As we saw earlier, one of the central claims of intelligent design (ID) is that "intelligent causes are necessary to explain the complex, information-rich structures of biology."
The more we learn about living organisms, the more they look like products of design rather than products of chance and natural law. Ironically, many opponents of intelligent design concede this fact. Oxford biologist Richard Dawkins, for example, says "Biology is the study of complicated things that give the appearance of having been designed for a purpose." 
Similarly, in a recent issue of the biology journal Cell, Bruce Alberts, a leading cell biologist and president of the National Academy of Sciences, wrote:
We have always underestimated cells. The entire cell can be viewed as a factory that contains an elaborate network of interlocking assembly lines, each of which is composed of a set of large protein machines. Why do we call the large protein assemblies that underlie cell function protein machines? Precisely because, like machines invented by humans to deal efficiently with the macroscopic world, these protein assemblies contain highly coordinated moving parts. 
Of course, biologists such as Dawkins and Alberts believe that the apparent design of living things is an illusionproduced not by an intelligent source, but by chance and natural law. Dawkins specifically states that Paleys "watchmaker" is natural selection, which produces complex systems by accumulating favorable genetic changes over time.
The notion of complex specified information (CSI) provides a way to test this claim. To see how this might work, lets consider one of the processes involved in human vision. (If this seems too technical to follow, you can skip this section without missing the main point.)
When light strikes a rod cell, a visual cell thats located in the retina, the rod cell produces an electrical charge that runs down a nerve cell and into the brain. How does the light set off the electrical charge?
In the absence of light, a rod cell maintains an electrically neutral state by letting sodium ions flow freely in and out of the cell. (An ion is an atom or group of atoms that carries an electric charge.) It does this by means of certain proteins embedded in the cell membrane. One protein, called an ion channel, acts like a gate, regulating the inflow of sodium ions. Another protein acts as a pump, pushing the sodium ions back out of the cell.
The ion channel opens and closes in response to another biomolecule, called cGMP. For convenience, well call it the opener. When the opener attaches to the ion channel, the channel opens up and lets positively charged sodium ions flow into the cell. When the opener falls off, the channel shuts and the flow of ions stops.
Under normal circumstances, there are so many opener molecules in the cell that they are continually attaching to the channel and then falling off. As a result, the channel is continually opening and closing.
That changes when light enters the cell. The light strikes a biomolecule that well call the trigger (its real name is 11-cis-retinal). This causes the trigger to change its shape, setting off a cascade of chemical reactions in the cell.
The result of all these reactions is that the opener gets snipped in two, and is no longer able to attach to the ion channel. Sodium ions are no longer able to enter the cell, and as the pump pushes more of them out, an electrical charge develops. When the charge gets strong enough, the cell fires off an electrical impulse. Then another cascade of reactions restores the trigger and opener molecules to their original state, allowing the ion channel to function again.
Is this system designed or was it produced by strictly natural processes?
Darwinists would say no: All biological systems were "created" by a stepwise accumulation of random genetic mutations that are preserved by natural selectionor survival of the fittest. Existing systems are merely modifications of earlier systems, which were modifications of even earlier systems and so on.
Design theorists, on the other hand, would say yesif the system exhibits specified complexity.
Whos right? Both sides would agree that this system is complex. It has lots of parts, and all these parts have to work together.
The real question, then, is how specified the system is: How broad are the requirements for a working system?
One way to answer this question is to tinker with the system and see what happens. How well does the system function when you start knocking out proteins or other biomolecules? How well do the molecules function when you alter them? If it can take a lot of hits and still work, then it isnt very specified, and could plausibly have been produced by an undirected, stepwise process. But if it can handle only minute changes, then the system is highly specifiedand extremely unlikely to have been produced by chance.
Some systems are so highly specified that they seem to tolerate no change at all. One such system is the bacterial flagellum, an outboard motor that bacteria use to navigate their environment. It requires about 50 proteins to build. If you knock out any of these proteins, the flagellum either wont be built or wont work. the flagellum doesnt work. The flagellum thus seems to display not only specified complexity, but irreducible complexity.
More fascinating is a study reported in the journal Science. A team of researchers wanted to discover how many genes were necessary for the simplest organism to survive and reproduce. If you think of an organisms genes as its parts list, the scientists wanted to know how small they could make the parts list and still have a living, reproducing organism.
They did this, in part, by tinkering with a bacterium called Mycoplasma genitalium, which is the simplest known organism (though the recently discovered Nanoarchaeum equitans looks like it may become the new champ in this category). The organisms genetic code is about 580,000 letters long and spells out 480 protein-producing genes plus 37 "species" of RNA. After "knocking out" various protein-coding genes, the scientists have estimated that 265 to 350 of this bacteriums genes are "essential" for the organism to live and reproduce under laboratory conditionsan extremely favorable environment that would not be found on the early earth.
Is this a designed system? It looks like it. But the main point is that specified complexity gives us a standard to guide our research.
ARN Recommends: For further study of intelligent design and biology see the following resources:
Darwin's Black Box The Biochemical Challenge to Evolution Michael J. Behe
Irreducible Complexity: The Biochemical Challenge to Darwinian Theory Michael J. Behe