Irreducible complexity is the cornerstone of the intelligent design movement, and it is a popular argument among young earth creationists as well. In simplest terms, this argument states that some systems are too complex to have evolved via natural selection because they have lots of parts and all of those parts need to be in place for the final product to function. Thus, no one part can be selected for until all of the other parts are already in place (it’s basically a chicken or the egg paradox). This is not a new argument. In fact, Darwin explained why this argument doesn’t work in Origin of the Species, and his rebuttal still works today. Nevertheless, despite the fact that this argument was defeated over 150 years ago, it is still a very popular argument today.
No one has done more to popularize this argument that Michael Behe. He championed this notion in his book, Darwin’s Black Box in which he shifted the target of this argument. You see, Darwin dealt with relatively large “irreducible” systems, such as eyes, whereas Behe focuses on tiny molecular machines such as the bacterial flagellum. Darwin’s explanation applies to these tiny machines just as well as it does to large systems, but given the immense popularity of this argument, I will explain the problems with Behe’s derivation of this argument (it irritates me to no end that I have to debunk an argument that was soundly defeated over 150 years ago).
The standard example of irreducible complexity is the mousetrap, so let’s start with that. A mousetrap is fairly simple and requires only a few pieces: a base, spring, arm, catch, and bait plate. According to Behe, this is an irreducibly complex system because all five pieces are necessary for it to function as a mousetrap. If even one piece was removed, it would not catch any mice. Therefore, according to Behe, a mousetrap could never have evolved via natural selection because natural selection is a gradual step-wise process, but no one step would be useful until all of the steps were in place.
If you have read my other posts on evolution, then the problem with this argument should be obvious: it ignores the fact that evolution is blind. I will fully admit that you need all five pieces for it to function as a mousetrap, but that is not the same thing as saying that you need all five pieces for it to function. This argument sets up a mousetrap as an ultimate endpoint that evolution is working towards, but that’s simply not how evolution works. It does not have any foresight or goal. So the parts don’t have to function as a mousetrap for nature to select them. Rather, they simply have to perform some useful function, and it is easy to think of uses for every single piece of a mousetrap. Springs are, for example, useful for many applications. Similarly, a block of wood has a nearly infinite number of uses. Thus, each piece of a mousetrap could be selected for a reason other than acting as a mousetrap. Then, random mutations could bring those pieces together, and as long as some useful function was performed, those combinations would be selected for. During a trial over teaching intelligent design, Kenneth Miller has famously illustrated this by wearing a partial mousetrap as a tie clip. It may not be the best fashion statement, but it makes an important point: you can have a useful function without having all five parts, and that’s all that natural selection needs.
Now, let’s apply this to a real world example. The poster child of irreducible complexity is the bacterial flagellum. This is an amazing structure that propels bacteria through their environment like a tiny motor. In most species, it requires 42 proteins to work, and if any one of them are missing, it will not function as a flagellum. Thus, according to Behe, the flagellum is irreducibly complex and could not have evolved because no one protein would be selected until all of the other proteins were already in place. The reality is that we know of useful functions that almost all of those proteins do elsewhere in the cell. Thus, each protein would have initially been selected in order to perform a function other than being part of a flagellum. Further, we know of multiple combinations of these proteins that are extremely important to cells. In fact, we have laid out an entire pathway that would result in the step-wise evolution of a flagellum, with each step requiring only one mutation and each step producing a useful product (you can see an animation of this pathway here). To put this another way, all 42 proteins are useful for other purposes, mutations can bring these proteins together to perform other functions, and further mutations can result in useful combinations of those clusters until we ultimately end up with a flagellum. In other words, the flagellum is not irreducibly complex.
We can clearly see from the bacterial example that the irreducible complexity argument is nonsense. It relies on a complete misunderstanding of evolution. Nevertheless, I have found that debating this topic with creationists is much like fighting a hydra. Every time that I explain how one supposedly irreducibly complex system could have evolved, they bring up some other system that they think is irreducibly complex. This brings up an important point. Namely, this argument is really just an argument from ignorance fallacy. It states the following, “I don’t understand how this evolved, therefore it didn’t evolve.” That’s not logically valid. Just because you don’t understand something doesn’t mean that it isn’t true.
At this point, I often get the response of, “fine, we have hypothetical mechanisms that would allow the immune system, flagella, the clotting cascade, etc. to evolve, but we have never proved that any of those pathways actually occurred.” This response totally misses the point, and it commits another argument from ignorance fallacy. Irreducible complexity states that these systems cannot evolve. This is a universal claim, and all that it takes to defeat a universal claim is a single example to the contrary. So, for example, I do not have to prove that the evolution of the flagellum followed the exact pathway outlined in the video, because the simple fact that a possible pathway exists demonstrates that the flagellum could have evolved. We don’t have to prove how it evolved, we just have to show that it could have evolved. Irreducible complexity is an all-or-nothing argument. Either these systems can evolve or they can’t, so by providing even one example of how they could have evolved, we have completely defeated the argument.
Further, even though we do have hypothetical pathways for essentially all of the supposedly irreducibly systems, having pathways for every single one is totally unnecessary because the argument itself is so fundamentally flawed. It relies entirely on the concept that evolution has a goal that it is working towards, and we know that evolution doesn’t operate that way. As long as a given mutation is in some way useful, it will be selected for.
In conclusion, irreducible complexity is based on a fundamental misunderstanding of one of the most basic and important concepts of evolution: mutations do not need to be useful for some ultimate endpoint in order to be selected, they just have to be useful. Further, we have hypothetical pathways for the evolution of systems that are supposedly irreducibly complex, and any one of these pathways is sufficient to complete debunk this argument. To demonstrate that a system is truly irreducibly complex, you would have to prove that its parts are only useful for the system in question, and no one has ever been able to do that. Until someone can do that, until someone finds a system in which it is clear and unambiguous that the parts involved are only useful for that system, this argument simply doesn’t work, so please stop using it.