I am by no means an expert on evolution (or really anything), but I can offer my thoughts on this very pertinent question.
Everything in the universe can be broken down into 4 quadrants, the interior individual (thoughts, intentions, etc.), the exterior individual (physical, behavioral, etc.), the interior collective (group thoughts,etc.), and exterior collective (group behavior, etc.). This Ken Wilber's AQAL theory and way to complex to fully explain here, but very interesting stuff.
Anyway, everything is made up of these 4 qualities, from the tiniest particle to the cells making up your tissues, to group entities such as governments, to solar systems and galaxies.
Nothing exists in a vacuum, and everything progresses in lines of development, from less complex to more complex. Atoms are more complex than particles, molecules are more complex than atoms, cells are more complex than molecules, etc. Each level transcends but also includes the previous level.
Now, all mass contains energy (E=mc^2), and all energy is the primordial consciousness (creator). So particles can't help gathering into groups to create atoms, atoms are compelled to form molecules, molecules group to make single celled organisms, etc. At some point in time single cell organisms found that gathering into groups increased their chances for survival, hence multi-cellular organisms, with individual cells taking on specialized functions over time to increase efficiency, which is the primary impulse of evolution. Same rules created individual organisms, groups, villages, etc.
At every level, micro and macro, efficiency rules. The organism that develops the greatest efficiency in terms of securing safety, food, reproduction (Maslow) will survive and re-create itself. What is efficient in one environment may not be efficient in another. So the environment is key. But those that survive and advance get to reproduce their newly developed traits into the next generation.
So survival of the fittest really means survival of that which figures out the environment and develops the traits necessary to overcome its challenges. Could be a set of claws, agility, defense mechanisms, plumage, a more intelligent brain, technology, etc. But the development of those traits, for the organism, begins at the cellular level.
And this is where the scientifically unexplained (so far) processes come into play. This is where the spiritual practices such as those in the bible play a role. Meditation/prayer helps to polarize the interior individual. IOW, a pure/whole body is one in which every cell is on the same page, pulling in the same direction, to overcome the challenges of its environment. An impure body is full of chaos, cells looking out for themselves only (can result in cancer or other diseases).
So to answer your question as briefly as I could (sorry so long), I feel the origin of mutations is the primordial consciousness of the universe, which cannot be proven, but still makes sense to me.
Didn't proof read so this may be a lot of nonsense.
