Planet Earth

The Blind Locksmith Continued: The Mushy Definition of Complexity

The LoomBy Carl ZimmerApr 7, 2006 12:39 PM


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Yesterday I blogged about a new study in which scientists reconstructed 450 million year old proteins in order to trace the evolution of some receptors for hormones. The paper itself does not comment on the implications these results have for intelligent design, which claims that some biological systems are too complex to have evolved. But in the accompanying commentary, Chris Adami does. (Adami is the brains behind Avida, an artificial life program that I wrote about in Discover in 2005.) He writes,

Although these authors have not directly addressed this controversy in the discussion of their work--because the work itself is intrinsically interesting to biologists--such studies solidly refute all parts of the intelligent design argument. Those "alternate" ideas, unlike the hypotheses investigated in these papers, remain thoroughly untested. Consequently, whatever debate remains must be characterized as purely political.

In a press release from the University of Oregon, Joe Thornton, the lead author, also raised the connection.

The stepwise process we were able to reconstruct is entirely consistent with Darwinian evolution," Thornton said. "So-called irreducible complexity was just a reflection of a limited ability to see how evolution works. By reaching back to the ancestral forms of genes, we were able to show just how this crucial hormone-receptor pair evolved.

The intelligent design advocates responded quickly with a statement. Michael Behe, author of Darwin's Black Box, says:

"The authors (including Christoph Adami in his Science commentary) are conveniently defining "irreducible complexity" way, way down. I certainly would not classify their system as anywhere near irreducibly complex (IC). The IC systems I discussed in Darwin's Black Box contain multiple, active protein factors. Their "system", on the other hand, consists of just a single protein and its ligand. Although in nature the receptor and ligand are part of a larger system that does have a biological function, the piece of that larger system they pick out does not do anything by itself. In other words, the isolated components they work on are not irreducibly complex."

In an article in the New York Times, Behe is reported to have commented that

a two-component hormone-receptor pair was too simple to be considered irreducibly complex. He said such a system would require at least three pieces and perform some specific function to fit his notion of irreducibly complex.

Time for a fact check! I looked at the index of Darwin's Black Box for "Irreducibly complexity, defined." It directed me to page 39, where I found the following passage:

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.

Hm. Behe's own definition does not refer to active protein factors, just parts. So that's not a valid objection. It does not specify "at least three" pieces must be involved. While he uses the word "several," he offers no explanation for why a two-part system is not complex. It certainly does meet his specification that removing one part makes the system non-functional. What's more, a hormone and its receptor certainly do carry out a function: they relay a signal from outside a cell to inside a cell. That signal is indeed carried further into the cell by other proteins. But Behe does not explain why that fact means that the two molecules studied by Thornton don't have a function. And in fact, as was revealed in the recent Dover intelligent design trial, Behe himself is happy to look at a part of a system. In the trial, the plaintiff's lawyer explored Behe's claim that blood-clotting is a complex system. Behe ignored part of the blood-clotting response in his argument. It just so happens that some proteins in the part he ignored are missing from some species, but they can still form blood clots. That raises doubts about whether the system is irreducibly complex. So, to recap: this system appears to match Behe's own original definition of irreducible complexity, while Behe's comments yesterday do not. And Thornton and his colleagues have presented evidence about how this irreducibly complex system could have evolved. The intelligent design statement promises more comments from others today. We'll have to see how well they hold up to a fact check as well. Update 4/10: Not so well, it turns out.

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