Here is a simple yet solid intelligent design rebuttal that would make any creationist pee a little.
Evolution by natural selection explains very complex things through gradual increases from the simplest beginnings. Religion fails at this explanation because it explains complex things through an even more complex creator hypothesis, which not only multiplies the improbability of the explained event, but leads to the problem of infinite regression [the creator made the universe, well who made him? and who made the original creator....etc].
Have you ever heard a creationist or biblical apologist say something like this:
I can’t imagine evolution making something so complex or amazing. And I can’t see it happening by accident [e.g. an eye or a butterfly's wing].
Leslie Orgel (famous biologist) had a number of observations that he coined into little rules that guided evolutionary thinking. Orgel’s 2nd rule: “evolution is smarter than you”. Therefore it is not in any way limited by the observer’s imagination. Just because you cannot even fathom how an eye was put together, does not mean a feedback loop of selection over billions of years could not (the eye is thought to have fully evolved in as little as 250,000 years).
The problem is that our brains just simply cannot comprehend the geologic timescale. Think of all of the trillions of little events that have accumulated in your whole life; can you imagine that number multiplied by 8 orders of magnitude? The reality is hard to grasp, but we really are operating with vast timescales. That isn’t to say that time is the only factor involved. The problem most creationists have is this:
It is too mathematically improbable that something as complex as a heart evolved. It must have been designed.
However, when realizing we are talking about geologic time, the evolution of a heart can be fully explained. What evolution does is it works with small improbabilities in the form of random genetic mutations. Of course it would be too unlikely that a heart would evolve in just one step and consequently this is not how evolution works. What happens is those small improbabilities accumulate over thousands and hundreds of thousands of generations, ratcheting up the improbability each time. What we are left with is something very improbable indeed, a result of accumulated mutations, but something that has had humble beginnings in the form of small genetic mutations.
Therefore an answer to “intelligent design” or “ID” presents itself: evolution “designs” biological systems over a vast timescale using trial and error. This is where most ID’ers stake their judgments. Many are under the assumption that everything on Earth happened in 6,000 years. However, through modern science we know that the Earth is much older than this. Therefore, to understand the vast complexity of life, one must evaluate change over millions of millenia.
The process of billions of years of trial an error is inherently superior to a simply top-down design, like one would expect from a creator, because of the actual feedback that the process gets from real world trial and error over massive time periods. In fact, this trial and error is exactly why we see imperfections, mutations, and evolution.
Any kind of system with seemingly infinite feedback will always be superior to something that was made in one swift motion. This is why you can see corrected imperfections throughout evolutionary history in all species. From eyes, to wings, to lungs, to nerves. The eye for example has to focus, has to flip the image, has to have evolutionary balances that make the eye as functional as say, a human eye. A creator would have certainly made an eye with no need for corrections, for the perfection is implied. But what we see is that the eye is not perfect, as is seen in our own eyes and in species with less developed eyes. The feedback loop of selection weeds out the misgivings of the eye, through the groups of animals with the mutations necessary to correct them. These groups in turn pass on their genes, and the small genetic improbability of a fully developed eye becomes slighty more probable. Multiply this process a billion times and there you go.
Just the fact that we can see and recognize these corrected imperfections in an eye or a wing is enough to dismiss the idea of a creator making perfect biological organisms. If we can see the logic and the process (and of course understand it), and the majority of evidence ever recorded on the subject points to this elegant theory, it must be held as the most probable explanation.
The theory of evolution has enough explanation and evidence to have no need for any design whatsoever. Until there is real, tested, scientific evidence to the contrary, there is no more likely explanation for the diversity of life (and its complexity) on Earth.