Here I shall argue that
intelligent design is an inadequate attempt to explain
biological structures. To do this, the nature of design must be considered; this means considering the designers (in decreasing order of intelligence):
- Unique, all powerful designer
God in other words. Although all seeing and all powerful, this designer is faced with some difficult design descisions. Create everything at once, and let the system run - or start from scratch and intervene at key moments. Allow species and individuals to kill each other (potentially risking their own lives) or remove their free will. Should the bacteria have free will? - Open source, mortal and distributed designers
Not a popular candidate theory for creation. Imagine, though, many designers making individual decisions about specific areas. Like in Time Bandits, where the dwarves make the details (like shrubbery). Multiple contributions to the project mean occasional clashes of design (not just the colours, mind). - Stupid self-design
Each organism designs itself. Descisions are made partly at random and sucessful designs are kept. Bad attempts are just thrown away - they die. Conflicts between designers are resolved in a similar fashion, but co-operation and hybrid designs are often helpful.
As you can see, as the design decisions are made more and more by the individuals - even though they cannot see "the whole picture" - the task becomes easier. Of course, the all-powerful designer presumably has no trouble with any task. Assuming the existence of this intelligent designer is unecessary, however, since stupid (self) design works just as well*.
How does stupid design account for irreducibly complex structures? It doesn't - they are reducible. That they appear not to be (to us, semi-intelligent mortals) is of no consequence to the stupid designers. If it can be modified, it will be; if it doesn't work, it's lost.
*To put it another way, an all-powerful designer could simply set everything in motion, allowing the system to design itself.