The Lying Stones of Marrakech (54 page)

Professionals also commit this common error of confusing well-documented single instances with statements about relative strength among plausible alternatives. For example, we would like to know how often small and isolated populations evolve differences as adaptive responses to local environments (presumably by Darwin's mechanism of natural selection), and how often such changes occur by the random process known as “genetic drift”—a potentially important phenomenon in small populations (just as a small number of coin flips can depart radically from fifty-fifty for heads and tails, while a million flips with an honest coin cannot stray too far from this ideal). Losos's study on lizard legs provides one vote for selection (if the change turns out to have a genetic basis)—because leg length altered in a predicted direction toward better adaptation to local environments on new islands. But even such an elegant case cannot prove the domination of natural selection in general. Losos has only shown the power of Darwin's process in a particular example. Yet the reporter for
Science
magazine made this distressingly common error in concluding: “If it [change in leg length] is rooted in the genes, then the study is strong evidence that isolated populations diverge by natural selection, not genetic drift as some theorists have argued.” Yes, strong evidence for these lizards on that island during those years—but not proof for the general domination of selection over drift. Single cases don't establish generalities, so long as alternative mechanisms retain their theoretical plausibility.

2.
The paradox of the visibly irrelevant

As a second reason for overstating the centrality of such cases in our general understanding of evolution, many commentators (and research scientists as well) ally themselves too strongly with one of the oldest (and often fallacious)
traditions of Western thought: reductionism, or the assumption that laws and mechanics of the smallest constituents must explain objects and events at all scales and times. Thus, if we can render the behavior of a large body (an organism or a plant, for example) as a consequence of atoms and molecules in motion, we feel that we have developed a “deeper” or “more basic” understanding than if our explanatory principles engage only large objects themselves, and not their constituent parts.

Reductionists assume that documenting evolution at the smallest scale of a few years and generations should provide a general model of explanation for events at all scales and times—so these cases should become a gold standard for the entire field, hence their status as front-page news. The authors of our two studies on decadal evolution certainly nurture such a hope. Reznick and colleagues end their publication on Trinidadian guppies by writing: “It is part of a growing body of evidence that the rate and patterns of change attainable through natural selection are sufficient to account for the patterns observed in the fossil record.” Losos and colleagues say much the same for their lizards: “Macroevolution may just be microevolution writ large—and, consequently, insight into the former may result from study of the latter.”

We tend to become beguiled by such warm and integrative feelings (for science rightly seeks unity and generality of explanation). But does integration by reduction of all scales to the rates and mechanisms of the smallest really work for evolution—and do we crave this style of unification as the goal of all science? I think not, and I also regard our best general reason for skepticism as conclusive for this particular subject—however rarely appreciated, though staring us in the face.

These shortest-term studies are elegant and important, but they cannot represent the general mode for building patterns in the history of life. The reason for their large-scale impotence strikes most people as deeply paradoxical, even quite funny—but the argument truly cannot be gainsaid. Evolutionary rates as measured for guppies and lizards are
vastly too rapid
to represent the general modes of change that build life's history through geological ages.

But how can I say such a thing? Isn't this statement ridiculous a priori? How could these tiny, minuscule changes—a little less leg, a minimally larger size—represent too much of anything? Doesn't the very beauty of these studies lie in their minimalism? We have always been taught that evolution is wondrously slow and cumulative—a grain-by-grain process, a penny a day toward the domain of Bill Gates. Doesn't each of these studies document a grain? Haven't my colleagues and I found the “atom” of evolutionary incrementation?

I believe that these studies have discerned something important, but they have discovered no general atom. These measured changes over years and decades are too fast by several orders of magnitude to build the history of life by simple cumulation. Reznick's guppy rates range from 3,700 to 45,000 darwins (a standard metric for evolution, expressed as change in units of standard deviation—a measure of variation around the mean value of a trait in a population—per million years). By contrast, rates for major trends in the fossil record generally range from 0.1 to 1.0 darwin. Reznick himself states that “the estimated rates [for guppies] are … four to seven orders of magnitude greater than those observed in the fossil record” (that is, ten thousand to ten million times faster).

Moreover and with complete generality—thus constituting the “paradox of the visibly irrelevant” in my title—we may say that any change measurable
at all
over the few years of an ordinary scientific study must be occurring far too rapidly to represent ordinary rates of evolution in the fossil record. The culprit of this paradox, as so often, can be identified as the vastness of time (a concept that we can appreciate “in our heads” but seem quite unable to place into the guts of our intuition). The key principle, however ironic, requires such a visceral understanding of earthly time: if a case of evolution proceeds with sufficient speed to be discerned by our instruments in just a few years—that is, if the change becomes substantial enough to stand out as a genuine and directional effect above the random fluctuations of nature's stable variation and our inevitable errors of measurement—then we have witnessed something far too substantial to serve as an atom of steady incrementation in a paleontological trend. Thus, to restate the paradox: if we can measure it at all (in a few years), it is too powerful to be the stuff of life's history.

If large-scale evolution proceeded by stacking Trinidad guppy rates end to end, then any evolutionary trend would be completed in a geological moment, not over the many million years actually observed. “Our face from fish to man,” to cite the title of a famous old account of evolution for popular audiences, would run its course within a single geological formation, not over more than 400 million years, as our fossil record demonstrates.

Evolutionary theory must figure out how to slow down these measured rates of the moment, not how to stack them up! In fact, most lineages are stable (
non
changing) nearly all the time in the fossil record. When lineages do change, their alteration usually occurs “momentarily” in a geological sense (that is, confined to a single bedding plane) and usually leads to the origin of a new species by branching. Evolutionary rates during these moments may match the
observed speed of Trinidadian guppies and Bahamian lizards—for most bedding planes represent several thousand years. But during most of a typical species's lifetime, no change accumulates, and we need to understand why. The sources of stasis have become as important for evolutionary theory as the causes of change.

(To illustrate how poorly we grasp this central point about time's immensity, the reporter for
Science
magazine called me when my
Cerion
article, coauthored with Glenn Goodfriend, appeared. He wanted to write an accompanying news story about the exception I had found to my own theory of punctuated equilibrium—an insensibly gradual change over ten to twenty thousand years. I told him that, although exceptions abound, this case does not He among them, but actually represents a strong confirmation of punctuated equilibrium! We found all twenty thousand years' worth of snails on a single mudflat—that is, on what would become a single bedding plane in the geological record. Our
entire
transition occurred in a geological moment and represented a punctuation, not a gradual sequence of fossils. We were able to “dissect” the punctuation in this unusual case—hence the value of our publication—because we could determine ages for the individual shells. The reporter, to his credit, completely revised his originally intended theme and published an excellent account.)

In conclusion, I suspect that most cases like the Trinidadian guppies and Bahamian lizards represent transient and momentary blips and fillips that “flesh out” the rich history of lineages in stasis, not the atoms of substantial and steadily accumulated evolutionary trends. Stasis is a dynamic phenomenon. Small local populations and parts of lineages make short and temporary forays of transient adaptation, but these tiny units almost always die out or get reintegrated into the general pool of the species. (Losos himself regards the new island populations of lizards as evolutionarily transient in exactly this sense—for such tiny and temporary colonies are almost always extirpated by hurricanes in the long run. How; then, can such populations represent atoms of a major evolutionary trend? The news report in
Science
magazine ends by stating: “But whether the lizards continue to evolve depends largely on the winds of fate, says Losos. These islets are periodically swept by hurricanes that could whisk away every trace of anolian evolution.”)

But transient blips and fillips are no less important than major trends in the total “scheme of things.” Both represent evolution operating at a standard and appropriate measure for a particular scale and time—Trinidadian blips for the smallest and most local moment, faces from fish to human for the largest and
most global frame. One scale doesn't translate into another. No single scale can be deemed more important than any other; and none operates as a basic model for all the others. Each scale embodies something precious and unique to teach us; none can be labeled superior or primary. (Guppies and lizards, in their exposition of momentary detail, give us insight, unobtainable at broader scales, into the actual mechanics of adaptation, natural selection, and genetic change.)

The common metaphor of the science of fractals—Mandelbrot's familiar argument that the coast of Maine has no absolute length, but depends upon the scale of measurement—epitomizes this principle well (see chapter 23). When we study guppies in a pond in Trinidad, we are operating at a scale equivalent to measuring the coastline by wrapping our string around every boulder on every headland of Acadia National Park. When we trace the increase in size of the human brain from Lucy (about four million years ago) to Lincoln, we are measuring the coastline as depicted on my page of Maine in
Hammond's Atlas
. Both scales are exactly right for their appropriate problems. You would be a fool to spend all summer measuring the details in one cove in Acadia, if you just wanted to know the distance from Portland to Machiasport for your weekend auto trip.

I find a particular intellectual beauty in such fractal models—for they invoke hierarchies of inclusion (the single cove embedded within Acadia, embedded within Maine) to deny hierarchies of worth, importance, merit, or meaning. You may ignore Maine while studying the sand grain, and be properly oblivious of the grain while perusing the map of Maine on the single page of your atlas. But you can love and learn from both scales at the same time. Evolution does not he patent in a clear pond on Trinidad any more than the universe (
pace
Mr. Blake) lies revealed in a grain of sand. But how poor would be our understanding—how bland and restricted our sight—if we could not learn to appreciate the rococo details that fill our immediate field of vision, while forming, at another scale, only some irrelevant and invisible jigglings in the majesty of geological time.

G
OLGOTHA, THE SITE OF CHRIST'S CRUCIFIXION,
appears in most paintings as a substantial hill in the countryside, far from the city walls of Jerusalem depicted in a distant background. In fact, if the traditional spot has been correctly identified, Golgotha is a tiny protuberance located just next to the old city limits but now inside the walls built by Suleiman the Magnificent in the early sixteenth century. These walls extended the boundaries of Jerusalem, and the old town now sits as a small “jewel” at the center of a much bigger, modern city. Golgotha is small and low enough to fit
within
the Church of the Holy Sepulchre, located
within
Suleiman's city walls. Visitors just have to climb an internal staircase to reach the top of Golgotha, located on the church's second story. (Several theories compete to explain the derivation of the name, for
golgotha
means “skull” in Aramaic, while the alternative label of
calvary
has the same definition in Latin. Most scholars think that the name designates the shape of the small hill, not the mortal remains of executions.)

As one of the most sacred sites on earth, the Church of the Holy Sepulchre might be expected to exude dignity, serenity, and a spirit of transcendence above merely earthly cares. Yet in maximal, almost perverse contrast, the church is a site of constant bickering and division. The etymology of
religion
may refer to “tying together,” but the actual experience, given the propensities
of Homo sapiens
, the earth's most various and curmudgeonly species, tends more often to separation and anathematization. The precious space is “shared” (in this case, a euphemism for “wrangled over”) by six old Christian groups—Greek Orthodox, Roman Catholic, Armenian, Syrian, Coptic, and Abyssinian. (The various Protestant denominations came upon the scene a few centuries too late and didn't even get a pew.)