latter Orc varieties of this sort is purely speculative and only part of the story.

As for the half-orcs, goblin-men and so forth that are attributed to Saruman's army and the Chief's Men. I would submit that these are really men, quite distinct from the Uruk-hai, which may have been bred by introducing orc-features into human lines.

This appears to be a very late-Third-Age effort by Saruman, with perhaps, no clear purpose other than experimentation, although it might have contributed to efforts to perfect his Uruk-hai.

But it is likely that he wanted to create groups with the rampant evil and ferocity of Orcs, who would have no other options but to serve someone like him, but who still had the trustworthiness, tallness, individuality and so forth of a man.

As Pippin and Merry figured out, he used Men as guards because he couldn't trust Orcs in that way, not being able to control them in the same way that Sauron could, and he probably had large number of Dunlenders working at Isengard for centuries before he went bad.

I think support of your theory Bill in this regard is the Uruks such as Ugluck that worked for Saruman were not so much slaves but in it as part of bargain relative to what he could offer. This may have signified a difference between them and other, earlier Uruks that served Sauron.

But I still believe that Saruman's Uruks were orcs first, and not really hybrids with Men, even if he used his hybrids in breeding more Uruks.

The hybrids seem to be solely Saruman's doing, but still primarily men, with greater penchant for evil and foulness.