Introduction

Michael Behe's term "irreducible complexity" is, to be frank, plainly silly — and here's why.

"Irreducible complexity" is a simple concept. According to Behe, a system is irreducibly complex if its function is lost when a part is removed1. Behe believes that irreducibly complex systems cannot evolve by direct, gradual evolutionary mechanisms. However, standard genetic processes easily produce these structures. Nearly a century ago, these exact systems were predicted, described, and explained by the Nobel prize-winning geneticist H. J. Muller using evolutionary theory2. Thus, as explained below, so-called "irreducibly complex" structures are in fact evolvable and reducible. Behe gave irreducible complexity the wrong name.

Behe's flawed argument

Behe claims that irreducibly complex systems cannot be produced directly by gradual evolution3. But why not? Behe's reckoning goes like this:

(P1) Direct, gradual evolution proceeds only by stepwise addition of parts.

(P2) By definition, an irreducibly complex system lacking a part is nonfunctional.

(C) Therefore, all possible direct gradual evolutionary precursors to an irreducibly complex system must be nonfunctional.

Of course, Behe's argument is invalid since the first premise is false: gradual evolution can do much more than just add parts. For instance, evolution can also change or remove parts (pretty simple, eh?). In contrast, Behe's irreducible complexity is restricted to only reversing the addition of parts. This is why irreducible complexity cannot tell us anything useful about how a structure did or did not evolve.

The Mullerian two-step

With Behe's error now in hand, we immediately have the following embarrassingly facile solution to Behe's "irreducible" conundrum. Only two basic steps are needed to gradually evolve an irreducibly complex system from a functioning precursor:

Add a part. Make it necessary.

It's that simple. After these two steps, removing the part will kill the function, yet the system was produced directly and gradually from a simpler, functional precursor. And this is exactly what Behe alleges is impossible.

As a scientific explanation, the Mullerian two-step is extremely general and powerful, since it is independent of the biological specifics of the system in question. In fact, both steps can happen simultaneously, in a single event, even a single mutation. The function of the system can remain constant during the process or it can change. The steps can be functionally beneficial (adaptive) or not (neutral). We don't even need to invoke natural selection in the process — genetic drift or neutral evolution will do4. The number of ways to add a part to a biological structure is virtually unlimited, as is the number of different ways to change a system so that a part becomes functionally essential. Plain, ordinary genetic processes can easily do both.

A historically and technically appropriate name for IC: "Interlocking Complexity"

For the preceding reasons, compelled by both scholarly ethics and scientific accuracy, I suggest that we avoid reference to "irreducibly complex" structures using Behe's misnomer. Rather, I propose the term "Mullerian interlocking complexity" (MIC), terminology similar to that used in H. J. Muller's much earlier evolutionary analyses of the same molecular phenomenon (Muller 1918; Muller 1939).

Example 1: The stone bridge

A clear example of the Mullerian two-step is given by a stone bridge. Consider a crude "precursor bridge" made of three stones. This bridge spans the area needed to be crossed and is thus functional. For step one of the Mullerian two-step, a part is added: a flat stone on top, covering all precursor stones. Whether this improves the functionality of the bridge is irrelevant — it may or may not, the bridge still functions. For step two of the Mullerian two-step, the middle stone is removed. Voilá, we have an irreducibly complex bridge, since the last step made the top-stone necessary for the function.

The precursor bridge: three stones.

Step #1, add a part: the top-stone.

Step #2, make it necessary: remove the middle stone. As promised, we now have an irreducibly complex stone bridge. None of the three stones can be removed without destroying the bridge's function.

Example 2: How to eat pentachlorophenol

An irreducibly complex system has evolved in bacteria within the past 70 years.