Ah, but the reverse is more important, nowadays. [...]
Oh, indeed. Actually said that at one point, but it got too long. You know my replies.
And loss of function is indeed very easy to happen. Change of function perhaps next easiest. Multiplication of function could be either 2nd or 3rd place, depending on the mechanism. Addition of all other completely new functions (from "nothing", or while keeping an original function intact) is probably the most unlikely thing to happen.
Well, sort of, as change of function must not mean that a vital chemical pathway isn't completely lost (another reason why multiplication might be 2nd most important), and losses of function might be most common but could be most commonly life-inhibiting in their different (thus chucking it down the list), etc, etc, but that'll do as a summary of the immediate effects, even though it'll represent itself differently in such surviving offspring that inherit such effect-changes.
(Edited even that down. Thus some parts of that attempt at an explanation are
wrong, but will do as a fairly advanced level of
Lies To Children, I'm hoping that those with even more expertise than me (i.e. have a still current involvement) would agree.)
((Sorry, far too many nested asides.))