Irreducible Complexity Revisited

Michael Behe’s concept of irreducible complexity, and in particular his use of this concept to critique Darwinism, continues to come under heavy fire from the biological community. The problem with Behe, so Darwinists inform us, is that he has created a problem where there is no problem. Far from constituting an obstacle to the Darwinian mechanism of random variation and natural selection, irreducible complexity is thus supposed to be eminently explainable by this same mechanism. But is it really? It’s been eight years since Behe introduced irreducible complexity in Darwin’s Black Box (a book that continues to sell 15,000 copies per year in English alone). I want in this essay to revisit Behe’s concept of irreducible complexity and indicate why the problem he has raised is, if anything, still more vexing for Darwinism than when he first raised it. The first four sections of this essay review and extend material that I’ve treated elsewhere. The last section contains some novel material. 1 The Definition of Irreducible Complexity Highly intricate molecular machines play an integral part in the life of the cell and are increasingly attracting the attention of the biological community. For instance, in February 1998 the premier biology journal Cell devoted a special issue to “macromolecular machines.” All cells use complex molecular machines to process information, convert energy, metabolize nutrients, build proteins, and transport materials across membranes. Bruce Alberts, president of the National Academy of Sciences, introduced this issue with an article titled “The Cell as a Collection of Protein Machines.” In it he remarked, We have always underestimated cells.... The entire cell can be viewed as a factory that contains an elaborate network of interlocking assembly lines, each of which is composed of a set of large protein machines.... Why do we call the large protein assemblies that underlie cell function protein machines? Precisely because, like machines invented by humans to deal efficiently with the macroscopic world, these protein assemblies contain highly coordinated moving parts. 2 • IRREDUCIBLE COMPLEXITY REVISITED Almost six years later (December 2003), BioEssays published its own special issue on “molecular machines.” In the introductory essay to that issue, Adam Wilkins, the editor of BioEssays, remarked, The articles included in this issue demonstrate some striking parallels between artifactual and biological/molecular machines. In the first place, molecular machines, like man-made machines, perform highly specific functions. Second, the macromolecular machine complexes feature multiple parts that interact in distinct and precise ways, with defined inputs and outputs. Third, many of these machines have parts that can be used in other molecular machines (at least, with slight modification), comparable to the interchangeable parts of artificial machines. Finally, and not least, they have the cardinal attribute of machines: they all convert energy into some form of ‘work’. Alberts and Wilkins here draw attention to the strong resemblance between molecular machines and machines designed by human engineers. Nevertheless, as neo-Darwinists, they regard the cell’s marvelous complexity as products of Darwinian evolution and thus as only apparently designed. In the 1990s, however, scientists began to challenge the neo-Darwinian view and argue that such protein machines could only have arisen by means of actual design. For example, in 1996 Lehigh University biochemist Michael Behe published a book titled Darwin’s Black Box. In that book he detailed the failure of neo-Darwinian theory to explain the origin of complex molecular machines in the cell. But he didn’t stop there. He also argued that these molecular machines exhibit actual design. Central to his argument was the idea of irreducible complexity. A functional system is irreducibly complex if it contains a multipart subsystem (i.e., a set of two or more interrelated parts) that cannot be simplified without destroying the system’s basic function. I refer to this multipart subsystem as the system’s irreducible core. This definition is more subtle than it might first appear, so let’s consider it closely. Irreducibly complex systems belong to the broader class of functionally integrated systems. A functionally integrated system consists of parts that are tightly adapted to each other and thus render the system’s function highly sensitive to isolated changes of those parts. For an integrated system, a change in one place often shuts down the system entirely or else requires multiple changes elsewhere for the system to continue to function. We can therefore define the core of a functionally integrated system as those parts that are indispensable to the system’s basic function: WILLIAM A. DEMBSKI • 3 remove parts of the core, and you can’t recover the system’s basic function from the other remaining parts. To say that a core is irreducible is then to say that no other systems with substantially simpler cores can perform the system’s basic function.