A common belief among IDers is that evolution depends on blind chance - mutations randomly producing small changes that gradually add together. Hence their claim that natural selection is the equivalent to asserting that a whirlwind could assemble a jumbo jet by blowing through a junkyard.
But they miss the basic point of Darwin's contribution - the mutations may be random, but it is natural selection that chooses the few effective ones.
This is basic
Existing genes are sometimes duplicated during DNA replication. Mutations can then occur in one of the copies without affecting the original function of that gene.
This is why (most) humans have tri-chromatic vision - parts of the opsin gene duplicated, then mutated to see different parts of the light spectrum. Stringly selected for because of the advantages of such things as being able to tell the difference between ripe and unripe fruit, for example