The Stone Bridge example and IC

Dr. Swamidass has offered two examples that he thinks refute Michael Behe’s concept of irreducible complexity. Behe offers his definition in his book, Darwin’s Black Box:

“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.” (DBB, p.39)

The first example that Dr. Swamidass invokes is from an article from Talk.Origins Archive,
“The Mullerian Two-Step: Add a part, make it necessary”

The idea is that one starts with a complex system that is not irreducibly complex, at first, then eventually evolve it into an irreducibly complex system. The example offered is of a bridge made of three stones. A fourth stone is added on top of the first three stones. Then the middle stone underneath is removed. The resulting bridge has become irreducibly complex. Remove any one of the stones, and the bridge ceases to function as a bridge.

The problem with the example is that the bridge was already irreducibly complex. Remove any one of the original three stones, and the bridge ceases to function as a bridge. So all we’ve done is change one irreducibly complex system that functions as a bridge into another irreducibly complex system that functions as a bridge. It dos not explain how the first irreducibly complex bridge came into existence. So this example has not explained irreducible complexity.

The stone bridge example is only one of three arguments presented in the article. I’ll get to the second argument next thread.

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There is no stone bridge example. This is merely an illustration of Muller’s Two-Step. I’d encourage you to engage the actual argument, rather than an illustration of how it works. A simple search on the internet shows that this has been put forward many times:

Of note, Prof. Orr appears the first to have noted this connection. Maybe we will invite him here.

@CaseyLuskin responds by speculatively appealing to Axe: Behe’s Critics’ Scaffolding Falls Down | Evolution News


Dr. Swamidass, it’s difficult to engage an argument that isn’t able to present an actual example. Since you say the stone bridge is not an actual example, could you provide one?

I’ll get to Orr. But first, I’ll go to the article’s second argument.

Also @Bilbo, to some extent, I am happy to engage with you. However, these are really questions for Behe. He is the only one capable of clearing this up. If we show you how he has made a substantial error, can you fix it? Doubtful. Would you even make a public effort to get him to respond? I’m doubtful about this as well, though you could surprise me.

@bilbo, I’m only going to engage with you to a limited extent, as I see fit. This is far more than Behe is engaging. If you feel I am evasive, I urge you to direct that frustration towards insisting Behe engage. You are merely an anonymous person on the Internet, and three scientists have been raising questions here for both Axe and Behe. Any response you get from us, heightens the fact that Behe needs to respond to us. Please keep this in mind as we proceed.

Dr. Swamidass, until someone presents me with an actual example to illustrate the argument, I’ll just conclude that the argument is vacuous and not worth considering. I’ll get back to the discussion of the definition of IC in the proper thread. Not here.


As for specific example, I have already given 2 examples. There are several more in the literature that have been identified by others. However, this request is a red herring, which seeks to apply a higher standard to Behe’s critics than the IC1 argument itself has met.

Moreover, the case that Orr and Muller make against IC1 is of equivalent empirical value as Behe’s case for IC1. They are both logical arguments, that are made with words, not data. If you ignore Orr and Muller’s case, you must also ignore Behe’s case for IC1. Otherwise you are not being consistent. If you think this is vacuous, you must also conclude that Behe’s case for IC1 is vacuous. In no place does he demonstrate or show that IC1 systems cannot evolve. The only thing he does is define IC1, and show that biology includes IC1 systems (and everyone agrees with him that IC1 exists).

I’d suggest you also rapidly clarify this issue on another thread:

Bilbo, concepts cannot be refuted. It appears that you are trying to skip over hypothesis testing.

Dr. Swamidass, without a clear example, it is difficult for me to understand what Muller’s point is. I thought the stone bridge was meant to be such an example. You say that it is not. And now you say you have provided two examples to illustrate Muller’s argument. What exactly are those two examples?

I was going to move on to Orr’s argument, but you say that you really want to hear from Behe, not from me. Since Behe already had an extended exchange with Orr that can be found in his list of articles, you have heard from Behe. How do you think Behe’s response failed?

And by the way, the article you cited calls the stone bridge an example of the Mullerian two step process:

“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.”

So you may not think the stone bridge is an example. But the article you cite thinks it is.

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Let’s use lungs as an example. Terrestrial vertebrates (e.g. reptiles, mammals, birds) can’t live without them which makes them necessary. However, there are species that have both gills and lungs (e.g. lungfish). They are able to extract oxygen from water, but are also able to extract oxygen from air if they find themselves in brackish water that is depeleted in oxygen. In the case of these fish, lungs are a benefit but not a requirement. When fish evolved to be land creatures they lost their gills. This resulted in a situation where lungs are now necessary.

This is an example of a Mullerian two step. Add lungs, which are beneficial. Later, make them necessary.


Hi TA, in that example, Behe would say that lungs and gills are very complex multi-systems, and wouldn’t qualify as counterexamples to IC, which considers single systems.

Perhaps he would. But it would be a dodge.


The ameba-bacteria obligate symbiosis is an excellent example to work out that shows why this is a dodge. Also, @Biblo, have you read these two linked critiques yet?

These articles have it spot on. No mention of these or rebuttal of them from Behe that I have found. Have you?


I have a feeling that nothing you show Behe would be considered a counterexample. He would find some excuse as to why each example does not count.

Nonetheless, lungs are still an example of the Mullerian Two Step which is ultimately what you were asking for.


Yes, TA, thanks for the example.

No, it is not a dodge. Behe’s point is that before we can answer the question of whether Darwinian evolution can account for the evolution of things like lungs and gills, we first need to answer the question of whether it can account for the evolution of “simple” molecular machines. If it has problems accounting for those, why assume it can account for more complex systems.

Make up your mind, Dr. Swamidass. Do you want me to try to answer your challenges, or do you just want to hear from Behe?

I’m wondering if there could be ANY single system examples of IC. All living things we know of are multisystems. Bacterial flagella are beneficial, but not necessary. Blood clotting is beneficial for a creature IF it has already developed a circulatory system, but not before. Even the flagellum itself is a multisystem, and most of the component protein have multiple roles within the cell.

Nothing evolves in isolation. Restricting the definition of IC to single systems is sort of like asking why evolution has not produced a Crockoduck.

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I want to hear from Behe. On another thread if you’d like to make your own case from IC, not speaking for or defending him, that might have value. This means you would have to make the case with logic and evidence, answering criticize consistently, without appealing to Behe’s authority. It would not be about whether or not we understand Behe, but whether or not you are making a coherent case. Is that what you want to do?

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That’s just an excuse. The only reason Behe excludes macroscopic adaptations is because we have a fossil record for those changes.

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