Not Necessarily a Wing
by Stephen Jay Gould
rom
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 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 Mirvart 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:
How does evolutionary theory as understood by
Darwin explain the emergence of items such as wings, since a small move toward
a wing could hardly promote survival? I seem to be stuck with the idea that a
significant quality of wing would have to spring forth all at once to have any
survival value.
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):
Natural selection utterly fails to account for
the conservation and development of the minute and rudimentary beginnings, the
slight and infinitesimal commencements of structures, however useful those
structures may afterwards become.
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 co-adapted parts, could arise all at once:
He who believes that some ancient form was
transformed suddenly through an internal force or tendency into, for
instance, one furnished with wings, will be…compelled to believe that
many structures beautifully adapted to all the other parts of the same
creature and to the surrounding conditions, have been suddenly produced;
and of such complex and wonderful co-adaptations, he will not be able to
assign a shadow of an explanation.…To admit all this is, as it seems
to me, to enter into the realms of miracle, and to leave those of
Science.
(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,"[1] 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. Kingsolver 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).
In preparing this essay, I spent several days reading the
classical literature on the evolution of insect flight—and emerged with
a deeper understanding of just how difficult Darwin's principle of functional
shift can be, even for professionals. Most of the literature hasn't even made
the first step of applying functional shift at all; not to mention the later
reform of substituting direct evidence for verbal speculation. Most
reconstructions are still trying to explain the incipient stages of insect
wings as somehow involved in airborne performance from the start—not for
flapping flight, of course, but still for some aspect of motion aloft rather
than, as Darwin's principle would suggest, for some quite different
function.
To appreciate the dilemma of such a position (so well
grasped by Mivart more than 100 years ago), consider just one recent study
(probably the best and most widely cited) and the logical quandaries that a
claim of functional continuity entails. In 1964, J. W. Flower presented
aerodynamic arguments or wings evolved from tiniest rudiment to elaborate
final form in the interest of airborne motion. Flower argues, supporting an
orthodox view, that wings evolved from tiny outgrowths of the body used for
gliding prior to elaboration for sustained flight. But Flower recognizes that
these incipient structures must themselves evolve from antecedents too small
to function as gliding planes. What could these very first, slight outgrowths
of the body be for? Ignoring Darwin's principle of functional shift, Flower
searches for an aerodynamic meaning even at this very outset. He tries to
test two suggestions: E. H. Hinton's argument that initial outgrowths served
for "attitude control," permitting a falling insect to land in a suitable
position for quick escape from predators; and a proposal of the great British
entomologist Sir Vincent Wigglesworth (wonderful name for an insect man, I
always thought) that such first stages might act as stabilizing or controlling
devices during takeoff in small, passively aerial insects.
Flower proceeded by performing aerodynamic calculations on
consequences of incipient wings for simple body shapes when dropped—and
he quickly argued himself into an inextricable logical corner. He found,
first of all, that tiny outgrowths might help, as Wigglesworth, Hinton, and
others had suggested. But the argument foundered on another observation: The
same advantages could be gained far more easily and effectively by another,
readily available alternative route—evolution to small size (where
increased surface/volume ratios retard failing and enhance the probability of
takeoff). Flower then realized that he would have to specify a reasonably
large body size for incipient wings to have any aerodynamic effect. But he
then encountered another problem: At such sizes, legs work just as well as,
if not better than, proto-wings for any suggested aerodynamic function.
Flower admitted:
The first conclusion to be drawn from these
calculations is that the selective pressure in small insects is towards
smaller insects, which would have no reason to evolve wings.
I would have stopped and searched elsewhere (in Darwin's
principle of functional shift) at this point, but Flower bravely continued
along an improbable path:
The main conclusions, however, are that
attitude control of insects would be by the use of legs or by very small
changes in body shape [i.e., by evolving small outgrowths, or
proto-wings].
Flower, in short, never considered an alternative to his
assumption of functional continuity based upon some aspect of aerial
locomotion. He concluded
At first they [proto-wings] would affect
attitude; later they could increase to a larger size and act as a true wing,
providing lift in their own right. Eventually they could move, giving the
insect greater maneuverability during descent, and finally they could "flap,"
achieving sustained flight.
As an alternative to such speculative reconstructions
that work, in their own terms, only by uncomfortable special pleading, may
I suggest Darwin's old principle of functional shift
(preadaptation—ugh—for something else). The physiological literature
contains voluminous testimony to the thermodynamic efficiency of modern insect
wings: in presenting, for example, a large surface area to the sun for quick
heating (see B. Heinrich, 1981). If wings can perform this subsidiary function
now, why not suspect thermoregulation as a primary role at the outset? M. M.
Douglas (1981), for example, showed that, in Colias butterflies, only the
basal one-third of the wing operates in thermoregulation—an area
approximately equal to the thoracic lobes (proto-wings) of fossil insects
considered ancestral to modern forms.
Douglas then cut down some Colias wings to the actual size
of these fossil ancestral lobes and found that insects so bedecked showed a
55 percent greater increase in body temperature than bodies deprived of wings
entirely. These manufactured proto-wings measured 5 by 3 millimeters on a
body 15 millimeters long. Finally, Douglas determined that no further
thermoregulatory advantage could be gained by wings longer than 10
millimeters on a 15-millimeter body.
Kingsolver and Koehl performed a host of elaborate and
elegant experiments to support a thermoregulatory origin of insect
proto-wings. As with so many examples of excellent science producing clear
and interesting outcomes, the results can be summarized briefly and cleanly.
Kingsolver and Koehl begin by tabulating all the
aerodynamic hypotheses usually presented in the literature as purely verbal
speculations. They arrange these proposals of functional continuity (the
explanations that do not follow Darwin's solution of Mivart's dilemma) into
three basic categories: proto-wings for gliding (aerofoils for steady-state
motion), for parachuting (slowing the rate of descent in a falling insect),
and attitude stability (helping an insect to land right side up). They then
transcended the purely verbal tradition by developing aerodynamic equations
for exactly how proto-wings should help an insect under these three
hypotheses of continuity in adaptation (increasing the lift/drag ratio as
the major boost to gliding, increasing drag to slow the descent rate in
parachuting, measuring the moment about the body axis produced by wings for
the hypothesis of attitude stability).
They then constructed insect models made of wire, epoxy,
and other appropriate materials to match the sizes and body shapes of flying
and nonflying forms among early insect fossils. To these models, they
attached wings (made of copper wire enclosing thin, plastic membranes) of
various lengths and measured the actual aerodynamic effects for properties
predicted by various hypotheses of functional continuity. The results of
many experiments in wind tunnels are consistent and consonant: Aerodynamic
benefits begin for wings above a certain size, and they increase as wings
get larger. But at the small sizes of insect proto-wings, aerodynamic
advantages are absent or insignificant and do not increase with growing wing
length. These results are independent of body shape, wind velocity, presence
or placement of legs, and mounting position of wings. In other words, large
wings work well and larger wings work better—but small wings (at the
undoubted sizes of Mivart's troubling incipient stages) provide no
aerodynamic edge.
 |
|
The thermoregulatory (upper curve) and aerodynamic
(lower curve) Advantages for increasing wing length in insects. Note that
thermodynamic benefits accrue rapidly when the wing is very small (too small
for flight), but scarcely increase at all for wings of larger size.
Aerodynamic advantages, on the other hand, are insignificant for small size,
but increase rapidly at larger wing dimensions, just as the thermodynamic
benefits cease. Ben Gamit. Adapted from Joe Lemonnier. Courtesy of
Natural History.
| |
Kingsolver and Koehl then tested their models for
thermoregulatory effects, constructing wings from two materials with
different thermal conductivities (construction paper and aluminum foil) and
measuring the increased temperature of bodies supplied with wings of various
lengths versus wingless models. They achieved results symmetrically opposite
to the aerodynamic experiments. For thermoregulation, wings work well at the
smallest sizes, with benefits increasing as the wing grows. However, beyond
a measured length, further increase of the wing confers no additional
effect. Kingsolver and Koehl conclude:
At any body size, there is a relative wing length above
which there is no additional thermal effect, and below which
there is no significant aerodynamic effect.
The accompanying chart illustrates these combined
results. Note how the thermoregulatory effect of excess body temperature
due to wings (solid line) increases rapidly at small wing sizes but not at
all above an intermediate wing length. Conversely, the aerodynamic effect
of lift/drag ratio does not increase at all until intermediate wing
length, but grows rapidly thereafter.
We could not hope for a more elegant experimental
confirmation of Darwin's solution to Mivart's challenge. Kingsolver and
Koehl have actually measured the functional shift by showing that incipient
wings aid thermoregulation but provide no aerodynamic benefit—while
larger wings provide no further thermoregulatory oomph but initiate
aerodynamic advantage and increase the benefits steadily thereafter. The
crucial intermediate wing length, where thermoregulatory gain ceases and
aerodynamic benefits begin, represents a domain of functional shift, as
aerodynamic advantages pick up the relay from waning thermoregulation to
continue the evolutionary race to increasing wing size.
But what might push an insect across the transition? Why
reach this crucial domain at all? If wings originally worked primarily for
thermoregulation, why not just stop as the length of maximum benefit
approached? Here, Kingsolver and Koehl present an interesting speculation
based on another aspect of their data. They found that the domain of
transition between thermal and aerial effects varied systematically with
body size: The larger the body, the sooner the transition (in terms of
relative wing length). For a body 2 centimeters long, the transition
occurred with wings 40 to 60 percent of body length; but a 10-centimeter
body switches to aerodynamic advantage at only 10 percent of body length.
Now suppose that incipient ancestral wings worked
primarily for thermoregulation, and had reached a stable, optimum size for
greatest benefit. Natural selection would not favor larger wings and a
transition to the available domain of aerodynamic advantage. But if body
size increased for other reasons, an insect might reach the realm of aerial
effects simply by growing larger, without any accompanying change of body
shape or relative wing length.
We often think, naively, that size itself should make no
profound difference. Why should just more of the same have any major effect
beyond simple accumulation? Surely, any major improvement or alteration
must require an extensive and explicit redesign, a complex reordering of
parts with invention of new items.
Nature does not always match our faulty intuitions.
Complex objects often display the interesting and paradoxical property of
major effect for apparently trifling input. Internal complexity can translate
a simple quantitative change into a wondrous alteration of quality. Perhaps
that greatest and most effective of all evolutionary inventions, the origin
of human consciousness, required little more than an increase of brain power
to a level where internal connections became rich and varied enough to force
this seminal transition. The story may be much more complex, but we have no
proof that it must be.
Voltaire quipped that "God is always for the big
battalions." More is not always better, but more can be very different.
Notes
This dreadful name has made a difficult
principle even harder to grasp and understand. Preadaptation seems to
imply that the proto-wing, while doing something else in its incipient stages,
knew where it was going—predestined for a later conversion to flight.
Textbooks usually introduce the word and then quickly disclaim any odor of
foreordination. (But a name is obviously ill-chosen if it cannot be used without
denying its literal meaning.) Of course, by "preadaptation" we only mean that
some structures are fortuitously suited to other roles if elaborated, not that
they arise with a different future use in view-now there I go with the standard
disclaimer. As another important limitation, preadaptation does not cover the
important class of features that arise without functions (as developmental
consequences of other primary adaptations, for example) but remain available for
later co-optation. I suspect, for example, that many important functions of the
human brain are co-opted consequences of building such a large computer for a
limited set of adaptive uses. For these reasons, Elizabeth Vrba and I have
proposed that the restrictive and confusing word "preadaptation" be dropped in
favor of the more inclusive term "exaptation"—for any organ not evolved
under natural selection for its current use—either because it performed a
different function in ancestors (classical preadaptation) or because it
represented a nonfunctional part available for later co-optation. See our
technical article, "Exaptation: A Missing Term in the Science of Form,"
Paleobiology, 1981.
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[ Stephen Jay Gould "Not Necessarily a Wing," Natural
History 94 (October 1985): 12-25; Reprinted here with permission from
Bully for
Brontosaurus, New York: W. W. Norton & Co., 1991, pp. 139-151. ]
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