Anti-evolutionists often come up with ridiculous numbers, representing what they call the "chance" of species occurring as they appear now. These are somewhere in the range of 1/10^147 chance of arranging all of the genes in the correct order, blah blah blah. Fortunately for those of us who think rationally, this is complete nonsense.
Take this for example. The following number set represents a perfect system:
1 2 9 8 7 1 6 9 0 2
But, I have only the following system:
5 0 8 9 3 5 0 5 6 5
Not a single one of those numbers matches, but what if I was to naturally select for them. Let's say the string mutated.
1 0 8 9 3 5 2 5 6 5
The 1 in the beginning and the 2 four from the end have mutated. One matches and works well for me, but the 2 doesn't. So, I'd wait for the system to mutate again, getting rid of those numbers that don't match and keeping the ones that do. In 21 generations of selection and mutation, I will have achieved the perfect system I was looking for, through the basic mechanisms of evolution (I evolved the string through mutation and natural selection).
Now, that in no way proves the legitimacy of evolution, but it is similar to how evolution works. Pieces that don't work get removed, while pieces that do stay. The error many creationists make when they calculate the probability of the occurrence of evolution is that they calculate the chance of assembling life as it appears now. Of course, life did not just begin as it appears now (ironically, those absurd probabilities are similar to what you would get if life spontaneously generated, i.e. creation). Life began simple, as a string of maybe 10 letters to make an analogy. Over time, more numbers get added (or, possibly, deleted) to that string as they reproduce. The sequences that work will live on, while those that don't will die. This makes for a much simpler process that makes those intensely high probability calculations worthless.