Life is probabilistic not deterministic. That is one of the toughest things for the human mind to grasp in my opinion. Growing up our numerous experiences that can be explained by Newtonian physics and our tendency to engage in pattern matching creates a mental model of the universe that does not reflect reality. What happens in any given instance, with any particular species, and in any biological or physical system will fall within some range of behaviors but predicting the exact behavior is impossible.
There is also the issue of never being able to prove something beyond a shadow of a doubt. This also is difficult to incorporate into an understanding of the universe when trained in a deterministic manner. However, the way that science works is to accept the model or theory that best fits the data until a better theory comes along. The problem with creationism and intelligent design is that it refutes the theory of evolution based on the fact that not all of the pieces are explained at this point. Resorting to god as a hypothesis is a non-explanation because none of the workings of the system are addressed. It is analogous to trying to reverse engineer a car. During the process all of the pieces are catalogued, the engineering drawings are made, and the manufacturers of all of the parts are identified. At a certain point in the process it is still an unknown where the screws that hold the taillights on are made. Therefore the team throws their hands in the air, trashes all of the parts lists and engineering drawings, and declares that there is no way Ford could have manufactured the vehicle but god must have magically created the car.
Similarly evolution explains the preponderance of the evidence now available. The knowledge base will continue to grow and expand and the theory will be tweaked. However, the theory of evolution is not likely to be thrown out and adjustments to the theory do not negate the overarching concept or most of the pieces.
In summary it is necessary in my opinion to become comfortable with uncertainty, probability, and chaos theory at various levels to be able to incorporate evolution into a world view. We will never know for sure but the statistical probability of being correct increases as the theories are argued, studied, and tested.
