The Panda’s Thumb

ON THE TITLE PAGE
of his classic book,
The Cell in Development and Inheritance
, E.B. Wilson inscribed a motto from Pliny, the great natural historian who died in his boots when he sailed across the Bay of Naples to study the eruption of Mt. Vesuvius in A.D. 79. He suffocated in the same vapors that choked the citizens of Pompeii. Pliny wrote:
Natura nusquam magis est tota quam in minimisâ
“Nature is to be found in her entirety nowhere more than in her smallest creatures.” Wilson, of course, commandeered Pliny's statement to celebrate the microscopic building blocks of life, minute structures unknown perforce to the great Roman. Pliny was thinking about organisms.

Pliny's statement captures the essence of what fascinates me about natural history. In an old stereotype (not followed nearly so often as mythology proclaims), the natural history essay restricts itself to describing the peculiarities of animalsâthe mysterious ways of the beaver, or how he spider weaves her supple web. There is exultation in this and who shall gainsay it? But each organism can mean so much more to us. Each instructs; its form and behavior embodies general messages if only we can learn to read them. The language of this instruction is evolutionary theory. Exultation
and
explanation.

I was lucky to wander into evolutionary theory, one of the most exciting and important of all scientific fields. I had never heard of it when I started at a rather tender age; I was simply awed by dinosaurs. I thought paleontologists spent their lives digging up bones and putting them together, never venturing beyond the momentous issue of what connects to what. Then I discovered evolutionary theory. Ever since then, the duality of natural historyârichness in particularities and potential union in underlying explanationâhas propelled me.

I think that the fascination so many people feel for evolutionary theory resides in three of its properties. First, it is, in its current state of development, sufficiently firm to provide satisfaction and confidence, yet fruitfully undeveloped enough to provide a treasure trove of mysteries. Second, it stands in the middle of a continuum stretching from sciences that deal in timeless, quantitative generality to those that work directly with the singularities of history. Thus, it provides a home for all styles and propensities, from those who seek the purity of abstraction (the laws of population growth and the structure of DNA) to those who revel in the messiness of irreducible particularity (what, if anything, did
Tyrannosaurus
do with its puny front legs anyway?). Third, it touches all our lives; for how can we be indifferent to the great questions of genealogy: where did we come from, and what does it all mean? And then, of course, there are all those organisms: more than a million described species, from bacterium to blue whale, with one hell of a lot of beetles in betweenâeach with its own beauty, and each with a story to tell.

These essays range broadly in the phenomena they treatâfrom the origin of life, to the brain of Georges Cuvier, to a mite that dies before it is born. Yet I hope that I have avoided that incubus of essay collections, diffuse incoherence, by centering them all upon evolutionary theory, with an emphasis on Darwin's thoughts and impact. As I stated in introducing my previous collection,
Ever Since Darwin:
“I am a tradesman, not a polymath. What I know of planets and politics lies at their intersection with biological evolution.”

I have tried to weld these essays into an integrated whole by organizing them into eight sections. The first on pandas, turtles & anglerfish, illustrates why we can be confident that evolution occurred. The argument embodies a paradox: the proof of evolution lies in imperfections that reveal history. This section is followed by a club sandwichâthree sections on major themes in the evolutionary study of natural history (Darwinian theory and the meaning of adaptation, the tempo and mode of change, and the scaling of size and time), and two intervening layers of two sections each (III and IV, and VI and VII) on organisms and the peculiarities of their history. (If anyone wants to pursue the metaphorical sandwich and divide these seven sections into supporting structure and meat, I will not be offended.) I have also impaled the sandwich with toothpicksâsubsidiary themes common to all sections, and intended to prick some conventional comforts: why science must be embedded in culture, why Darwinism cannot be squared with hopes for intrinsic harmony or progress in nature. But each pinprick has its positive consequence. An understanding of cultural bias forces us to view science as an accessible, human activity, much like any form of creativity. An abandonment of the hope that we might read a meaning for our lives passively in nature compels us to seek answers within ourselves.

These essays are lightly edited versions of my monthly columns in
Natural History
Magazine, collectively titled “This View of Life.” I have added postscripts to a few: additional evidence of Teilhard's possible involvement in the Piltdown fraud (essay 10); a letter from J Harlen Bretz, controversial as ever at 96 (19); confirmation from the southern hemisphere for an explanation of magnets in bacteria (30). I thank Ed Barber for persuading me that these essays might be less ephemeral than I thought.
Natural History
's editor in chief Alan Ternes and copy editor Florence Edelstein have greatly helped in deconvolution of phrase and thought and in devising some good titles. Four essays would not have been, without the gracious help of colleagues: Carolyn Fluehr-Lobban introduced me to Dr. Down, sent me his obscure article, and shared her insights and writing with me (essay 15). Ernst Mayr has urged the importance of folk taxonomy for years and had all the references on hand (essay 20). Jim Kennedy introduced me to Kirkpatrick's work (essay 22); otherwise I would never have penetrated the veil of silence surrounding it. Richard Frankel wrote me an unsolicited four-page letter explaining lucidly to this physical dunce the magnetic properties of his fascinating bacteria (essay 30). I am always cheered and delighted by the generosity of colleagues; a thousand untold stories overbalance every eagerly recorded case of nastiness. I thank Frank Sulloway for telling me the true story of Darwin's finches (essay 5), Diane Paul, Martha Denckla, Tim White, Andy Knoll, and Carl Wunsch for references, insights, and patient explanation.

Fortunately, I write these essays during an exciting time in evolutionary theory. When I think of paleontology in 1910, with its wealth of data and void of ideas, I regard it as a privilege to be working today.

Evolutionary theory is expanding its domain of impact and explanation in all directions. Consider the current excitement in such disparate realms as the basic mechanics of DNA, embryology, and the study of behavior. Molecular evolution is now a full-fledged discipline that promises to provide both strikingly new ideas (the theory of neutrality as an alternative to natural selection) and resolution of many classical mysteries in natural history (see essay 24). At the same time, the discovery of inserted sequences and jumping genes reveals a new stratum of genetic complexity that must be pregnant with evolutionary meaning. The triplet code is only a machine language; a higher level of control must exist. If we can ever figure out how multicellular creatures regulate the timing involved in the complex orchestration of their embryonic growth, then developmental biology might unite molecular genetics with natural history into a unified science of life. The theory of kin selection has extended Darwinian theory fruitfully into the realm of social behavior, though I believe that its more zealous advocates misunderstand the hierarchical nature of explanation and try to extend it (by more than permissible analogy) to realms of human culture where it does not apply (see essays 7 and 8).

Yet, while Darwinian theory extends its domain, some of its cherished postulates are slipping, or at least losing their generality. The “modern synthesis,” the contemporary version of Darwinism that has reigned for thirty years, took the model of adaptive gene substitution within local populations as an adequate account, by accumulation and extension, of life's entire history. The model may work well in its empirical domain of minor, local, adaptive adjustment; populations of the moth
Biston betularia
did turn black, by substitution of a single gene, as a selected response for decreased visibility on trees that had been blackened by industrial soot. But is the origin of a new species simply this process extended to more genes and greater effect? Are larger evolutionary trends within major lineages just a further accumulation of sequential, adaptive changes?

Many evolutionists (myself included) are beginning to challenge this synthesis and to assert the hierarchical view that different levels of evolutionary change often reflect different kinds of causes. Minor adjustment within populations may be sequential and adaptive. But speciation may occur by major chromosomal changes that establish sterility with other species for reasons unrelated to adaptation. Evolutionary trends may represent a kind of higher-level selection upon essentially static species themselves, not the slow and steady alteration of a single large population through untold ages.

Before the modern synthesis, many biologists (see Bateson, 1922, in bibliography) expressed confusion and depression because the proposed mechanisms of evolution at different levels seemed contradictory enough to preclude a unified science. After the modern synthesis, the notion spread (amounting almost to a dogma among its less thoughtful lieutenants) that all evolution could be reduced to the basic Darwinism of gradual, adaptive change within local populations. I think that we are now pursuing a fruitful path between the anarchy of Bateson's day and the restriction of view imposed by the modern synthesis. The modern synthesis works in its appropriate arena, but the same Darwinian processes of mutation and selection may operate in strikingly different ways at higher domains in a hierarchy of evolutionary levels. I think that we may hope for uniformity of causal agents, hence a single, general theory with a Darwinian core. But we must reckon with a multiplicity of mechanisms that preclude the explanation of higher level phenomena by the model of adaptive gene substitution favored for the lowest level.

At the basis of all this ferment lies nature's irreducible complexity. Organisms are not billiard balls, propelled by simple and measurable external forces to predictable new positions on life's pool table. Sufficiently complex systems have greater richness. Organisms have a history that constrains their future in myriad, subtle ways (see essays of section I). Their complexity of form entails a host of functions incidental to whatever pressures of natural selection superintended the initial construction (see essay 4). Their intricate and largely unknown pathways of embryonic development guarantee that simple inputs (minor changes in timing, for example) may be translated into marked and surprising changes in output (the adult organism, see essay 18).

Charles Darwin chose to close his great book with a striking comparison that expresses this richness. He contrasted the simpler system of planetary motion, and its result of endless, static cycling, with the complexity of life and its wondrous and unpredictable change through the ages:

There is grandeur in this view of life, with its several powers, having been originally breathed into a few forms or into one; and that, whilst this planet has gone cycling on according to the fixed law of gravity, from so simple a beginning endless forms most beautiful and most wonderful have been, and are being, evolved.

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