The first thing you need to understand about Behe's argument is that it's just plain wrong. It's not that he botched some stray fact about evolution, or that he doesn't know his biochemistry, but that his argument—as an argument—is fatally flawed. To see this we need to first get clear about what kinds of solutions to irreducible complexity are not open to Darwinism.

First it will do no good to suggest that all the required parts of some biochemical pathway popped up simultaneously by mutation. Although this "solution" yields a functioning system in one fell swoop, it's so hopelessly unlikely that no Darwinian takes it seriously. As Behe rightly says, we gain nothing by replacing a problem with a miracle. Second, we might think that some of the parts of an irreducibly complex system evolved step by step for some other purpose and were then recruited wholesale to a new function. But this is also unlikely. You may as well hope that half your car's transmission will suddenly help out in the airbag department. Such things might happen very, very rarely, but they surely do not offer a general solution to irreducible complexity.

Behe's colossal mistake is that, in rejecting these possibilities, he concludes that no Darwinian solution remains. But one does. It is this: 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.

The point is there's no guarantee that improvements will remain mere improvements. Indeed because later changes build on previous ones, there's every reason to think that earlier refinements might become necessary. The transformation of air bladders into lungs that allowed animals to breathe atmospheric oxygen was initially just advantageous: such beasts could explore open niches—like dry land—that were unavailable to their lung-less peers. But as evolution built on this adaptation (modifying limbs for walking, for instance), we grew thoroughly terrestrial and lungs, consequently, are no longer luxuries—they are essential. The punch line is, I think, obvious: although this process is thoroughly Darwinian, we are often left with a system that is irreducibly complex. I'm afraid there's no room for compromise here: Behe's key claim that all the components of an irreducibly complex system "have to be there from the beginning" is dead wrong.

It's worth noting that our scenario is neither hypothetical nor confined to the often irretrievable world of biological history. Indeed it's a common experience among computer programmers. Anyone who programs knows how easy it is to write yourself into a corner: a change one makes because it improves efficiency may become, after further changes, indispensable. Improvements might be made one line of code at a time and, at all stages, the program does its job. But, by the end, all the lines may be required. This programming analogy captures another important point: If I were to hand you the final program, it's entirely possible that you would not be able to reconstruct its history—that this line was added last and that, in a previous version, some other line sat between these two. Indeed, because the very act of revising a program has a way of wiping out clues to its history, it may be impossible to reconstruct the path taken. Similarly, we have no guarantee that we can reconstruct the history of a biochemical pathway. But even if we can't, its irreducible complexity cannot count against its gradual evolution any more than the irreducible complexity of a program does—which is to say, not at all.

I wish I could claim credit for this Darwinian model of irreducible complexity, but I'm afraid I've been scooped by eighty years. This scenario was first hinted at by the geneticist H. J. Muller in 1918 and worked out in some detail in 1939.⁶ Indeed, Muller gives reasons for thinking that genes which at first improved function will routinely become essential parts of a pathway. So the gradual evolution of irreducibly complex systems is not only possible, it's expected. For those who aren't biologists, let me assure you that I haven't dug up the half-baked lucubrations of some obscure amateur. Muller, awarded the Nobel Prize in 1946, was a giant in evolution and genetics.

Although Muller's essay isn't as well known as it should be, the gist of his idea is common wisdom in evolutionary biology. Here's an important application: Molecular evolutionists have shown that some genes are duplications of others. In other words, at some point in time an extra copy of a gene got made. The copy wasn't essential—the organism obviously got along fine without it. But through time this copy changed, picking up a new, and often related, function. After further evolution, this duplicate gene will have become essential. (We're loaded with duplicate genes that are required: myoglobin, for instance, which carries oxygen in muscles, is related to hemoglobin, which carries oxygen in blood. Both are now necessary.) The story of gene duplication—which can be found in every evolution text—is just a special case of Muller's theory. But it's an immensely important case: it explains how new genes arise and, thus, ultimately, how biochemical pathways get built.

So how does Behe explain duplicate genes? He doesn't. He reluctantly admits that different genes often have similar sequences. He even admits that some genes in his favorite pathway—blood clotting—are similar.⁷ But he refuses to draw the obvious conclusion: some genes are copies of others. Does Behe think their similarity is a coincidence—they just happen to look alike? It is, I think, clear why Behe fails to face up to duplicate genes: were he to admit that one gene is a copy of another, he'd have to admit that a copy was made at some point in time and thus that the organism once got along without it. But this implies that such systems can arise step by step. Behe avoids this conclusion only by sheer evasion: he brands gene duplication a "hypothesis," leaves the similarity of his favorite genes unexplained, and quickly moves on to safer turf.

Irreducible Confusion

In truth, we're done. Behe's chief objection to Darwinism is flat wrong, and, bereft of this, he's got little to say. But when you do look at what else he says, you find a bizarre string of confusions and contradictions.

For instance, while Behe claims that evidence for design had to await the new science of biochemistry, he never really explains what's so special about biochemistry. It's true that molecules provide some nice examples of irreducible complexity, but why can't we find such complexity at other levels? The answer is we can. Here's one: the heart. The human heart is built of a pump and valves. Remove either one and you're dead. But Behe seems terribly unclear about whether such non-molecular examples are kosher. In one breath, he tells us that "one has to examine molecular systems for evidence of design," but in the next, he assures us that the theologian William Paley's description of the heart as irreducibly complex was right on the money. So which is it? If Paley's example is "exactly correct," why did we have to await biochemistry? The issue is not trivial. For if anatomy didn't topple Darwinism (and it seems not to have), why should biochemistry?

Behe's one attempt to explain what's so special about molecules only hurls us into deeper confusion. He suggests that biochemical examples are best because they're simpler and thus clearer. But I, for one, have a hard time reconciling this argument with Behe's main claim—that biochemistry is very, very complicated. I suspect the real reason Behe finds biochemistry so special is that he has confused two senses of "reducible." Irreducible complexity is a formal property of a system, having nothing to do with physical scale. You might say we can't "reduce" the function of the system to its parts if they're all required. But if we like, we can always "reduce" such a system into its molecular bits and pieces (the heart is made of myosin, etc.). When Behe worries that an anatomical structure is made of so many different molecules that it's hard to know if it's irreducibly complex, he is mixing up these two senses of reducible. There is absolutely no reason to think we get truer irreducible complexity at the micro than macro scale. This is made perfectly clear by Behe's own example: to see that a mousetrap is irreducibly complex, we don't have to work out its chemistry! It remains irreducibly complex whether made of one kind of molecule or one million. The upshot is that Behe's grand claim that biochemistry poses some qualitatively new challenge to Darwinism just doesn't make sense. Irreducible complexity lives at all scales.

Behe offers up yet another contradiction when he tells us that he finds the descent of all species from a common ancestor "fairly convincing" but that he's not so sure about macroevolution.⁸ Now macroevolution is the process of getting species from a common ancestor. You can no more believe in one but not the other than you can believe in beer but not brewing. The strange thing is that Behe seems to understand the meaning of both words. He says sensible things about common descent and then about macroevolution. He just doesn't see that the two sets of statements are flatly contradictory.

Last, in one of the stranger passages of his book, Behe speculates that the designer provided the Primal Cell with all the genes modern organisms might need (i.e., the first bacterium carried genes for human speech centers). If a lineage didn't need some genes, they got lost or silenced. This notion leaves so much of molecular evolution unexplained that it's hard to know where to start. Here's just one problem: Although some genes do get killed or silenced over time (producing non-functional "pseudogenes"), how come we only carry pseudogenes that are wrecked copies of our real genes? In other words, why don't I carry pseudogenes for chlorophyll or flower structure? Why don't azaleas carry pseudogenes for brain cells? Behe's it-was-all-there-from-day-one hypothesis is flatly falsified by this and every other known pattern in molecular evolution.

I'll be the first to admit that such non sequiturs are not fatal to Behe's central argument. But they do betray remarkable confusion or, worse, a powerful tendency to see what he wants, contradictions be damned. In any case, strings of such contradictions eat away at Behe's case, and, in the end, make it hard to believe that Darwin will be getting company in Westminster Abbey any time soon.

Know Thy Enemy

One of the most interesting questions about Behe's book is why he feels especially qualified to critique Darwinism. (And not just to quibble over details, but to announce that "Darwinism is not science," as he did in a recent letter to Commentary.)⁹ To a historian or electrician, Behe certainly looks qualified. He is a biologist. But it's not that simple, as can be seen by turning the tables for a moment. If I, an evolutionary biologist, were to announce that biochemistry is deeply flawed—I've shown, for instance, that enzymes are not catalysts—I doubt I'd get a listen. I surely wouldn't get a publisher. Nor would any jurist consider my ruminations worthy of attention. But Behe stars in public debates, has a fancy publisher (Free Press, a division of Simon & Schuster) and the ear of the likes of Judge Bork. Why the difference? Why is everyone an expert witness when the topic is Darwinism but not when it's biochemistry?

The answer is complicated, but a few things are clear. First, Darwinism matters. Many people will inevitably have questions about Darwinism because many people will inevitably think about it. By comparison, I doubt many Sunday school classes get worked up over enzyme kinetics. Second—and this has more to do with attacks from scientists such as Behe's—there's a striking asymmetry in molecular versus evolutionary education in American universities. Although many science, and all biology, students are required to endure molecular courses, evolution—even introductory evolution—is often an elective. The reason is simple: biochemistry and cell biology get Junior into med school, evolution doesn't. Consequently, many professional scientists know surprisingly little about evolution.

Now I don't pretend to know the details of Behe's education, but I do know this: he is not at home in the technical evolution literature. His book reveals that his grasp of evolution derives mostly from the pop literature (Gould, Dawkins—good stuff, but no stand-in for the real thing) and from computer searches of the scientific literature that he strangely makes a big deal of. While I have utter confidence in Behe's biochemistry, I am less confident that he can say what soft selection, or Muller's ratchet, or the Fundamental Theorem of Natural Selection is—all bread and butter of evolutionary biology. It would be easy, of course, to get carried away here, and I want to emphasize that I'm not saying that outsiders can offer nothing of value (it's worth remembering that Darwin himself was trained primarily as a geologist, not a biologist). I'm simply saying that any would-be critic of Darwinism should know as much about evolution as any critic of biochemistry must know about molecules. (An idea that apparently never occurred to Free Press, who presumably will next treat us to a botanist's musings on the flat earth.)

Finally, Behe and others may feel obliged to sling mud Darwin's way because they suspect evolutionary biologists won't do so. Evolutionists are widely perceived as uncritical ideologues, devoted to suppressing all doubt about evolution. It's easy to see how this impression arose: evolutionists, after all, spend most of their public lives defending Darwin against endlessly recycled creationist arguments. So of course we appear hide-bound reactionaries. (So would physicists if the theory of gravity were dragged into court every other year.) But the truth is, I think, quite different. It would be fatuous to deny that scientists can be intellectually conservative or prone to hero worship. And it would be equally absurd to suggest that evolutionists have resolved every major problem facing us; many remain. But the fact is that, as in any science, evolutionists often sharply disagree. And, as in any science, these disagreements sometimes concern fundamentals. In the 1930s, for example, Sewall Wright championed the role of "genetic drift" in evolution. Parting with accepted wisdom, he argued that random changes in the genetic composition of populations—not natural selection—account for many of the differences we see between species. More recently, Motoo Kimura championed the neutral theory, arguing that a good deal of evolution at the molecular level does not reflect natural selection. Here were overt attempts to circumscribe the role of selection. And the attempts were largely successful: Wright and Kimura were not hooted down, gagged or shot. Instead genetic drift and the neutral theory are now enshrined in every evolution text.

So when the Christian Right tries to tell you that evolutionists instinctively circle the wagons whenever anyone dares question the Darwinian status quo, you should ask yourself why Wright and Kimura got through, but Behe not. The answer is, I think, straightforward: Wright and Kimura knew what they were talking about. <

1 Notebooks, 1914–16, ed. G. H. von Wright and G. E. M. Anscombe (Oxford: Blackwell, 1961), p. 47.

2 Robert H. Bork, Slouching Towards Gomorrah: Modern Liberalism and American Decline (New York: Regan Books, 1996), p. 294.

3 Also see Behe's New York Times op-ed (October 26,1996, p. A25), where he speaks more plainly about his religion.

4 New York Times, October 25, 1996, p. A1.

5 Behe waffles a bit here. He typically claims that irreducibly complex systems "cannot" by natural selection as all the parts have to be there from the beginning. But, once or twice, he contradicts these strong claims, asserting that no Darwinian explanation seems possible. The same waffling shows up in his New York Times op-ed, where he boldly states that we know of "no other mechanism [besides design], including Darwin's, which produces such complexity," and that "such a systemprobably cannot be put together in a Darwinian manner" (my italics). For present purposes, the distinction doesn't matter much: we'll see that it's neither impossible nor difficult to gradually evolve irreducible complexity.

6 H. J. Muller, "Reversibility in Evolution Considered from the Standpoint of Genetics," Biological Reviews 14 (1939): 261–80. Muller does not, of course, use Behe's term "irreducible complexity." Rather he speaks in terms of irreversibility: you can add something extra because it's merely advantageous. But, once it becomes essential, you can't remove it. Irreducible complexity means that evolution is not reversible.

7 The situation is slightly more complex than this applies. Sometimes just parts of genes get duplicated. But the point remains: parts of some genes in the blood-clotting pathway are copies of parts of other genes. See W.-H. Li and D. Graur, Fundamentals of Molecular Evolution (Sunderland, MA: Sinauer, 1991) for a discussion of partial v. complete gene duplicaion and the evolution of new gene function.

8 See J. A. Coyne, "God in the Details," Nature 383 (1996): 227–28. Coyne also emphasizes that Behe's theory is unfalsifiable: Since Behe admits both evolution and design, proof that a pathway was built gradually can't stump him. He can always claim that some other pathway was designed.

9 Commentary (September 1996), p. 22.