Analysis

According to creationist Michael Behe, who coined the term "irreducible complexity," an irreducibly complex system is "a single system which is composed of several interacting parts that contribute to the basic function, and where the removal of any one of the parts causes the system to effectively cease functioning" (Behe 1997). However, there is no reason at all why such systems could not evolve. Allen Orr explains:

An irreducibly complex system can be built gradually by adding parts that, while initially just advantageous, become — because of later changes — essential. The logic is very simple. Some part (A) initially does some job (not very well, perhaps). Another part (B) later gets added because it helps A. This new part isn't essential, it merely improves things. But later on, A (or something else) may change in such a way that B now becomes indispensable. (Orr 1998)

As Orr notes, gene duplication often provides the route by which irreducibly complex processes arise:

Molecular evolutionists have shown that some genes are duplications of others. In other words, at some point in time an extra copy of a gene got made. The copy wasn't essential — the organism obviously got along fine without it. But through time this copy changed, picking up a new, and often related, function. After further evolution, this duplicate gene will have become essential. (Orr 1998)

Not only are there mechanisms whereby irreducibly complex systems can be gradually evolved, but Russell Doolittle (who was unfairly criticized in Behe's book) also describes experimental proof that one of Behe's favorite examples of an irreducibly complex system (the blood-clotting process) is not even irreducibly complex to begin with:

Recently the gene for plasminogen was knocked out of mice, and, predictably, those mice had thrombotic complications because fibrin clots could not be cleared away. Not long after that, the same workers knocked out the gene for fibrinogen in another line of mice. Again, predictably, these mice were ailing, although in this case hemorrhage was the problem. And what do you think happened when these two lines of mice were crossed? For all practical purposes, the mice lacking both genes were normal! [A footnote here refers to Bugge et al. 1996.] Contrary to claims about irreducible complexity, the entire ensemble of proteins is not needed. (Doolittle 1998)

For the bacterial flagellum, which probably is the most-often touted example of an alleged irreducibly complex system, consult 4-3.

As for the claim that evolutionists have not even tried to posit and research mechanisms whereby allegedly irreducibly complex systems could have evolved, John Catalano (2004) gives short shrift to that claim.

For extensive links to articles by and on Michael Behe and irreducible complexity, see John Catalano's "Behe's Empty Box". For a beautifully illustrated critique of Behe along the same lines as Orr, see Keith Robison's "Darwin's Black Box: Irreducible Complexity or Irreproducible Irreducibility?"

References

Behe M. 1997. Molecular machines: experimental support for the design inference. www.arn.org/docs/behe/mb_mm92496.htm.

Catalano J. 2004. Publish or perish: some published works on biochemical evolution. www.talkorigins.org/faqs/behe/publish.html.

Doolittle RF. 1998. A delicate balance. bostonreview.mit.edu/br22.1/doolittle.html.

Orr AH. 1998. Darwin v intelligent design (again). bostonreview.mit.edu/br21.6/orr.html.