Orcs breed after the manner of Elves and Men, albeit rather quickly and maybe more like bunnies.
Orcs probably were made from Elves in the beginnings, but there may have been more in the mix than just that, if that at all. But if you want to look at it from an evolutionary standpoint, then orcs may have been bread to be ugly and squat and dumb and able to propagate quickly. Each of these may have a very good reason behind them, and perhaps the loss of strength was just a bad side affect.
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