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The Mullerian Two-Step: Add a part, make it necessary

or, Why Behe's "Irreducible Complexity" is silly

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Copyright © 2006-2007 by Douglas Theobald, Ph.D.

"... an irreducibly complex system might arise by gradually co-opting parts that initially were dispensable but eventually become indispensable ...."
William A. Dembski 2004, p. 24.


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:

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:

  1. Add a part.
  2. 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.

[Figure 1: Three square stones]

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

[Figure 2: A cap-stone added to the three-stone bridge]

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.

[Figure 3: The middle stone removed]

Example 2: How to eat pentachlorophenol

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


1: Behe has defined his usage of "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. "
Behe 1996 p. 39.

"... 'irreducibly complex' means roughly that if one removes a component from a system, function is lost; ..."
Behe 2001 p. 686.


2: H. J. Muller predicted and discussed M. J. Behe's "irreducibly complex" structures in two different papers, one in 1918 and one in 1939. This prediction was made long before the genetic material was known or anyone had seen the structure of a "molecular machine".

"... thus a complicated machine was gradually built up whose effective working was dependent upon the interlocking action of very numerous different elementary parts or factors, and many of the characters and factors which, when new, were originally merely an asset finally became necessary because other necessary characters and factors had subsequently become changed so as to be dependent on the former. It must result, in consequence, that a dropping out of, or even a slight change in any one of these parts is very likely to disturb fatally the whole machinery; for this reason we should expect very many, if not most, mutations to result in lethal factors ..."
Muller 1918 pp. 463-464. (emphasis in the original)

"V. The role of interlocking and diffusion of gene functions in hindering true reversal of evolution

"... an embryological or physiological process or structure newly arisen by gene mutation, after becoming once established (with or without the aid of selection), later takes more and more part in the whole complex interplay of vital processes. For still further mutations that arise are now allowed to stay if only they work in harmony with all genes that are already present, and, of these further mutations, some will naturally depend, for their proper working, on the new process or structure under consideration. Being thus finally woven, as it were, into the most intimate fabric of the organism, the once novel character can no longer be withdrawn with impunity, and may have become vitally necessary."
Muller 1939 pp. 271-272.


3: Behe explains why he imagines "irreducible complexity" is a barrier to gradual evolution:

"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."
Behe 1996 p. 39.

"Irreducibly complex systems appear very unlikely to be produced by numerous, successive, slight modifications of prior systems, because any precursor that was missing a crucial part could not function. Natural selection can only choose among systems that are already working, so the existence in nature of irreducibly complex biological systems poses a powerful challenge to Darwinian theory."
Behe 2002.


4: H. Allen Orr has explained Muller's explanation for "irreducible complexity" in several articles in the Boston Review criticizing Behe's and William Dembski's writings. Orr has emphasized the adaptive possibilities in the Mullerian two-step (i.e. improvement of function at each step). However, the mechanism is more general and does not even require selection, a point that Muller himself made originally, 50 years before neutral evolution was found to be important in molecular evolution.

"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 (and 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. This process continues as further parts get folded into the system. And at the end of the day, many parts may all be required."
Orr 1996

"... gradual Darwinian evolution can easily produce irreducible complexity: all that's required is that parts that were once just favorable become, because of later changes, essential. "
Orr 1997



Behe, M. J. (1996) Darwin's Black Box: The Biochemical Challenge to Evolution. New York, Touchstone.

Behe, M. J. (2001) "Reply to my critics: A response to reviews of Darwin's Black Box: The Biochemical Challenge to Evolution." Biology and Philosophy 16:685-709.

Behe, M. J. (2002) "The challenge of irreducible complexity." Natural History, 111(3):74.

Darwin, C. (1872) The Origin of Species. Sixth Edition. The Modern Library, New York.

Dembski, W. A. (2004) "Irreducible Complexity Revisited." Progress in Complexity, Information, and Design (PCID) 3.1.4, November. [PDF]

Muller, H. J. (1918) "Genetic variability, twin hybrids and constant hybrids, in a case of balanced lethal factors." Genetics 3:422-499. [Free Text, Genetics Online]

Muller, H. J. (1939) "Reversibility in evolution considered from the standpoint of genetics." Biological Reviews of the Cambridge Philosophical Society 14:261-280.

Orr, H. A. (1996) "Darwin v. Intelligent Design (Again)." Boston Review, December 1996/January 1997. [Free Text, Boston Review]

Orr, H. A. (1997) "Is Darwin in the Details?: H. Allen Orr Responds" Boston Review, February/March 1997. [Free Text, Boston Review]

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