Darwinian evolution is widely advertised to be a fact, as firmly established as the shape of the Earth. Defenders of the theory insist that there is no scientific controversy over it, and people who question or criticize it are typically accused of being ignorant or religiously motivated. Yet every few years a book comes along—written by scientists—claiming to remedy some major flaw in evolutionary theory.

The Plausibility of Life, by Marc W. Kirschner and John C. Gerhart, is such a book. Gerhart is an eminent cell and developmental biologist, now retired after a distinguished career at the University of California at Berkeley; Kirschner—once Gerhart's graduate student (as was I)—is an equally eminent biologist, now a department chair at Harvard University. According to the book's jacket, the authors close "a major gap in Darwin's theory" and thereby provide "a timely scientific rebuttal to critics of evolution who champion 'intelligent design.'"

The "major gap" in Darwin's theory is the origin of novelty: new variations, physiologies, anatomies, and behaviors. According to Kirschner and Gerhart, Darwin's theory rests on three pillars: natural selection, heredity, and variation. Darwin provided the first, and Mendelian and molecular genetics provided the second, but until now "a major weakness remained, casting all else in doubt." That weakness has been the origin of new variations: "Ignorance about novelty is at the heart of skepticism about evolution, and resolving its origins is necessary to complete our understanding of Darwin's theory." Indeed, the "cardinal issue in evolution is the origin of complex and heritable variations," and the authors maintain that until now no one has dealt adequately with this issue. "The plausibility of life," they write, "rests on the plausibility of generating novelty, and that in turn rests on mechanisms newly uncovered in biology."

Such variations must arise in the embryo, so Kirschner and Gerhart write that "understanding the organization, growth and development of the organism is essential to complete Darwin's theory." Although Darwinists largely ignored embryology for a hundred years, many biologists in the late 20th century realized that we need to understand embryo development before we can hope to understand evolution. This realization led to the field of "evolutionary developmental biology," or "evo-devo." The Plausibility of Life is a contribution to this field.

To overcome the weakness in existing evolutionary theory, Kirschner and Gerhart propose what they call "a major new scientific theory: facilitated variation." The authors note that organisms consist of "core processes" that have apparently been "conserved" since their evolution from a common ancestor. The most basic of these are found in all living organisms from bacteria to humans; for example, dna replication, protein synthesis, and metabolic pathways. Kirschner and Gerhart acknowledge that they have no explanation for the origin of these in the first living cell. "Evidence is completely lacking about what preceded this early cellular ancestor," they write. "Everything about evolution before the bacteria-like life forms is sheer conjecture."

There are other major transitions in the history of life that Kirschner and Gerhart also concede remain unexplained. One of these was the "invention" of the first eukaryotes, cells with nuclei that are very different from bacteria. "Generating the first eukaryotic cell was a major and enduring accomplishment," they write. "Extensive innovation showed up in the complexity and organization of the eukaryotic ancestor." Another major transition was the origin of multicellular organisms, which require complex mechanisms for cells to aggregate and communicate with each other. Still another unexplained transition was the origin of animal body plans in the Cambrian explosion. "Once again," write Kirschner and Gerhart, "a new suite of cellular and multicellular functions emerged rather quickly and was conserved to the present."

Some other major transitions that Kirschner and Gerhart concede remain unexplained are the origin of limbs in the first land vertebrates, the origin of neural crest cells that sculpt the heads and nervous systems of vertebrates, and the origin of the neocortex in vertebrate brains. "The origin of these processes and properties would seem to be the primary events of evolution, requiring high novelty," but the authors admit they cannot explain them. So, what does their theory explain?

Their theory purports to explain how organisms that share a given set of conserved core processes—say, land vertebrates—can diversify into a wide variety of forms. Kirschner and Gerhart argue that although the core processes themselves are conserved over evolutionary time, and invariant ("constrained") in the process of embryo development, they are interconnected by "exploratory behaviors" and "weak linkages" that are "compartmentalized" in the embryo. These three characteristics "deconstrain" development and facilitate the emergence of new variations. This "facilitated variation," they argue, makes it "easy" for organisms to evolve.

For example, when a limb develops in the embryo of a land vertebrate, nerve cells (core processes) in the spinal cord send out extensions in many directions (exploratory behavior), but only those that encounter signals from targets (more core processes) in the developing limb survive and establish functioning connections (weak linkages). If an extra embryonic limb is artificially grafted onto the side of a chick embryo, the added targets cause the stabilization and connection of more nerve extensions, and the chick develops five limbs instead of four. The core processes are the nerve cells and the targets; the exploratory behavior is the sending out of extensions; the weak linkages are the signals; and compartmentalization enables forelimbs and hindlimbs to develop differently.

Kirschner and Gerhart propose that slight changes in the regulation of these mechanisms would make it "easy" to rearrange core processes in the course of evolution. "Throughout biology," they write, "individual core processes are constructed so that new linkages can easily be forged and broken… . The implications for evolution are powerful, for if complex development is elicited by simple signals, then changes in complex development may be achieved by changing the amount or location of these simple signals, rather than by changing a highly integrated and complex process."

The authors compare exploratory behavior in an embryo to random variation in a species, and they compare stabilization by target signals to natural selection. But variation and selection, whether in the laboratory, on the farm, or in the wild, have never been observed to produce a new species. Variation and selection produce only minor changes within existing species.

Similarly, exploratory behavior and weak linkage have never been observed to change the species of an embryo. Any embryo that completes its development becomes the organism it was programmed to become. Exploratory behavior and weak linkage notwithstanding, the endpoint of embryo development appears to be predetermined—by some factor or factors missing from Kirschner and Gerhart's equation.

So the analogy between variation and exploratory behavior, between selection and weak linkage, may be more revealing than the authors realize. Like variation and selection, exploratory behavior and weak linkage have never been observed to produce a new species, much less species with new kinds of limbs.

Where, then, is the evidence that "facilitated variations" can produce heritable changes that lead to evolutionary novelty? Kirschner and Gerhart write:

At some point, such heritable regulatory changes will be created in a test animal in the laboratory, generating a trait intentionally drawing on various conserved processes. At that point, doubters would have to admit that if humans can generate [such] variation in the laboratory in a manner consistent with known evolutionary changes, perhaps it is plausible that facilitated variation has generated change in nature. Such experiments are just now becoming feasible.

The authors describe only one such experiment. In 2004, biologists at Harvard and Princeton reported a correlation between beak differences in Galápagos finches and levels of a particular protein in their embryos. Birds with deeper beaks had more of the protein. The same biologists then altered the amount of that protein in chicken embryos and found corresponding changes in beak shape. Yet the researchers presented no evidence that the changes were heritable, and they did not produce a breed of chickens with modified beaks, much less anything approaching an evolutionary novelty.

In the 1970s, two of the researchers (Peter and Rosemary Grant) had conducted a field study of finches on the Galápagos Islands. When a severe drought killed most of the finches in 1977, the survivors had, on average, slightly deeper beaks that enabled them to crack the tough surviving seeds. The Grants had witnessed natural selection in action. Nevertheless, when the rains returned the average beak size went back to normal. No net evolution occurred.

In the 2004 laboratory experiment, the Grants and their colleagues merely identified a protein that may have been involved in producing differences in beak sizes in the finches. Like the field study, the laboratory study found no real evolution—not even within an existing species.

Nevertheless, Kirschner and Gerhart are optimistic: "We can expect that the increased study of evolution and development, coupled with genomic analysis, will provide more and more examples of the use of facilitated variation in specific evolutionary events." With this iou in hand, the authors proclaim that "the apparent weaknesses surrounding novelty… have now been corrected." This "should help open-minded people at all levels of scientific sophistication to realize that although many scientific questions remain in evolution and embryology, there are no apparent gaping holes in our theoretical understanding of evolution." Thus "it is no longer accurate to say that science cannot explain the generation of novelty."

While the bulk of their book is devoted to making the case for their theory of facilitated variation, Kirschner and Gerhart also apply their argument to current controversies regarding the theory of intelligent design (id). They claim that "some traditional religious groups" have "exaggerated and fabricated weaknesses in evolutionary theory in order to discredit it." According to the authors, after the U.S. Supreme Court prohibited the teaching of biblical creationism in public school science classes, opponents of evolution turned to id to find "ways around the U.S. Constitution to question evolution and to force its removal from the public school curriculum."

Yet Kirschner and Gerhart misunderstand intelligent design and its history. id maintains that it is possible to infer from empirical evidence that some features of the natural world are best explained by an intelligent cause rather than unguided natural processes. Since design is inferred from evidence, not deduced from Scripture or religious doctrine, id is clearly not the same as biblical creationism. Nor have major proponents of id tried to force the removal of evolution from public school curricula; instead, they advocate a "teach the controversy" approach that exposes students to the evidence and scientific arguments for and against evolutionary theory.

Nevertheless, according to Kirschner and Gerhart the "promulgators of intelligent design are merely covertly advocating their own religious agenda and have no desire to hear alternatives based on modern molecular and developmental research." They hope that "other more open-minded people may be influenced by arguments for the plausibility or implausibility of generating novelty in evolution."

The authors mention only one id proponent by name in the main text of their book: Michael Behe. They write: "Behe uses elaborate biochemical examples to intimidate us into believing that the complexity of living cells is beyond understanding." But this misrepresents Behe's position, which is that complexity is understandable—as the result of intelligent design.

In his 1996 book Darwin's Black Box: The Biochemical Challenge to Evolution, Behe quotes Darwin: "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." Behe then asks: "What type of biological system could not be formed by 'numerous, successive, slight modifications'?" And he answers: "Well, for starters, a system that is irreducibly complex. 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."

As the title of Behe's book indicates, the inner workings of the cell were a mystery (a "black box") for Darwin. Modern biochemistry, however, has uncovered many irreducibly complex systems inside living cells. Not only do these pose a problem for Darwin's theory, but according to Behe they also point to design: "Inferring that biochemical systems were designed by an intelligent agent is a humdrum process that requires no new principles of logic or science. It comes simply from the hard work that biochemistry has done over the past forty years, combined with a consideration of the way in which we reach conclusions of design every day."

Behe describes several examples of irreducible complexity. One of these is the biochemistry of vision, which involves a series of specialized molecules that detect light and convert it to nerve impulses. This is the only one of Behe's examples that Kirschner and Gerhart take on.

"In Behe's particular example," they write,

we know that the signaling pathway from the visual pigment (which is itself conserved from bacteria to humans) to the electrical channel in the cell that receives the light impulses in the retina is, in fact, a concatenation of conserved processes common to eukaryotic cells. Furthermore, these processes all have a capacity for weak linkage so that they can be easily wired in different circuits… . Behe sees the constraint in particular designs, but not the deconstraint these designs provide.

By acknowledging that Behe "sees the constraint in particular designs," Kirschner and Gerhart implicitly concede Behe's main point, which concerns only the irreducible complexity of the conserved core processes. Behe's argument is untouched by the fact that the basic components may be wired together in a variety of ways. Yet Kirschner and Gerhart do not even attempt to explain the complexity in those components; they merely assert that intelligent design was unnecessary. "The great innovations of core processes were not magical moments of creation," they write, "but periods of extensive modification of both protein structure and function." Like Topsy in Uncle Tom's Cabin, the core processes just grow'd.

Kirschner and Gerhart also criticize Phillip Johnson and me (without mentioning us by name, except in the notes). Darwin thought that "the embryos of the most distinct species belonging to the same class are closely similar, but become, when fully developed, widely dissimilar," and that this provided "by far the strongest" evidence for his theory that all vertebrates are descended from a common ancestor. In the revised 1993 edition of Darwin On Trial, however, Johnson pointed out that Darwin was mistaken: Vertebrate embryos actually start out very dissimilar, then they become similar midway through development before diverging again. Darwinists typically dismiss this inconvenient discrepancy by arguing that early development can evolve easily. In other words, they simply assume the truth of their theory, then use it to explain why early vertebrate embryos are so different. What had been the strongest evidence for the theory turns out to be false, but the theory is taken to be true anyway and the anomalous evidence is explained away. According to Kirschner and Gerhart, this somehow transforms dissimilarities in early vertebrate embryos "from a confounding paradox of evolution to one of its strongest arguments."

In my book Icons of Evolution, I pointed out that using structural similarity ("homology") as evidence for Darwinian evolution is problematic. Without an unguided natural mechanism, it is impossible to establish that similarities are due to common ancestry rather than common design. Kirschner and Gerhart argue that their theory solves the problem. Maybe. Maybe not. It would help if they could provide good evidence for their theory, but the best they can do is promise us that such evidence will be forthcoming. In the meantime, they expect us to believe that "the modern molecular evidence for homology, its development, and its evolution, is unassailable."

So what are we to make of The Plausibility of Life? Its authors claim to complete Darwin's theory by closing its last remaining major gap, yet they concede that the completed theory has no explanation for the origin of core processes in the first cells, the first eukaryotes, the first multicellular organisms, animal body plans, or vertebrate limbs, heads and brains. There seem to be more gaps in evolutionary theory now than there were before Kirschner and Gerhart got started.

Perhaps it would be fairer to overlook the authors' inflated rhetoric and judge them merely on the basis of their limited theory of facilitated variation. Even if we grant the existence of conserved core processes, have Kirschner and Gerhart succeeded in explaining how land vertebrates diversified into lizards, birds, mice, whales, bats, and humans? Although they assure us that evidence will be forthcoming, the mechanisms they propose—exploratory behavior, weak linkages, and compartmentalization—have never been observed to produce anything like the novelties needed by evolution. If a century of embryology has taught us anything, it is that we can fiddle with these mechanisms all we want in a mouse embryo, and there are only three possible outcomes: a normal mouse, a deformed mouse, or a dead mouse.

Despite the dubious nature of their theoretical proposal, Kirschner and Gerhart imply that anyone who continues to be skeptical of Darwinian evolution is close-minded. In particular, people who think that intelligent design might provide a better explanation for some features of living things are dismissed as ignorant, religiously motivated, and covertly seeking ways to evade the law. Like many of their fellow Darwinists, Kirschner and Gerhart ultimately resort to personal insults.

Does the theory of facilitated variation make life plausible? Not at all, since it assumes the existence of life in the first place. Does the theory of facilitated variation rebut intelligent design? Not at all, since it assumes the existence of irreducibly complex core processes in the first place. The principal take-home lesson from The Plausibility of Life is that evolutionary theory still suffers from major weaknesses, but anyone who says so without reaffirming Darwinism and condemning id is a close-minded, ignorant, Bible-thumping subverter of the Constitution.

Where's the novelty in that?

Jonathan Wells has a Ph.D. in biology from the University of California at Berkeley and a Ph.D. in theology from Yale University. Currently a Senior Fellow at the Discovery Institute in Seattle, he is author most recently of The Politically Incorrect Guide to Darwinism and Intelligent Design, just published by Regnery.

Copyright © 2006 by the author or Christianity Today/Books & Culture magazine.
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