The Panda’s Thumb (4 page)

I am afraid that the turtles illustrate another aspect of historical science—this time a frustration, rather than a principle of explanation. Results rarely specify their causes unambiguously. If we have no direct evidence of fossils or human chronicles, if we are forced to infer a process only from its modern results, then we are usually stymied or reduced to speculation about probabilities. For many roads lead to almost any Rome.

This round goes to the turtles—and why not? While Portuguese sailors hugged the coast of Africa,
Chelonia mydas
swam straight for a dot in the ocean. While the world's best scientists struggled for centuries to invent the tools of navigation,
Chelonia
looked at the skies and proceeded on course.

NATURE MARKS
Izaak Walton as a rank amateur more often than I had imagined. In 1654, the world's most famous fisherman before Ted Williams wrote of his favorite lure: “I have an artificial minnow…so curiously wrought, and so exactly dissembled that it would beguile any sharpsighted trout in a swift stream.”

An essay in my previous book,
Ever Since Darwin
, told the tale of
Lampsilis
, a freshwater clam with a decoy “fish” mounted on its rear end. This remarkable lure has a streamlined “body,” side flaps simulating fins and tail, and an eyespot for added effect; the flaps even undulate with a rhythmic motion that imitates swimming. This “fish,” constructed from a brood pouch (the body) and the clam's outer skin (fin and tails), attracts the real item and permits a mother clam to shoot her larvae from the brood pouch toward an unsuspecting fish. Since the larvae of
Lampsilis
can only grow as parasites on a fish's gill, this decoy is a useful device indeed.

I was astounded recently to learn that
Lampsilis
is not alone. Ichthyologists Ted Pietsch and David Grobecker recovered a single specimen of an amazing Philippine anglerfish, not as a reward for intrepid adventures in the wilds, but from that source of so much scientific novelty—the local aquarium retailer. (Recognition, rather than
machismo
, is often the basis of exotic discovery.) Anglerfish lure their dinner, rather than a free ride for their larvae. They carry a highly modified dorsal fin spine affixed to the tips of their snouts. At the end of this spine, they mount an appropriate lure. Some deep-sea species, living in a dark world untouched by light from the surface, fish with their own source of illumination: they gather phosphorescent bacteria in then lures. Shallow-water species tend to have colorful, bumpy bodies, and look remarkably like rocks encrusted with sponges and algae. They rest inert on the bottom and wave or wiggle their conspicuous lures near their mouths. “Baits” differ among species, but most resemble—often imperfectly—a variety of invertebrates, including worms and crustaceans.

Anglerfish
DAVID B. GROBECKER

Pietsch and Grobecker's anglerfish, however, has evolved a fish lure every bit as impressive as the decoy mounted on
Lampsilis
's rear—a first for anglerfish. (Their report bears as its appropriate title “The Compleat Angler” and cites as an epigraph the passage from Walton quoted above.) This exquisite fake also sports eyelike spots of pigment in the right place. In addition, it bears compressed filaments representing pectoral and pelvic fins along the bottom of the body, extensions from the back resembling dorsal and anal fins, and even an expanded rear projection looking for all the world like a tail. Pietsch and Grobecker conclude: “The bait is nearly an exact replica of a small fish that could easily belong to any of a number of percoid families common to the Philippine region.” The angler even ripples its bait through the water, “simulating the lateral undulations of a swimming fish.”

These nearly identical artifices of fish and clam might seem, at first glance, to seal the case for Darwinian evolution. If natural selection can do this twice, surely it can do anything. Yet—continuing the theme of the last two essays and bringing this trilogy to a close—perfection works as well for the creationist as the evolutionist. Did not the psalmist proclaim: “The heavens declare the glory of God; and the firmament showeth his handiwork.” The last two essays argued that imperfection carries the day for evolution. This one discusses the Darwinian response to perfection.

The only thing more difficult to explain than perfection is repeated perfection by very different animals. A fish on a clam's rear end
and
another in front of an anglerfish's nose—the first evolved from a brood pouch and outer skin, the second from a fin spine—more than doubles the trouble. I have no difficulty defending the origin of both “fishes” by evolution. A plausible series of intermediate stages can be identified for
Lampsilis
. The fact that anglerfish press a fin spine into service as a lure reflects the jury-rigged, parts-available principle that made the panda's thumb and the orchid's labellum speak so strongly for evolution (see the first essay of this trilogy). But Darwinians must do more than demonstrate evolution; they must defend the basic mechanism of random variation and natural selection as the primary cause of evolutionary change.

Anti-Darwinian evolutionists have always favored the
repeated
development of very similar adaptations in different lineages as an argument against the central Darwinian notion that evolution is unplanned and undirected. If different organisms converge upon the same solutions again and again, does this not indicate that certain directions of change are preset, not established by natural selection working on random variation? Should we not look upon the repeated form itself as a cause of the numerous evolutionary events leading toward it?

Throughout his last half-dozen books, for example, Arthur Koestler has been conducting a campaign against his own misunderstanding of Darwinism. He hopes to find some ordering force, constraining evolution to certain directions and overriding the influence of natural selection. Repeated evolution of excellent design in separate lineages is his bulwark. Again and again, he cites the “nearly identical skulls” of wolves and the “Tasmanian wolf.” (This marsupial carnivore looks like a wolf but is, by genealogy, more closely related to wombats, kangaroos, and koalas.) In
Janus
, his latest book, Koestler writes: “Even the evolution of a single species of wolf by random mutation plus selection presents, as we have seen, insurmountable difficulties. To duplicate this process independently on island and mainland would mean squaring a miracle.”

The Darwinian response involves both a denial and an explanation. First, the denial: it is emphatically not true that highly convergent forms are effectively identical. Louis Dollo, the great Belgian paleontologist who died in 1931, established a much misunderstood principle—“the irreversibility of evolution” (also known as Dollo's law). Some ill-informed scientists think that Dollo advocated a mysterious directing force, driving evolution forward, never permitting a backward peek. And they rank him among the non-Darwinians who feel that natural selection cannot be the cause of nature's order.

In fact, Dollo was a Darwinian interested in the subject of convergent evolution—the repeated development of similar adaptations in different lineages. Elementary probability theory, he argued, virtually guarantees that convergence can never yield anything close to perfect resemblance. Organisms cannot erase their past. Two lineages may develop remarkable, superficial similarities as adaptations to a common mode of life. But organisms contain so many complex and independent parts that the chance of all evolving twice toward exactly the same result is effectively nil. Evolution is irreversible; signs of ancestry are always preserved; convergence, however impressive, is always superficial.

Consider my candidate for the most astounding convergence of all: the ichthyosaur. This sea-going reptile with terrestrial ancestors converged so strongly on fishes that it actually evolved a dorsal fin and tail in just the right place and with just the right hydrological design. These structures are all the more remarkable because they evolved from nothing—the ancestral terrestrial reptile had no hump on its back or blade on its tail to serve as a precursor. Nonetheless, the ichthyosaur is no fish, either in general design or in intricate detail. (In ichthyosaurs, for example, the vertebral column runs through the lower tail blade; in fish with tail vertebrae, the column runs into the upper blade.) The ichthyosaur remains a reptile, from its lungs and surface breathing to its flippers made of modified leg bones, not fin rays.

Ichthyosaur
COURTESY OF THE
A
MERICAN
M
USEUM OF
N
ATURAL
H
ISTORY

Koestler's carnivores tell the same tale. Both placental wolf and marsupial “wolf” are well designed to hunt, but no expert would ever mistake their skulls. The numerous, small marks of marsupiality are not obliterated by convergence in outward form and function.

Second, the explanation: Darwinism is not the theory of capricious change that Koestler imagines. Random variation may be the raw material of change, but natural selection builds good design by rejecting most variants while accepting and accumulating the few that improve adaptation to local environments.

The basic reason for strong convergence, prosaic though it may seem, is simply that some ways of making a living impose exacting criteria of form and function upon any organism playing the role. Mammalian carnivores must run and stab; they do not need grinding molar teeth since they tear and swallow their food. Both placental and marsupial wolves are built for sustained running, have long, sharp, pointed canine teeth and reduced molars. Terrestrial vertebrates propel themselves with their limbs and may use their tails for balance. Swimming fish balance with their fins and propel from the rear with their tails. Ichthyosaurs, living like fish, evolved a broad propulsive tail (as whales did later—although the horizontal flukes of a whale's tail beat up and down, while the vertical flukes offish and ichthyosaurs beat from side to side).

No one has treated this biological theme of repeated, exquisite design more eloquently than D'Arcy Wentworth Thompson in his 1942 treatise,
On Growth and Form
, still in print and still as relevant as ever. Sir Peter Medawar, a man who eschews hype and exaggeration, describes it as “beyond comparison the finest work of literature in all the annals of science that have been recorded in the English tongue.” Thompson, zoologist, mathematician, classical scholar, and prose stylist, won accolades as an old man but spent his entire professional life in a small Scottish university because his views were too unorthodox to win prestigious London and Oxbridge jobs.

Thompson was more a brilliant reactionary than a visionary. He took Pythagoras seriously and worked as a Greek geometrician. He took special delight in finding the abstract forms of an idealized world embodied again and again in the products of nature. Why do repeated hexagons appear in the cells of a honeycomb and in the interlocking plates of some turtle shells? Why do the spirals in a pine cone and a sunflower (and often of leaves on a stem) follow the Fibonacci series? (A system of spirals radiating from a common point can be viewed either as a set of left- or right-handed spirals. Left and right spirals are not equal in number, but represent two consecutive figures of the Fibonacci series. The Fibonacci series is constructed by adding the previous two numbers to form the next: 1, 1, 2, 3, 5, 8, 13, 21, etc. The pine cone may, for example, have 13 left spirals and 21 right spirals.) Why do so many snail shells, ram's horns, and even the path of a moth to light follow a curve called the logarithmic spiral?

Thompson's answer was the same in each case: these abstract forms are optimal solutions for common problems. They are evolved repeatedly in disparate groups because they are the best, often the only, path to adaptation. Triangles, parallelograms, and hexagons are the only plane figures that fill space completely without leaving holes. Hexagons are often favored because they approximate a circle and maximize area within relative to the supporting walls (minimum construction for greatest storage of honey, for example). The Fibonacci pattern emerges automatically in any system of radiating spirals built by adding new elements at the apex, one at a time in the largest space available. The logarithmic spiral is the only curve that does not change its shape as it grows in size. I can identify the abstract Thompsonian forms as optimal adaptations, but to the larger metaphysical issue of why “good” form often exhibits such simple, numerical regularity, I plead only ignorance and wonder.

So far, I have only spoken to half the issue embodied in the problem of repeated perfection. I have discoursed on the “why.” I have argued that convergence never renders two complex organisms completely identical (a circumstance that would strain Darwinian processes beyond their reasonable power) and I have tried to explain close repeats as optimal adaptations to common problems with few solutions.