Sunday, June 19, 2005

OBT: Stephen Jay Gould IV

Natural History, December 1997/January 1998, Vol. 106, No. 11, pp.12-18,
60-66 [Reprinted in Gould S.J., "The Paradox of the Visibly Irrelevant,"
in "The Lying Stones of Marrakech: Penultimate Reflections in Natural
History," [2000], Vintage: London, 2001, reprint, pp.333-)

The paradox of the visibly irrelevant
Stephen Jay Gould

Abstract: The belief that evolution is too slow to be observed is refuted by
the rapid evolution of bacteria, Trinidad guppies, and Bahama lizards and
snails. This can be blamed in part on experts who generalize the results of
short-term research. However, the long-term evolution of species remain

An odd principle of human psychology, well known and exploited by the
full panoply of prevaricators, from charming barkers like Barnum to evil
demagogues like Goebbels, holds that even the silliest of lies can win
credibility by constant repetition. In current American parlance, these
proclamations of "truth" by xeroxing -- if sufficiently benign to do little
harm, yet embraced with all the force of a dictum running "from God's
mouth to your ear" -- fall into the fascinating domain of "urban legends."

My favorite bit of nonsense in this category hits me daily, and in very large
type, thanks to a current billboard campaign by a company that win remain
nameless. The latest version proclaims: "Scientists say we use 10 percent of
our brains. That's way too much." Just about everyone regards the "truth"
of this proclamation as obvious and incontrovertible -- although you might
still start a barroom fight over whether the correct figure should be 10, 15,
or 20 percent (I have heard all three asserted with utter confidence). But
this particular legend is even worse than false, for the statement is truly
meaningless and nonsensical. What do we mean by "90 percent unused"?
What is all this superfluous tissue doing? The claim, in any case, can have
no meaning until we develop an adequate theory about how the brain
works. We don't even have a satisfactory account for the neurological basis
of memory and its storage -- surely the sine qua non for formulating any
sensible notion about unused percentages of brain matter! (I think that the
legend developed because we rightly sense that we ought to be behaving
with far more intelligence than we seem willing to muster -- and the
pseudoquantification of the urban legend acts as a falsely rigorous version
of this legitimate, but vague, feeling.)

In my field of evolutionary biology, the most prominent urban legend --
another "truth" known by "everyone" -- holds that evolution may well be
the way of the world, but one has to accept the idea with a dose of faith
because the process occurs far too slowly to yield any observable result in a
human lifetime. Thus, we can document evolution from the fossil record
and infer it from the taxonomic relationships of living species, but we
cannot see evolution on human timescales "in the wild."

In fairness, we professionals must shoulder some blame for this utterly false
impression about evolution's invisibility in the here and now of everyday
human life. Darwin himself -- although he knew and emphasized many
cases of substantial change in human time (including the development of
breeds in his beloved pigeons) -- tended to wax eloquent about the
inexorable and stately slowness of natural evolution. In a famous passage
from the Origin of Species, he even devised a striking metaphor about
clocks to underscore the usual invisibility:

It may be said that natural selection is daily
and hourly scrutinizing, throughout the
world, every variation, even the slightest;
rejecting that which is bad, preserving and
adding up all that is good, silently and
invisibly working... We see nothing of
these slow changes in progress until the hand
of time has marked the long lapse of ages.

Nonetheless, the claim that evolution must be too slow to see can only rank
as an urban legend -- although not a completely harmless tale in this case,
for our creationist incubi can then use the fallacy as an argument against
evolution at any scale, and many folks take them seriously because they just
"know" that evolution can never be seen in the immediate here and now. In
fact, a precisely opposite situation prevails: biologists have documented a
veritable glut of cases for rapid and eminently measurable evolution on
timescales of years and decades.

However, this plethora of documents -- while important for itself, and
surely valid as a general confirmation for the proposition that organisms
evolve -- teaches us rather little about rates and patterns of evolution at the
geological scales that build the history and taxonomic structure of life. The
situation is wonderfully ironic -- a point that I have tried to capture in the
title of this article. The urban legend holds that evolution is too slow to
document in palpable human lifetimes. The opposite truth has been a firmed
by innumerable cases of measurable evolution at this minimal scale -- but,
to be visible at all over so short a span, evolution must be far too rapid
(and transient) to serve as the basis for major transformations in geological
time. Hence, the "paradox of the visibly irrelevant" -- or, if you can see it at

all, it's too fast to matter in the long run.

Our best and most numerous cases have been documented for the dominant
and most evolutionarily active organisms on our planet -- bacteria. In the
most impressive of recent examples, Richard E. Lenski and Michael
Travisano (Proceedings of the National Academy of Sciences, vol. 91,
1994) monitored evolutionary change for 10,000 generations in twelve
laboratory populations of the common human gut bacterium, Escherichia
coli. By placing an twelve populations in identical environments, they could
study evolution under ideal experimental conditions of replication -- a rarity
for the complex and unique events of evolutionary transformation in
nature. In a fascinating set of results, they found that each population
reacted and changed differently, even within environments made as
identical as human observers know how to do. Yet, Lenski and Travisano
did observe some important and repeated patterns within the diversity. For
example, each population increased rapidly in average cell size for the first
2,000 generations or so, but then remained nearly stable for the last 5,000

But a cynic might still hold: Fine, I'll grant you substantial observable
evolution in the frenzied little world of bacteria, where enormous
populations and new generations every hour allow you to monitor 10,000
episodes of natural selection in a manageable time. But a similar
"experiment" would consume thousands of years for multicellular
organisms that measure generations in years or decades rather than minutes
or hours. So we may still hold that evolution cannot be observed in the big,
fat, furry, sexually reproducing organisms that serve as the prototype for
"life" in ordinary human consciousness. (A reverse cynic would then reply
that bacteria truly dominate life and that vertebrates represent only a late-
coming side issue in the full story of evolution, however falsely promoted
to centrality by our own parochial focus. But we must leave this deep issue
to another time.)

I dedicate this essay to illustrating our cynic's error. For obvious reasons,
bacteria may provide our best and most consistent cases, but measurable
(and substantial) evolution has also, and often, been documented in
vertebrates and other complex multicellular organisms. The classic cases
have not exactly been hiding their light under a bushel, so I do wonder why
the urban legend of evolution's invisibility persists with such strength.
Perhaps the firmest and most elegant examples involve a group of
organisms named to commemorate our standard bearer himself -- Darwin's
finches of the Galapagos Islands, where my colleagues Peter and Rosemary
Grant have spent many years documenting fine-scale evolution in such
adaptively important features as size and strength of the bill (a key to the
mechanics of feeding) as rapid climatic changes force an alteration of food
preferences. This work formed the basis for Jonathan Weiner's excellent
and best-selling book, The Beak of the Finch -- so the story has certainly
been well and prominently reported in both the technical and popular press.

Nonetheless, new cases of such short-term evolution still maintain
enormous and surprising power to attract public attention -- for interesting
and instructive, but utterly invalid, reasons as I shall show. Let us consider
the three most prominent examples published during the past year. (One
derives from my own research and publication, so at least I can't be
accused of sour grapes in the debunking that will follow -- although I trust
that readers will also grasp the highly Positive twist that I will ultimately
impose upon my criticisms.) The coincident geography of all three cases
formed no part of my intention for this essay. I did not even know that the
editor of Natural History had planned a special issue on the Caribbean. But
all three cases -- Trinidadian fishes and Bahamian snails and lizards --
happen to fall within the spatial focus of this issue. As I said, measurable.
short-term evolution is not a rare phenomenon at all, urban legends
notwithstanding. I shall briefly describe each case, then present my two
general critiques of their prominent reporting by the popular press, and
finally explain why such cases teach us so little about evolution in the large,
yet remain so important for themselves, and at their own equally legitimate

1. Guppies from Trinidad. In many drainage systems on the island of
Trinidad, populations of guppies live in downstream pools, where several
species of fish can feed upon them. "Some of these species prey
preferentially on large, mature-size classes of guppies." (I take all quotes
from the primary technical article that inspired later press accounts.
"Evaluation of the Rate of Evolution in Natural Populations of Guppies
[Poecilia reticulata]," by D. N. Reznick, F. H. Shaw, F. H. Rodd, and R. G.
Shaw, published in Science, vol. 275, 1977.) Other populations of the same
species live in "upstream portions of each drainage," where most
"predators are excluded ... by rapids or waterfalls, yielding low-predation

In studying both kinds of populations, Reznick and colleagues found that
"guppies from high-predation sites experience significantly higher mortality
rates than those from low-predation sites." They then reared both kinds of
guppies under uniform conditions in the laboratory and found that fishes
from high-predation sites in lower drainages matured earlier and at a
smaller size. "They also devote more resources to each litter, produce
more, smaller offspring per litter, and produce litters more frequently than
guppies from low-predation localities."

This combination of observations from nature and the laboratory yields two
important inferences. First, the differences make adaptive sense, for
guppies subjected to greater predation would fare better if they could grow
up fast and reproduce both copiously and quickly before the potential
boom falls -- a piscine equivalent of the old motto for electoral politics in
Boston: vote early and vote often. On the other hand, guppies in little
danger of being eaten might do better to bide their time and grow big and
strong before engaging their fellows in any reproductive competition.
Second, since these differences persist when both kinds of guppies are
reared in identical laboratory environments, they must be based upon
evolved and inherited distinctions between the populations.

In 1981, Peznick transferred some guppies from high-predation
downstream pools into low-predation upstream waters then devoid of
guppies. These transplanted populations evolved rapidly to adopt the
reproductive strategy favored by indigenous populations in neighboring
upstream environments: delayed sexual maturity at a larger size, and longer
life. Moreover, Reznick and colleagues made the interesting observation
that males evolved considerably more rapidly in this favored direction. In
one experiment, males reached their fun extent of change within four years,
while females were still evolving after eleven years. Since the laboratory
populations had shown higher heritability for these traits in males than in
females, these results make good sense. (Heritability is, roughly, the
correlation between traits in parent and offspring due to genetic
differences. The greater the heritable basis of a trait, the faster it can
by natural selection.)

This favorable set of circumstances -- rapid evolution in a predictable and
presumably adaptive direction based on traits known to be higly heritable --
provides a "tight" case for well-documented (and sensible) evolution at
scales well within the purview of human observation, a mere decade in this
case. The headline for the news report of this paper in Science magazine
(March 28, 1997) read "Predator-free Guppies Take an Evolutionary Leap

2. Lizards from the Exuma Cays, Bahama Islands. During most of my
career, my fieldwork has centered on the biology and paleontology of the
land snail Cerion in the Bahama Islands. During these trips, I have often
encountered fellow biologists devoted to other creatures; we all, I trust,
feel a shared bond of collegiality and comradeship. In one major program
of research, Tom Schoener (a biology professor at the University of
California, Davis) has, with numerous students and colleagues, been
studying the biogeography and evolution of the ubiquitous little lizard
Anolis (see "Darwin's Lizards," page 34) -- for me just a fleeting shadow
running across a snail-studded ground, but for them a focus of utmost
fascination (while my beloved snails must just blend into their immobile

In 1977 and 1981, Schoener and colleagues transplanted populations of
five to ten lizards from Staniel Cay in the Exuma chain to fourteen small,
neighboring islands that housed no lizards. In 1991, they found that the
lizards had thrived (or at least survived and bred) on most of these islands,
and they collected samples of adult males from each island with an
adequate population. In addition, they gathered a larger sample of males
from areas on Staniel Cay that had served as the source for original

This study then benefits from general principles learned by extensive
research on numerous Anolis species throughout the Bahamas. In
particular, relatively longer limbs permit greater speed, a substantial
advantage provided that preferred perching places can accommodate long
legged lizards. Trees and other "thick" perching places therefore favor the
evolution of long legs. Staniel Cay itself includes a predominant forest, and
the local Anolis tend to be long legged. But when lizards must live on thin
twigs in bushy vegetation, the agility provided by shorter legs (on such
precarious perches) may outweigh the advantages in speed that longer legs
would provide. Thus, lizards living on narrow twigs tend to be shorter
legged. The small cays that received the fourteen transported populations
tend to lack forest and to be covered with bushy vegetation (and narrow

J.B. Losos, the principal author of the new study, therefore based an
obvious prediction on these generalities. The populations had been
transferred from forests with wide perches to bushy islands covered with
narrow twigs. "From the kind of vegetation on the new islands," Losos
stated, "we predicted that the lizards would develop shorter hindlimbs."
The study, published in Nature, vol. 387, 1997, validates this expected
result: a clearly measurable change, in the predicted and adaptive direction,
in less than twenty years.

This study lacks a crucial piece of documentation that the Trinidadian
guppies did provide -- an absence immediately noted by friendly critics and
fully acknowledged by the authors. Losos and colleagues have not studied
the heritability of leg length in Anolis sagrei and therefore cannot be certain
that their results record a genetic process of evolutionary change. Perhaps
leg length is development mentally plastic, so that the same genes yield
longer legs if lizards grow up on trees and shorter legs if they always
cavort in the bushes (just as the same genes can lead to a thin or fat human
being depending upon a personal history of nutrition and exercise). In any
case, however, a sensible and apparently adaptive change in average leg
length has occurred within twenty years on several islands, whatever the
cause of modification.

3. Snails from Great Inagua, Bahama Islands. Most of Great Inagua, the
second largest Bahamian island (Andros wins first prize), houses a large
and ribby Cerion species named C. rubicundum. But fossil deposits of no
great age lack this species entirely and feature instead an extinct form
named C. excelsior, the largest of all Cerion species. Several years ago, on
a mud flat in the southeastern comer of Great Inagua, David Woodruff (of
the University of California, San Diego) and I corrected a remarkable series
of shells that seemed to span (and quite smoothly) the entire range of form
from extinct C. excelsior to modern C. rubicundum. Moreover, and in
general, the more eroded and "older looking" the shell, the closer it seemed
to lie to the anatomy of extinct C. excelsior.

This situation suggested a local evolutionary transition by hybridization, as
C. rubicundum, arriving on the island from an outside source, interbred
with indigenous C. excelsior. Then, as C. excelsior declined toward
extinction while C. rubicundum thrived and increased, the average anatomy
of the population transformed slowly and steadily in the direction of the
modern form. This hypothesis sounded good and sensible, but we could
devise no way to test our idea -- for all the shells had been collected from a
single mud flat (analogous to a single bedding plane of a geological
stratum), and we could not determine their relative ages. The pure C.
excelsior shells "looked" older, but such personal impressions (subject as
they are to a researcher's bias) count for less than nothing in science. So we
were stymied and put the specimens in a drawer.

Several years later, I teamed up with paleontologist and geochemist Glenn
A. Goodfriend, from the Carnegie Institution of Washington. He had
refined a dating technique based on changes in the composition of amino
acids in the shell over time. By keying these amino acid changes to
radiocarbon dates for some of the shells, we were able to estimate the age
of each shell. A plot of shell age versus position on an anatomical spectrum
from extinct C. excelsior to modern C. rubicundum produced a beautiful
correlation between age and anatomy: the younger the specimen, the closer
to the modern anatomy.

This 10,000- to 20,000-year transition by hybridization exceeds the
Trinidad and Exuma studies by three orders of magnitude in time (that is,
1,000-fold), but even 10,000 years represents a geological eyeblink in the
fullness of evolutionary time. The transformation marks a full change from
one species to another, not just a small decrement of leg length or a change
in the timing of breeding within a single species. (For details, see G. A.
Goodfriend and S.J. Gould, "Paleontology and Chronology of Two
Evolutionary Transitions by Hybridization in the Bahamian Land Snail
Cerion," Science, vol. 274, 1996). Harvard University's press release (with
no input from me) carried the headline "Snails Caught in Act of Evolving."

A scanning of any year's technical literature in evolutionary biology would
yield numerous, well-documented cases of such measurable, small-scale
evolutionary change -- thus disproving the urban legend that evolution
must always be too slow to observe in the geological microsecond of
human lifetime. These three studies, all notable for their documentation and
for their wrapping up of details, do not really rank as "news" in the
journalist's prime sense of novelty or deep surprise. Nonetheless, these
three stories share another interesting feature, sociological rather than
scientific this time: all became subjects for front-page stories in either the
New York Times or Boston Globe.

Now please don't get me wrong. I am not one of those rarefied academics
who cringes at every journalistic story about science for fear that the work
reported might thereby become tainted with popularity. And, in a purely
"political" sense, I certainly won't object if major newspapers choose to
feature any result of my profession as a lead story -- especially, if I may be
self-serving for a moment, when one of the tales reports my own work!
Nonetheless, this degree of public attention for workaday results in my
field (however elegantly done) does fill me with wry amusement -- if only
for the general reason that most of us feel a tickle in the funny bone when
we note a gross imbalance between public notoriety and the true novelty or
importance of an event, as when Hollywood spinmeisters manage to depict
their client's ninth marriage as the earth's first example of true love
triumphant and permanent.

So thanks, fellas. I'm really glad you reported some ordinary, but
particularly well done, studies of small-scale evolution as front-page news.
But I still feel compelled to ask why these studies, rather than one hundred
others of equal care and merit that appear in our literature every month,
caught your fancy and inspired such prime attention. When I muse over this
issue, I can only come up with two reasons -- based on deep and interesting
fallacies well worth identifying and discussing. In this sense, the
miselevation of everyday good work to surprising novelty may teach us
something important about public attitudes toward evolution and toward
science in general. We may, I think, resolve each of the two fallacies by
contrasting the supposed meaning of these studies, as reported in public
accounts, with the significance of such work as viewed by professionals in
the field.

1. The fallacy of the crucial experiment. In high-school physics classes, we
all learned a heroically simplified version of scientific progress based upon
a model that does work sometimes, but by no means always -- the
experimentum crucis, or crucial experiment. Newton or Einstein? Ptolemy
or Copernicus? Special creation or Darwin? To find out, perform a single
decisive experiment with a clearly measurable result full of power to decree
yea or nay. Throw the accused witch in the pond; if she sinks, she was
innocent (however dead by drowning).

The decision to treat a limited and particular case as front-page news must
be rooted in this fallacy. Reporters must imagine that evolution can be
proved by a single crucial case, so that any of these stories may provide
decisive confirmation of Darwin's truth -- a matter of some importance
given the urban legend that evolution, even if valid, must be invisible on
human timescales.

But two counter-arguments vitiate this premise. First, as a scientific or
intellectual issue, we hardly need to "prove" evolution by discovering new
and elegant cases (the analog, perhaps, of the Marine Corps's search for "a
few good men" -- read "people"). We do not, after all, expect to encounter
a page-one story with the headline "New Experiment Proves Earth Goes
Around Sun, Not Vice Versa. Galileo Vindicated." The fact of evolution
has been equally well documented for more than a century.

Second, and more generally, single "crucial" experiments rarely decide
major issues in science -- especially in natural history, in which nearly all
theories require data about "relative frequencies" (or percentage of
occurrences), not pristine single cases. Of course, for a person who
believes that evolution never occurs at all, one good case can pack
enormous punch, but this basic issue was adequately resolved more than
one hundred years ago. Nearly every interesting question in evolutionary
theory asks "how often" or "how dominant in setting the pattern of life" --
not "does this phenomenon occur at all?" For example, on the most
important issue of all -- the role of Darwin's own favored mechanism of
natural selection -- single examples of selection's efficacy advance the
argument very little. We already know, by abundant documentation and
rigorous theorizing, that natural selection can and does operate in nature.
We need to determine the relative strength of Darwin's mechanism among a
set of alternative modes for evolutionary change -- and single cases,
however elegant, cannot establish a relative frequency.

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 potent 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 hones coin cannot stray too far from this

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 dominance of natural
selection in general. Losos has only shown the power of Darwin's process
in this 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 animal 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 refer only to large objects themselves, and not to
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 for science in any case? I think not, and I also regard our best
general reason for skepticism as conclusive for this subject -- however
rarely appreciated, although 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 strikes most people as deeply paradoxical, even funny -- but the
argument truly cannot be gainsaid. Evolutionary rates of a moment, 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 my 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?

We have discerned something important, but we 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 darwins. 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 -- 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, is the vastness of time (a concept that
we can appreciate "in our heads" but seem quite unable to get into the guts
of our intuition). The key principle, however ironic, requires such a visceral
understanding of earthly time: if evolution is fast enough to be discerned by
our instruments in just a few years -- that is, 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
such evolution is far too fast to serve as an atom of steady incrementation
in a paleontological trend. Thus, 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, 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! Most lineages are stable
(nonchanging) nearly all the time in the fossil record. When lineages do
change, the alteration is usually "momentary" in a geological sense (that is,
confined to a single bedding plane of a stratum) 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 no change accumulates most of the time, and we need to
understand why. The sources of stasis are as important for evolutionary
theory as the causes of change.

(To illustrate how poorly we grasp this central point of 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 how I had found an exception to my own
theory of punctuated equilibrium -- an insensibly gradual change over
10,000 to 20,000 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 20,000 years' worth of snails on a
single mud flat -- 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, such as 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 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
just 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 is more important than any other; none operates as a basic
model for all the others. Each has something precious and unique to teach
us; none is 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 chief illustration of last month's essay -- Mandelbrot's familiar
argument that the coast of Maine has no absolute length, but depends upon
the scale of measurement -- also epitomizes this month's theme. When we
study guppies in a pond in Trinidad, we are 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 4 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
an summer measuring the details in one cove of Acadia if you just wanted
to know the distance from Portland to Machiasport for your weekend auto

I find a particular intellectual beauty in such fractal models, for they use
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 single-page map of

But you can love and learn from both scales at the same time. Evolution
does not lie 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 geology's irrelevant and invisible jigglings [in the majesty
of geological time]*.


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