Evolution is limited

When I look at evolution, or the theory of, and consider the evolution of species. I have to ask the question why evolution is so limited. What or who determines what evolves and what doesn’t? Why when a donkey and a horse are mated, do we get a mule, and every mule is sterol? Where did these so called road blocks come from? In horticulture you can have a big robust plant, let’s say an orchid, and it produces a wimpy ugly flower. I’ve also seen a wimpy ugly little orchid plant produce a magnificent specimen of a flower. Its like evolution is being directed with its own set of checks and balances. Yes I know there is always the exception to the rule that can be argued, but in general everything seems to have its purpose like a giant puzzle. Now when you look at God’s Creation, you can’t help observing the magnificence of it. There are so many things that obviously are road signs pointing to Him.