Happy accidents are selected for, unhappy accidents are selected against. There isn’t really a plan...

There is no plan or design, it is a series of accidents and the best ones survive. The book, "The Blind Watchmaker: Why the Evidence of Evolution Reveals a Universe without Design" by Richard Dawkins gives a very good explanation.

Read the comment given by HeathenFarmer!!!

That's a vague statement. But evolution does not have goals set. It favours the ones that tend to survive

Think about trees that put out many thousands and thousands of seeds, as they do not know what the environment around them is like, random differences in genetics, where some luckily are suitable to grow in that environment. This is chance, and the only way to prepare for a blind guess, is huge variety.

Neither, it is a process based on addaptation to changing environment over time.

Evolution is only the expression of genes in subsequent generations.

Evolution of life seems to be built into the universe due to the creation of the needed chemical ingredients blasted into interstellar space.