The Argument from Irreducible Complexity

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Bacterial flagella

When I was a student, well over a decade ago now, intelligent design was all the rage. It was the latest religiously-inspired threat to Darwinism (though it tried to hide its religious origins). It argued that Darwinism could never account for certain forms of adaptation that we see in the natural world.
What made intelligent design different from its forebears was its seeming scientific sophistication. Proponents of intelligent design were often well-qualified scientists and mathematicians, and they dressed up their arguments with the latest findings from microbiology and abstruse applications of probability theory. My sense is that the fad for intelligent design has faded in the intervening years, though I have no doubt that it still has its proponents.

That’s all really by way of an apology for the following post, which is going to revisit some of the arguments touted by intelligent design proponents, arguments that have long been challenged and dismissed by scientists and philosophers alike. My excuse for this is that I have recently been reading Benjamin Jantzen’s excellent book An Introduction to Design Arguments which goes through pretty much every single design argument in the history of Western thought, and subjects them all to fair and serious criticism. He has two chapters on arguments from the intelligent design movement: one based on Michael Behe’s argument from irreducible complexity and one based on William Dembski’s argument from specified complexity. Both arguments get at the same basic point, but arrive there by different means. I want to look at the argument from irreducible complexity in the remainder of this post, summarising some of Jantzen’s thoughts on it.

I’m hoping that this is of interest to people who are familiar with the idea of irreducible complexity as well as those who are not. If nothing else, the following analysis helps to clarify the structure of the argument from irreducible complexity, and to highlight some important conceptual issues when it comes to interpretation of natural phenomena.


1. The Argument Itself
Let’s start by clarifying the structure of the argument. The basic idea is that certain natural phenomena, specifically features of biological organisms, display a property that cannot be accounted for by mainstream evolutionary theory. In Behe’s case the relevant property is that of irreducible complexity. But what is this property? To answer that, we’ll need to look at Behe’s definition of an ‘irreducibly complex system’:

Irreducibly complex system (ICS) = Any single system which is composed of several well-matched, interacting parts that contribute to the basic function, and where the removal of any one of the parts causes the system to cease functioning. 

(Behe 1996, 39)

There is a problem with this definition, but we’ll get to that in the next section. For now, it would probably help us to wrap our heads around the notion if we had an example of an ICS. Behe’s favourite example is the bacterial flagellum. This is a thin, filament-like, appendage that protrudes from the cell membrane of many species of bacteria. It is used to help propel the bacteria through liquid. When observed with the aid of a microscope, one of the remarkable features of a bacterial flagellum is that it functions like a rotary motor, where the flagellum is like a freely-rotating axle, supported by a complex assemblage of protein parts. Behe’s contention, and we can accept this, is that if you removed one component from the complex assemblage it would cease to function as a rotary motor.

A slightly more familiar example, and one also used by Behe, is a mousetrap (an old-fashioned, spring-loaded one). This is made up of fewer functional parts, but every one of them is essential if the mousetrap is going to perform its desired function of trapping — and unfortunately killing — mice and other small vermin. Thus, it is an ICS because if you remove one of the parts it ceases to function as intended.

Hopefully this suffices for understanding the property of irreducible complexity. What about the argument from irreducible complexity? That argument begins by identifying an ICS and then works like this:

• (1) X is an irreducibly complex system. (For illustrative purposes say ‘X= bacterial flagellum’)

• (2) If X is an irreducibly complex system, then X must have been brought about by intelligent design.

• (3) Therefore, X (the bacterial flagellum) must have been intelligently designed.

Two important interpretive points about this argument. First, note that the use of the variable-term X is significant. While the bacterial flagellum is the most widely-discussed example, the idea behind the argument is that there are many such ICSs in nature and hence many things in need of an explanation in terms of intelligent agency. Second, note the conclusion. The claim is not that God must have created the bacterial flagellum but, rather, that an intelligent designer did. For tactical reasons, proponents of intelligent design liked to hide their religious motivations, trying to claim that their theory was scientific, not religious in nature. This was largely done in order to get around certain legal prohibitions on the teaching of religion under US constitutional law. I’m not too interested in that here though. I view the intelligent design movement as a religious one, and hence the arguments they proffer as on a par with pretty much all design arguments.

Now that we are clear on the structure of the argument, we can proceed to critically evaluate it. There are two major criticisms I want to discuss, both drawn from Jantzen’s book.


2. The First Criticism: Problems with the concept of irreducible complexity
The first criticism takes issue with the first premise. Appropriately enough. That premise claims that there are readily identifiable ICSs in the natural world. But is this true? Go back for a moment to Behe’s definition (given above). It defines an ICS in relation to a so-called ‘basic function’. The idea is that the basic function of the bacterial flagellum is to propel a bacterium through liquid. All the protein parts of the rotary-motor are directed towards the performance of that basic function, and this is what makes it right and proper to say that removal of one of those parts would cause the system to cease functioning. The same goes for the mousetrap. The basic function of the mousetrap is to capture and kill mice. All the parts of the system are geared toward that end.

That probably sounds fine, but there’s a subtle interpretive problem lurking in the background. It’s easy enough to say that the basic function of the mousetrap is to trap and kill mice. After all, we know the purpose for which it was designed. We know why all the parts are arranged in the format that they are. When it comes to natural objects, it’s a very different thing. Every object, organism, or event in the physical world causes many effects. A mouth is a useful food-grinding device, but it is also a breeding ground for bacteria, a signalling tool (e.g. smiles and smirks), a pleasure organ, and more. To say that one of these effects constitutes its ‘basic function’ is contentious. As Jantzen puts it:

Physical systems that were not crafted by human hands do not come with inscriptions telling us what they are for. 

(Jantzen 2014, 191)

We cannot read the basic function of an alleged ICS off the book of nature. We need interpretive principles. One such principle would be to appeal to the intentions of an intelligent designer. But proponents of intelligent design don’t like to do this because they try to remain agnostic about who the designer is. Furthermore, even if they admitted to being orthodox theists, there would be problems. The mind of God is mysterious thing. Many a theologian has squandered a career trying to guess at His intentions. Some say we should not even try: God has beyond-our-ken reasons for action.

Another possibility is to try to naturalise the concept of a basic function. But this too poses a dilemma for the proponent of intelligent design. One popular way of naturalising basic functions is to appeal to the theory of evolution by natural selection — i.e. to argue that the basic function of a system is the one that was favoured by natural selection — but since the goal of intelligent design theorists is to undermine natural selection this solution is not available to them. The other way to do it is to define the basic function of a system in terms of the causal contribution that the system makes to some larger system. Thus, for instance, you can say that the basic function of the lens of the eye is to focus light rays because this contributes to the larger system that enables us to see.

The main problem with this second approach is that it simply pushes the problem back a further step. It defines the basic functionality of a sub-system by reference to the functionality of the larger system of which it is a part. But then the question becomes: what is the function of that larger system? It’s only once we have settled the answer to that question that we can figure out whether the sub-system is indeed an ICS, which lands us back with the original problem: that basic functions cannot simply be read off the book of nature.

To summarise:

• (4) In order to successfully identify an ICS, you must be able to identify the basic function of the system in question.

• (5) In order to determine the basic function of a system you must either: (a) appeal to the intentions of the designer of the system; (b) appeal to the purpose for which the system has been naturally selected; or (c) identify the causal contribution that the system makes to some super-system with function y.

• (6) A proponent of intelligent design cannot appeal to the intentions of the designer, since they wish to remain agnostic about the intentions of the designer.

• (7) A proponent of intelligent design cannot appeal to natural selection, since their goal is to deny its truth.

• (8) Appealing to the causal contribution that the system makes to some super-system simply pushes the problem back a step.

This leads to the negation of premise (1), i.e. the claim that we have successfully identified an ICS.
This is a somewhat technical objection and its unlikely to have much intuitive appeal. It just seems too obvious to most people that the basic function of something like the bacterial flagellum is to propel a bacterium; that the basic function of the eye is to see; that the basic function of the teeth is to grind food; and so on. It’s only if you really interrogate our reasons for thinking that this is obvious that you begin to see the problem.

Fortunately, there are other ways to object to the argument.


3. Second criticism: The problem of evolutionary co-optation
The main criticism of the argument from irreducible complexity focuses on premise (2) of the argument. That premise claims that the only possible explanation for the existence of an ICS is that it was brought into being by an intelligent designer. But why think that? Aren’t there other plausible explanations for the existence of an ICS? Couldn’t natural selection do the trick?

• (9) Natural selection can explain the existence of an ICS.

The proponent of intelligent design says ‘no’. They argue that natural selection — if they accept the idea at all — can only work in a gradual, step-wise fashion. This might enable the development of some systems that display interesting adaptations and functionality, but it can only work if every step in the chain has a function (i.e. contributes positively to the organism’s survival and reproduction). The problem is that an ICS cannot evolve in a gradual, step-wise fashion. Suppose you have forty different protein parts that need to be arranged in a very precise way in order for the bacterial flagellum to function as it does. It is beyond the bounds of credibility to believe that this could happen through random mutations in an organism’s genetic code. Too many things have to line up in a precise order for that to happen. It would be like having a forty-wheeled combination lock, randomly spinning each wheel, and then hoping to end up with the right sequence. You might get two or three in the right place, but not all forty. You need intelligent designers to bring about improbable (and functional) arrangements.

• (10) Natural selection cannot explain the existence of an ICS because natural selection only accounts for gradual, step-wise changes. An ICS cannot emerge from gradual stepwise changes.

The evolutionist’s response to this is pretty straightfoward: you’re thinking about it in the wrong way. It may well be true that the bacterial flagellum is, currently, irreducibly complex, such that if you altered or removed one part it would no longer function as a rotary motor. But that doesn’t mean that the parts that currently make up the flagellum couldn’t have had other functions over the course of evolutionary history, or couldn’t have contributed to other systems that are not irreducibly complex over that period of time. The flagellum is the system that has emerged at the end of an evolutionary sequence, but evolution did not have that system in mind when it started out. Evolution isn’t an intelligently directed process. Anything that works (that contributes to survival or reproduction) gets preserved and multiplied, and the bits and pieces that work can get merged into other systems that work. So one particular protein may have contributed to a system that helped an organism at one point in time, but then get co-opted into another, newer, system at a later point in time.

That’s the critical point. The history of evolution is a history of co-optation. Just like the mechanic who might take a part from an old car engine in order to make a new improved one, so too does evolution repurpose and reorganise parts into new, improved systems. This is effectively what other microbiologists have pointed out in response to Behe. They’ve noted that the proteins in the bacterial flagellum have other uses in other biological systems. Furthermore, many evolutionary texts are filled with examples of the co-optation process. Jantzen has a very nice example in his book about the evolution of flying insects. He highlights research showing how they evolved from sea-dwelling crustacean ancestors. In the process, the thoracic gill plates of the ancestors (whose original purpose was to facilitate oxygen respiration under water) were repurposed in order to enable the insects to push themselves along the surface of the water. They then evolved to enable the insects to ‘sail’ along the surface of the water, before finally (and I’m skipping several steps here) evolving into full-blown wings.

• (11) Natural selection can explain the evolution of an ICS through the process of co-optation, i.e. through the fact that the component parts of biological systems often get repurposed and reorganised into new systems over the course of evolutionary history.

This might still leave a puzzle as to why natural selection has favoured the creation of ICSs. After all, ICSs are highly vulnerable to change: mess around with one component and the system ceases to function. Why wouldn’t there be some inbuilt redundancy of parts? There are many responses to this. It is quite possible that an organism (or, rather, species) could survive the loss of one ICS. There are, after all, many ways of making a living, as the diversity of life on earth proves. But also, vulnerable and fragile systems can emerge from less vulnerable ones. A.G. Cairns-Smith famously used the example of a stone arch to illustrate the point. An arch is irreducibly complex. Remove one stone and the whole thing collapses. But arches are built by having scaffolding in place during the construction process. It’s only once the keystone is in place that the scaffolding is removed and the system becomes more vulnerable to change. Many alleged ICSs could have emerged through an analogous process.

Okay so that’s it for this post. Hopefully this has effectively explained the concept of irreducible complexity and the two main criticisms of the argument. If you have read this far, I trust it has been of interest to you, even if it does retread old ground.

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