Two principal arguments are made for intelligent design: one based on complexity, the other based on information. These arguments for intelligent design may be seen in the work of two key proponents of the theory, Michael Behe and William Dembski. Behe, a practicing Catholic and microbiologist, was for a long time a Darwinist who saw no theological or scientific problems with the theory of the common descent of living beings by a process of random change and natural selection. That changed abruptly when he read the geneticist Michael Denton’s Evolution: A Theory in Crisis. He describes the experience as world-changing: “When I laid the book down, I lived in a different world.”1 He began reading with a skeptical eye the claims for evolution in the scientific literature, and volunteered to lead a seminar titled “Popular Arguments on Evolution,” in which he and his students read and discussed pro- and anti-evolution books and articles, particularly Denton’s Evolution: A Theory in Crisis and Richard Dawkin’s The Blind Watchmaker. The next stage in his engagement with the theory of evolution came when he read the lawyer Phillip Johnson’s book Darwin on Trial, which argued that if one did not assume materialism was true, then the evidence for random mutation and natural selection as the explanation of life on earth is very small indeed.2 In the following months Behe became involved in debates on evolution with Phillip Johnson, and worked out the arguments that later became the basis for his own book Darwin’s Black Box,3 arguments that he believed made a unique contribution from the perspective of biochemistry.
The key concept in Behe’s argument is irreducible complexity. The argument begins with two premises: (1) certain parts of organisms, such as the mechanism for blood clotting, or the bacterial flagellum, are complex, that is, they are composed of many different parts; (2) all of these parts are necessary in order to achieve the function; in other words, the mechanism is irreducibly complex; the structure cannot be simpler and have the same function. The second step of the argument is that such an irreducibly complex mechanism cannot be built up gradually, by a process of natural selection. If the mechanism producing the function is irreducibly complex, then intermediate structures would have no function, and thus no value for the organism; they would not contribute to its living or reproducing, and so would not be promoted by natural selection.
The concept of irreducible complexity as an objection to evolution by natural selection is not really new. Darwin himself recognized it: “If it could be demonstrated that any complex organ existed which could not possibly have been formed by numerous, successive, slight modifications, my theory would absolutely break down.”4 Behe’s contribution was the application of this idea at the molecular level rather than at the level of large organs such as the eye. Complex bio-chemical processes are supposed to exemplify exactly such a complexity as Darwin spoke of. Behe proclaimed that design is clearly evident at the cellular level, and that this discovery “must be ranked as one of the greatest achievements in the history of science,” rivaling those of Newton, Einstein, Lavoisier, Schrödinger, Pasteur, and Darwin.5
While the general principle is sound, its application is weak. A “molecular machine” that requires each of its parts in order to perform its function could have been built up from parts with different functions. Indeed, this is just what the theory of evolution would predict! As the ancestors of horses were not simply “imperfect horses,” but were something other than horses, so one could expect the precursor of many biological systems to be not merely imperfect systems of the same type, but systems functioning somewhat differently.
More generally, the argument that some biological system could not have been formed gradually is an argument based on ignorance: we don’t know how, or at least don’t know exactly how such-and-such a function evolved; therefore, it couldn’t have evolved gradually. This argument is weak, unless we suppose that we know biochemistry so well that if there were a gradual way for the function to evolve, we would know it. Since our knowledge of biochemistry remains quite imperfect regarding many detailed points, the fact that we do not know in detail how gradual evolution of various functional systems could have happened is a weak argument that it is impossible. But in fact, possible paths of evolution have been sketched out for the very things, such as blood clotting, that Behe claims are irreducibly complex!6
1Michael J. Behe, “From Muttering to Mayhem: How Phillip Johnson Got Me Moving” in Darwin’s Nemesis, ed. William A. Dembski, 42.
3Behe, Darwin’s Black Box: The Biochemical Challenge to Evolution (New York: The Free Press, 1996).
4Charles Darwin, On the Origin of Species, ch. 6.
5Darwin’s Black Box, 232–233.
6See, for example, Kenneth Miller, Finding Darwin’s God: A Scientist’s Search for Common Ground Between God and Evolution (New York: Harper Perennial, 2000), 134–158.