Bully for Brontosaurus (15 page)

Now, having wrestled with the question of adaptation for many years, I understand the wisdom of Crick’s remark. If all structures had a “why” framed in terms of adaptation, then my original dismissal would be justified for we would know that “whys” exist whether or not we had elucidated the “how.” But I am now convinced that many structures (including male nipples and clitoral orgasm) have no direct adaptational “why.” And we discover this by studying pathways of genetics and development—or, as Crick so rightly said to me, by first understanding how a structure is built. In other words, we must first establish “how” in order to know whether or not we should be asking “why” at all.

I began with Charles Darwin’s grandpa Erasmus and end with his namesake, Desiderius Erasmus, the greatest of all Renaissance scholars. Of more than 3,000 proverbs from antiquity collected in his
Adagia
of 1508, perhaps two are best known and wonderfully apt for the point of this essay (which is not a diatribe against adaptation but a plea for expansion by alternative hypotheses and for fruitful competition and synthesis between functional and structural perspectives). First a comment on limitations of outlook: “No one is injured save by himself.” Second, probably the most famous of zoological metaphors about human temperament: “The fox has many tricks, and the hedgehog only one, but that is the best of all.” Some have taken the hedgehog’s part in this dichotomy, but I will cast my lot for a diversity of options—for our complex world may offer many paths to salvation, and the hounds of hell press continually upon us.

FROM
Flesh Gordon
to
Alex in Wonderland
, title parodies have been a stock-in-trade of low comedy. We may not anticipate a tactical similarity between the mayhem of
Mad
magazine’s movie reviews and the titles of major scientific works, yet two important nineteenth-century critiques of Darwin parodied his most famous phrases in their headings.

In 1887, E. D. Cope, the American paleontologist known best for his fossil feud with O. C. Marsh (see Essay 5) but a celebrated evolutionary theorist in his own right, published
The Origin of the Fittest
—a takeoff on Herbert Spencer’s phrase, borrowed by Darwin as the epigram for natural selection: survival of the fittest. (Natural selection, Cope argued, could only preserve favorable traits that must arise in some other manner, unknown to Darwin. The fundamental issue of evolution cannot be the differential survival of adaptive traits, but their unexplained origin—hence the title parody.)

St. George Mivart (1817–1900), a fine British zoologist, tried to reconcile his unconventional views on religion and biology but ended his life in tragedy, rejected by both camps. At age seventeen, he abandoned his Anglican upbringing, became a Roman Catholic, and consequently (in a less tolerant age of state religion) lost his opportunity for training in natural history at Oxford or Cambridge. He became a lawyer but managed to carve out a distinguished career as an anatomist nonetheless. He embraced evolution and won firm support from the powerful T. H. Huxley, but his strongly expressed and idiosyncratic anti-Darwinian views led to his rejection by the biological establishment of Britain. He tried to unite his biology with his religion in a series of books and essays, and ended up excommunicated for his trouble six weeks before his death.

Cope and Mivart shared the same major criticism of Darwin—that natural selection could explain the preservation and increase of favored traits but not their origin. Mivart, however, went gunning for a higher target than Darwin’s epigram. He shot for the title itself, naming his major book (1871)
On the Genesis of Species
. (Darwin, of course, had called his classic
On the Origin of Species
.)

Mivart’s life may have ended in sadness and rejection thirty years later, but his
Genesis of Species
had a major impact in its time. Darwin himself offered strong, if grudging, praise and took Mivart far more seriously than any other critic, even adding a chapter to later editions of the
Origin of Species
primarily to counter Mivart’s attack.

Mivart gathered, and illustrated “with admirable art and force” (Darwin’s words), all objections to the theory of natural selection—“a formidable array” (Darwin’s words again). Yet one particular theme, urged with special attention by Mivart, stood out as the centerpiece of his criticism. This argument continues to rank as the primary stumbling block among thoughtful and friendly scrutinizers of Darwinism today. No other criticism seems so troubling, so obviously and evidently “right” (against a Darwinian claim that seems intuitively paradoxical and improbable).

Mivart awarded this argument a separate chapter in his book, right after the introduction. He also gave it a name, remembered ever since. He called his objection “The Incompetency of ‘Natural Selection’ to Account for the Incipient Stages of Useful Structures.” If this phrase sounds like a mouthful, consider the easy translation: We can readily understand how complex and fully developed structures work and how their maintenance and preservation may rely upon natural selection—a wing, an eye, the resemblance of a bittern to a branch or of an insect to a stick or dead leaf. But how do you get from nothing to such an elaborate something if evolution must proceed through a long sequence of intermediate stages, each favored by natural selection? You can’t fly with 2 percent of a wing or gain much protection from an iota’s similarity with a potentially concealing piece of vegetation. How, in other words, can natural selection explain the incipient stages of structures that can only be used in much more elaborated form?

I take up this old subject for two reasons. First, I believe that Darwinism has, and has long had, an adequate and interesting resolution to Mivart’s challenge (although we have obviously been mightily unsuccessful in getting it across). Second, a paper recently published in the technical journal
Evolution
has provided compelling experimental evidence for this resolution applied to its most famous case—the origin of wings.

The dilemma of wings
—the
standard illustration of Mivart’s telling point about incipient stages—is set forth particularly well in a perceptive letter that I recently received from a reader, a medical doctor in California. He writes:

Interestingly, my reader’s proposal that much or most of the wing must arise all at once (because incipient stages could have no adaptive value) follows Mivart’s own resolution. Mivart first enunciated the general dilemma (1871, p. 23):

After fifty pages of illustration, he concludes: “Arguments may yet be advanced in favor of the view that new species have from time to time manifested themselves with suddenness, and by modifications appearing at once.” Advocating this general solution for wings in particular, he concludes (p. 107): “It is difficult, then, to believe that the Avian limb was developed in any other way than by a comparatively sudden modification of a marked and important kind.”

Darwin’s theory is rooted in the proposition that natural selection acts as the primary creative force in evolutionary change. This creativity will be expressed only if the fortuitous variation forming the raw material of evolutionary change can be accumulated sequentially in tiny doses, with natural selection acting as the sieve of acceptance. If new species arise all at once in an occasional lucky gulp, then selection has no creative role. Selection, at best, becomes an executioner, eliminating the unfit following this burst of good fortune. Thus, Mivart’s solution—bypassing incipient stages entirely in a grand evolutionary leap—has always been viewed, quite rightly, as an anti-Darwinian version of evolutionary theory.

Darwin well appreciated the force, and potentially devastating extent, of Mivart’s critique about incipient stages. He counterattacked with gusto, invoking the standard example of wings and arguing that Mivart’s solution of sudden change presented more problems than it solved—for how can we believe that so complex a structure as a wing, made of so many coordinated and coadapted parts, could arise all at once:

(This essay must now go in other directions but not without a small, tangential word in Mivart’s defense. Mivart did appreciate the problem of complexity and coordination in sudden origins. He did not think that any old complex set of changes could arise all at once when needed
—that
would be tantamount to miracle. Most of Mivart’s book studies the regularities of embryology and comparative anatomy to learn which kinds of complex changes might be possible as expressions and elaborations of developmental programs already present in ancestors. He advocates these changes as possible and eliminates others as fanciful.)

Darwin then faced his dilemma and developed the interestingly paradoxical resolution that has been orthodox ever since (but more poorly understood and appreciated than any other principle in evolutionary theory). If complexity precludes sudden origin, and the dilemma of incipient stages forbids gradual development in functional continuity, then how can we ever get from here to there? Darwin replies that we must reject an unnecessary hidden assumption in this argument—the notion of functional continuity. We will all freely grant that no creature can fly with 2 percent of a wing, but why must the incipient stages be used for flight? If incipient stages originally performed a different function suited to their small size and minimal development, natural selection might superintend their increase as adaptations for this original role until they reached a stage suitable for their current use. In other words, the problem of incipient stages disappears because these early steps were not inadequate wings but well-adapted something-elses. This principle of
functional change in structural continuity
represents Darwin’s elegant solution to the dilemma of incipient stages.

Darwin, in a
beau geste
of argument, even thanked Mivart for characterizing the dilemma so well—all the better to grant Darwin a chance to elaborate his solution. Darwin writes: “A good opportunity has thus been afforded [by Mivart] for enlarging a little on gradations of structure, often associated with changed functions—an important subject, which was not treated at sufficient length in the former editions of this work.” Darwin, who rarely added intensifiers to his prose, felt so strongly about this principle of functional shift that he wrote: “In considering transitions of organs, it is so important to bear in mind the probability of conversion from one function to another.”

Darwin presented numerous examples in Chapters 5 and 7 of the final edition of the
Origin of Species
. He discussed organs that perform two functions, one primary, the other subsidiary, then relinquish the main use and elaborate the formerly inconspicuous operation. He then examined the flip side of this phenomenon—functions performed by two separate organs (fishes breathing with both lungs and gills). He argues that one organ may assume the entire function, leaving the other free for evolution to some other role (lungs for conversion to air bladders, for example, with respiration maintained entirely by gills). He does not, of course, neglect the classic example of wings, arguing that insects evolved their organs of flight from tracheae (or breathing organs—a minority theory today, but not without supporters). He writes: “It is therefore highly probable that in this great class organs which once served for respiration have been actually converted into organs of flight.”

Darwin’s critical theory of functional shift, usually (and most unfortunately) called the principle of “preadaptation,”
^*
has been with us for a century. I believe that this principle has made so little headway not only because the basic formulation seems paradoxical and difficult, but mainly because we have so little firm, direct evidence for such functional shifts. Our technical literature contains many facile verbal arguments—little more than plausible “just-so” stories. The fossil record also presents some excellent examples of sequential development through intermediary stages that could not work as modern organs do—but we lack a rigorous mechanical analysis of function at the various stages.

Let us return, as we must, to the classic case of wings.
Archaeopteryx
, the first bird, is as pretty an intermediate as paleontology could ever hope to find—a complex mélange of reptilian and avian features. Scientists are still debating whether or not it could fly. If so,
Archaeopteryx
worked like the Wrights’ biplane to a modern eagle’s Concorde. But what did the undiscovered ancestors of
Archaeopteryx
do with wing rudiments that surely could not produce flight? Evolutionists have been invoking Darwin’s principle of functional shift for more than 100 years, and the list of proposals is long. Proto-wings have been reconstructed as stabilizers, sexual attractors, or insect catchers. But the most popular hypothesis identifies thermoregulation as the original function of incipient stages that later evolved into feathered wings. Feathers are modified reptilian scales, and they work very well as insulating devices. Moreover, if birds evolved from dinosaurs (as most paleontologists now believe), they arose from a lineage particularly subject to problems with temperature control.
Archaeopteryx
is smaller than any dinosaur and probably arose from the tiniest of dinosaur lineages. Small animals, with high ratios of surface area to volume, lose heat rapidly and may require supplementary devices for thermoregulation. Most dinosaurs could probably keep warm enough just by being large. Surface area (length × length, or length squared) increases more slowly than volume (length × length × length, or length cubed) as objects grow. Since animals generate heat over their volumes and lose it through their surfaces, small animals (with their relatively large surface areas) have most trouble keeping warm.

There I go again—doing what I just criticized. I have presented a plausible story about thermoregulation as the original function of organs that later evolved into wings. But science is tested evidence, not tall tales. This lamentable mode of storytelling has been used to illustrate Darwin’s principle of functional shift only
faute de mieux
—because we didn’t have the goods so ardently desired. At least until recently, when my colleagues Joel G. King-solver and M. A. R. Koehl published the first hard evidence to support a shift from thermoregulation to flight as a scenario for the evolution of wings. They studied insects, not birds—but the same argument has long been favored for nature’s smaller and far more abundant wings (see their article, “Aerodynamics, Thermoregulation, and the Evolution of Insect Wings: Differential Scaling and Evolutionary Change,” in
Evolution
, 1985).