The 'irreducible complexity' argument advocated by the intelligent design community often cites the specific example of the eye. It is argued that an eye is a complex organ with many different individual parts that all must work together perfectly and that this implies it could not have been gradually built out of small gradual random changes.

This argument has been around a long time but it has been well answered within the scientific literature and the vast majority of biologist consider the issue settled.

Dawkins' book 'Climbing mount improbable' provides a summary of the science for the lay reader and uses the eye as a detailed example.

Darwin was the first to explain how the the eye could have evolved via natural selection. I quote the wikipedia article:

Charles Darwin himself wrote in his Origin of Species, that the evolution of the eye by natural selection at first glance >seemed "absurd in the highest possible degree". However, he went on to explain that despite the difficulty in imagining it, >this was perfectly feasible:

...if numerous gradations from a simple and imperfect eye to one complex and perfect can be shown to exist, each >grade being useful to its possessor, as is certainly the case; if further, the eye ever varies and the variations be inherited, >as is likewise certainly the case and if such variations should be useful to any animal under changing conditions of life, >then the difficulty of believing that a perfect and complex eye could be formed by natural selection, though insuperable by >our imagination, should not be considered as subversive of the theory.

The argument of 'irreducible complexity' has been around since Darwin first proposed natural selection and it has been conclusively answered within the scientific literature (for a good summary see the Wikipedia article). Those who believe that all life was created by God cannot believe the scientific explanation. In my view the real problem is that they tend to argue that they have superior scientific evidence which proves that the scientific consensus is wrong. In other words the intelligent design community argues they are scientifically superior to the science community. This reduces their position to a undignified one of deception or perhaps even fraud.