Sauron the Gene-maker

[Zigûr]

Originally Posted by Nerwen

However, it could also be an entirely passive side-effect of magical "pollution"- there are many other examples of Sauron's general corrupting influence on the surrounding natural world.

This is how I think I always felt about it, like it was something that happened spontaneously as a result of Sauron's presence and the general malaise of Mordor, sort of like Mount Doom erupting at dramatic moments.