[The extent to which most or all mutations are triggered by these or by other environmental stimuli, possibly right down to the subatomic level, is not clear, but it would be reasonable to hypothesise that most, if not all, are so caused.

A bit vague and hedged with caveats to be proposed as the default position, however humbly.]

I was only trying to be honest. Science tries not to deal in certainties.

[In my view, "random" sits very uneasily among that lot.

I'm sorry, but your discomfort with randomness is your problem. If you are going to include stimuli down to the subatomic level, you have to accept that we live in a non-deterministic universe.]

No, you're seeking refuge in the unknown. Input values can never be made to be identical (at least, we can never be sure they're identical) in the cases we're talking about so it's impossible to state that the outcomes are non-deterministic. Not in this case. It's the trouble with all biological systems, as many a frustrated physicist has dicovered.

[OK, I looked it up in four places. It doesn't mean "random."

Try looking up random.]

Haha, you asked me to look up unpredictable. There are many reasons why things might be unpredictable, randomness being just one. You want it both ways.