American Scientist

Darwin and Design: Does Evolution Have a Purpose? Michael Ruse. x + 371 pp. Harvard University Press, 2003. $29.95.

Eyes are for seeing; leaves are for capturing sunlight; the plates on the back of Stegosaurus may have been for radiating heat. For at least 2,000 years, from Plato until Darwin, these contrivances of organisms were considered strong evidence for the existence of a higher power. They are functional, seeming to exist because they serve a purpose, like human artifacts, especially machines. Before Darwin, the only way imaginable for a purposeful device to arise was by design—that is, for it to be produced intentionally, by a mind like ours. And thus it seemed reasonable to conclude that such functional features of organisms must be the product of a divine designing mind. In 1802, this argument was proffered compellingly by one of the foremost exponents of natural theology, William Paley. He argued that if one stumbles on a stone in a field, one can say very little about its origins; but if one finds a watch, it is immediately clear that "its several parts are framed and put together for a purpose." Paley went on to argue that a purposeful watch requires a watchmaker, and that the equivalent functional complexity of organisms likewise requires a higher power, a designing mind.

A half-century later Charles Darwin demolished this 2,000-year-old argument, showing that an unthinking process, natural selection, could produce function and therefore the appearance of design. In the Darwinian view, of course, function arises iteratively, beginning with chance modification, followed by preservation of those modifications that promote survival and reproduction, further chance modification, and so on. Thus, in effect, the functional structures of an organism exist because of their past contributions to reproductive success. No intentionality, no designing mind, is required.

Michael Ruse's latest book, Darwin and Design: Does Evolution Have a Purpose?, is an intellectual history of the design argument and its Darwinian solution. Ruse is one of the leading philosopher-historians of biology today, and his story is a fascinating one, enlivened especially by his accounts of various imaginative attempts before Darwin to solve the design problem without recourse to a deity. Aristotle accepted purposefulness in nature (his "final causes") but placed it in the organization of the organisms themselves rather than in an external entity. David Hume pointed out that intelligent design cannot be inferred even from our own technology, much of which is the product of a long history of bumbling trial and error. And Immanuel Kant saw the necessity of a notion of final cause in biology but understood it as an intellectual device for making scientific sense out of experience and not necessarily as a real feature of the world.

Equally fascinating are some of the theological positions taken since Darwin. For example, Ruse argues that natural theology, deprived by Darwinism of God-as-designer-of-organisms, substituted God-as-designer-of-evolution, the cause and guarantor of the evolution of humans or of evolutionary progress generally.

The heart of Ruse's analysis is a new parsing of the design question. Historically, the entire chain of reasoning, from the appearance of design in nature to the existence of a divine designer, has been called the "argument from design." Ruse breaks the chain up into smaller pieces. First, there is the recognition of "adaptive complexity," or just "complexity" for short, as a phenomenon to be explained somehow. The trilobite eye, for example, is too complex and exquisitely engineered to have arisen by chance, and so its existence demands an explanation. Ruse calls this the "argument to complexity." Second, there is the independent claim that adaptive complexity implies design, which Ruse calls the "argument to design." Paley accepted both the argument to complexity and the argument to design. Darwinians accept the argument to complexity. For them, the adaptive complexity of the trilobite eye does require an explanation. But they reject the argument to design, because they have natural selection as a ready alternative. One virtue of this parsing is that it reveals a third position, in which Ruse places the entire lineage of evolutionary formalists, from the transcendental morphologists to modern structuralists, from Goethe and Geoffroy St. Hillaire to Stephen Jay Gould and Brian Goodwin. The formalists accept that adaptation occurs and recognize that it requires explanation, but for them it is not the central problem of biology. In effect, they deny the argument to complexity. Or at least they are not much interested in it, and so of course they also take very little interest in the argument to design.

Ruse's parsing suggests a refinement, one that he doesn't make and might not endorse. The argument to complexity—which for Ruse, you'll recall, is a shorthand for adaptive complexity—can be further broken down into what might be called an "argument to adaptation" and a true or pure "argument to complexity." What this does is cleanly separate questions about adaptation and complexity. Both still require explanations, but those explanations can differ. Clearly, the trilobite eye is an adaptation, and natural selection explains it. What about the eye's complexity?

One possibility is that eyes of this sort need to be complex to function properly, and in that case the complexity question gets the same answer as the adaptation question: natural selection. But another possibility is that complexity is governed by evolutionary forces operating, at least in part, independently of natural selection. For example, complexity could be the result of an internal randomizing tendency—a tendency for the parts of a system to multiply and differentiate, with the passive accumulation of accidents (an idea with roots in the work of Herbert Spencer). In other words, perhaps the variational properties of trilobites were such that all or most of the options available to selection were complex—that is, all or most contained many differentiated parts (and perhaps an increasing number over time). Then, given variational options that are mainly or only complex, suppose that natural selection found a way to make one of them work. In that case, we might say that selection produced adaptation but was stuck with complexity, so to speak.

I am not claiming that this is in fact what happened in the evolution of trilobite eyes, or any other particular complex adaptive structure for that matter. But this further parsing does point to a study of complexity independent of adaptation, thereby opening up a new range of explanatory possibilities. My hunch is that the payoff will be an explanatory scheme that makes adaptive complexity seem even less improbable and our naturalistic explanations even more satisfying.

Inevitably Ruse must confront the most recent version of the intelligent design argument, championed by Michael Behe and William Dembski. And he does this in a final chapter appropriately titled "Turning Back the Clock." He rebuts them, perhaps too briefly, showing that natural selection is fully competent to produce adaptive complexity (using a heuristic device borrowed from Richard Dawkins), and showing how adaptive complexity arises spontaneously in artificial-life experiments (drawing on Thomas Ray's Tierra world).

Then in closing, Ruse takes an interesting turn, implying that the whole design argument is rather old and quaint, not to be taken seriously nowadays. What he considers not to be old and quaint, though—what is still vital and serious—is the sense of awe that organismal design inspires, the feeling of wonder that launched the whole debate long ago. The passage is worth quoting:

Behe, Dembski, and their nemesis, Dawkins, share a desire to return to the high Victorian era, when Britain ruled the waves and science and religion could never agree. . . . But the world has moved on. The old disjunction is gone. Evolution has been proven true, and is widely accepted as such. God did not intervene miraculously to make each species separately. Christians can and do accept this fact. Yet, in another sense, I come to praise these old-fashioned people and to argue that they have grasped something from the past that too many who write on the science/religion relationship have lost or never even seen. This is the argument to organized, adaptive complexity.

What is the argument to organized, adaptive complexity really, at bottom? As Ruse closes, he suggests that it may best be thought of not as argument at all, but rather as a special kind of feeling, which he describes as "an uncommon delight in the intricate workings of the organic world," a feeling of being "overwhelmed by the glory of creation."—Daniel W. McShea, Biology, Duke University