The Mullerian Two-Step - - How Evolution Creates Irreducible Complexity - Type 2!

I wanted to set aside a copy of an old/closed thread to make it as easy as possible to find the topic in the archives. The term “Mullerian Two-Step” is probably not a standard reference, but it is clearly a unique label by which we can remember this topic - - how long before the phrase “Irreducible Complexity” was coined, Muller had predicted the appearance of such surprisingly complex evolutionary outcomes!

Prof. @swamidass has done a commendable job summarizing these points below. Every time I accidentally came across the phrase “Mullerian Two-Step”, I would tell myself (most insistently!) that I would memorialize the topic by creating a stand-alone thread.

Well, today is the day I actually do it!

[Aside from some format additions, this all comes from Joshua Swamidass!]

This article has some really important historical information. The key point is that IC1 was shown evolvable well before Behe even proposed it. Quoting from this article below…

The Mullerian Two-Step

“Irreducible complexity” is a simple concept. According to Behe, a system is irreducibly complex if its function is lost when a part is removed. 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 theory. Thus, as explained below, so-called “irreducibly complex” structures are in fact evolvable and reducible. Behe gave irreducible complexity the wrong name.

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.

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

Note from @swamidass: “Darwinian” evolution was falsified a long time ago (neutral processes are important too), this is a quote of a scientist writing before that was known. Do not be confused by that anachronism. Modern evolutionary science is not Darwinian evolution. It turns out that Muller was wrong on this:


we should expect very many, if not most, mutations to result in lethal factors …”

Turns out that most mutations are neutral, not lethal. See The Neutral Theory of Evolution . It seems that over 100 years, we’ve learned a thing or two of about evolution, because ware actively studying it. Remember, at this point in history, Muller didn’t even know what DNA was. No surprise he had a thing or two wrong.

Now, why exactly does anyone still think that IC1 is a good argument? How can we trust anyone advances yet again the IC argument and is ignorant of this history?


Here is the URL to the original posting, and the overall thread:

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Here’s a nice exhibit from out of the FAQ link above:

The bridge fails as an illustration unfortunately.


The bridge fails? How so? I can “extract” the other example from that link… But what is the “snag” with the bridge?

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It starts out as an IC system with 3 parts, and ends up as an IC system with 3 parts.

The game of Jenga might be a useful analogy. If you have ever played the game you will notice that the weight of the stack can shift from one side to the other during the game. In the early game you may be able to remove a specific block because it isn’t bearing any weight, but later in the game it may become integral to the stability of the stack and can no longer be moved. The same for the blocks that are added to the top. At first they may not be integral, but later they are. This is also a good analogy for epistasis within a single protein.


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