well, take the motion system as example. can you as a designer made a motion system that is base on a single part? i mean, can you add a single part to an object (that cant move) and it will start to move from A to B on its ownf?
i guess that any new and different bump that serve a new function might be a good start.
ok. so i will say that the average rate to get a new anatomical trait is one in about 1000 births, and probably more then that. why? because we have seen billions of living creatures (human population alone is about 8 billion). and as far as im aware only in rare case (if any) we see a new function. let alone a begining of a new organ. so i think that a rate of one in 1000 births is very generous and even contradict empirical reality.
i refer to the cases that its true. which is a lots of cases.