The variation is not random - as one might say about a flash-in-the-pan 'accidental' happenstance - but rather a self-organizing process driven by natural selection.
To use an analogue that's been common around here lately, evolution is no more "random" than is the food supply network of New York City.
Right. Variation isn't really "random." I didn't want to get into that. The evidence that variation plus natural selection results in speciation is overwhelming. Darwin didn't even know what caused variation, but it was obviously going on.