Mutations can be triggered by ionising radiation and by ultra-violet light and by numerous chemicals that we even call mutagens, among other things. If a mutstion arises that can't immediately be assigned to such a cause, I'm sure you'd think it reasonable to try to find one. Good college, was it?

"(or at least, nothing accessible to calculation or prediction)"

Exactly.

Tia, back to you after I've had me tea!