The mutation is the “random” chance, the selection is hardly random. All the increasing understanding of genetics does is give us the understanding that there are many more things subject to mutation than was previously thought. It makes evolutionary adaptation more likely rather than less.
For instance, some people have larger molar teeth than other people. If we all become vegetarians eating a great deal of raw veggies, the people with larger molars will have an advantage in making use of the new diet. Evolution will select for large molars and each generation will have fewer people with small molars. So it usually isn’t some huge difference like a third arm or some such that is the subject of evolution.
Drawin’s specific conclusions don’t really hold much relevancy today. His importance in more in the direction he pointed. He made a lot of incomplete observations where he was often correct in the trend but lacking in the, as yet, undiscovered details. He didn’t have any knowledge of the function of DNA which was technically discovered in 1869 but little understood until well into the 20th Century.
Constant selection for higher sugar content reached a natural limit in the genetic code and nothing could be produced with higher sugar content. The genes had a hard coded limit, and nothing would go past that. When they removed the imperative for sugar producing, the beets went back very quickly to the genetic mid-point.
The genes are “designed” to keep the organism at a steady point and all of the evidence shows this to be true. Look up all the attempts to change the fruit fly through actual mutations, and within a couple of generations, the genes would “heal” the genetic mutation forced into the system.
The mutation is the “random” chance