Irreducible complexity is one of the main design inferences that ID proponents use (specified complexity is the other). So let’s see how Michael Behe defines irreducible complexity.
By irreducibly complex I mean a single system composed of several well-matched, interacting parts that contribute to the basic function, wherein the removal of any one of the parts causes the system to effectively cease functioning. An irreducibly complex system cannot be produced directly (that is, by continuously improving the initial function, which continues to work by the same mechanism) by slight, successive modifications of a precursor system, because any precursor to an irreducibly complex system that is missing a part is by definition nonfunctional. An irreducibly complex biological system, if there is such a thing, would be a powerful challenge to Darwinian evolution. Since natural selection can only choose systems that are already working, then if a biological system cannot be produced gradually it would have to arise as an integrated unit, in one fell swoop, for natural selection to have anything to act on (Behe. p. 39.).
The obvious rejoinder to irreducible complexity is to talk about function switching in evolution. Behe doesn’t deny this as he says:
Yet there’s no reason that individual components of an irreducibly complex system could not be used for separate roles, or multiple separate roles, and I never wrote that they couldn’t (Behe. p. 260).
So Behe recognizes that parts could have had others function; it’s just that he thinks it’s incredibly unlikely that natural processes could have produced certain systems.
The key point in the definition of irreducible complexity that “the removal of any one of the parts causes the system to effectively cease functioning”. (By function, I assume he means the current function.) But, as Sober points out, every system can be irreducibly complex depending on how you remove a part. My computer has dual BIOS. If one BIOS fails, then it can still boot up on the other BIOS. Depending on how I remove parts, the computer will be multiply redundant or be irreducibly complex. If the part I remove is one BIOS, then the computer is multiply redundant. If the part I remove consists of half of each BIOS (in effect breaking both BIOSes), then the computer is irreducibly complex. So, without further qualification, irreducible complexity turns out to be a trivial truth even for multiply redundant systems.
One plausible way to remove the parts is to remove them in the order that they developed in their evolutionary history. But this puts the burden on the ID proponent to show the history of the system. For example, Behe would need to show that history of the flagellum in its system, and show that that system is irreducibly complex through its evolutionary history. (But this may be unfair and unnecessary as I’ll explain later.) Instead, what happens is that Behe wants to put the burden on the opponent to provide the evolutionary history.
Not only would I need a step-by-step, mutation by mutation analysis, I would also want to see relevant information such as what is the population size of the organism in which these mutations are occurring, what is the selective value for the mutation, are there any detrimental effects of the mutation, and many other such questions. (Kitzmiller vs Dover, p. 19)
Suppose a full history of the evolution of the eye is provided. ID would not be refuted because they can move the target to the flagellum. Suppose a full history of the flagellum is provided, and suppose a full history of every organism in its entirely is provided. ID would still not be refuted because there is cosmology. After all the Discovery Institute says:
The theory of intelligent design holds that certain features of the universe and of living things are best explained by an intelligent cause, not an undirected process such as natural selection.
In other words, until we explain everything they want to know about the universe, ID will remain unrefuted. This is certainly not in the Popperian spirit of sticking your necks out for criticism.
So who has the burden of proof? Earlier I said that Behe doesn’t actually need to show the full history of the flagellum, because that requirement is unnecessary. What he does need to show is that his theory is more plausible than evolution. Novel predictions generated by his theory would be a good start.
But in support of IC, Behe has produced three examples of irreducible complexity: blood clotting cascade, eye, and flagellum. There have been plausible rejoinders to all three; but suppose that Behe finds something where we can’t find evidence to propose a pathway for. Is that evidence for ID? This seems like nothing more than God of the gaps. Even if we currently don’t have evidence for a pathway for the evolution of something, we would still be justified in holding out for a naturalistic solution. Why should we accept naturalism of the gaps? Because naturalistic research programmes have been fecund, they have the explanatory virtues of testability, fit with background knowledge, past success, non ad hoc hypotheses, and ontological parsimony. By contrast, the ID research programme lack these things.
Behe, Michael. Darwin’s Black Box. (2006).
Boudry, Maarten. Irreducible Incoherence and Intelligent Design.
Kitzmiller vs Dover Area School District. Transcript of proceedings. Afternoon session (U.S. District Court for the Middle District of Penssylvania 2005)
Sober, Elliott. Evidence and Evolution