Opus 200
by Stephen Jay Gould
n
my adopted home of Puritan New England, I have learned that personal indulgence
is a vice to be tolerated only at rare intervals. Combine this stricture with
two further principles and this essay achieves its rationale: first, that we
celebrate in hundreds and their easy multiples (the Columbian quincentenary and
the fiftieth anniversary of DiMaggio's hitting streak—both about equally
important, and only the latter an unambiguous good); second, that geologists
learn to take the long view.
This is my 200th essay in "This View of Life," a series
begun in January 1974 (with never an issue missed). If once is an incident and
twice a tradition, I now establish an indulgence for even multiples of 100
only. Essays must express the personal thoughts and prejudices of
authors—for this has been the genre's definition ever since Montaigne. But
I have tried not to abuse the bully pulpit of this forum to act as a shill for
my own professional research and theorizing. Still, once in a hundred shouldn't
subvert my bonae fides, so I indulge, as I did once before at my own
centenary.
Essay 100 treated the Bahamian land snails that usurp the
bulk of my time for empirical research. Opus 200 shall discuss the theoretical
idea most central to my work—punctuated equilibrium. Moreover, Niles
Eldredge and I formulated the theory of punctuated equilibrium in 1971, and I
can scarcely resist the double whammy of 200 essays on the
twentieth
anniversary of "punk eke," the affectionate nickname used by supporters,
while detractors have parried with "evolution by jerks."
Punctuated equilibrium began, as so much else that later
looms large in our lives, as a little path that might never have opened.
Paleontology, as the study of life's history, should be a jewel among the
geological sciences; what subject could be more fascinating? Yet, until
recently, it languished with an unjust reputation as a dull exercise in
descriptive cataloging. Paleontologists were stereotyped as narrow specialists
in mind-numbing particulars of favorite groups, places, and times.
Nature, the British professional journal of science, editorialized about
us in 1969:
Scientists in general might be excused for
assuming that most geologists are paleontologists and most paleontologists have
staked out a square mile as their life's work. A revamping of the geologist's
image is badly needed.
During the 1960s, tumultuous for other reasons, a group of
young paleontologists (including Eldredge and myself, then in that blessed
stage of ontogeny) worked hard to reverse this image and to recapture the high
reputation merited by more than 3 billion years of evolution. We felt that
evolutionary theory provided the context for such a revitalization, and that
the exclusively geological training then so traditional for paleontologists had
fostered a reputation for dullness by excising the intellectual heart of the
subject and leaving only the descriptive task of identifying fossils to tell
the age of rocks and the environments of their formation.
We pursued our studies in biology as well and tried to use
the latest concepts of evolutionary theory as a new foundation for interpreting
life's history. We called our study paleobiology and eventually founded a
journal of the same name,
intellectually vital (sometimes frenetically so) and even profitable since its
initial year in 1975. As a prelude, the late Tom Schopf sponsored a symposium
at the 1971 meeting of the Geological Society of America in Washington,
D.C.—Models in Paleobiology (published as a book in 1972).
Tom gathered all the young Turks and asked us each to apply
a subject of evolutionary theory to the fossil record. He assigned me
"speciation." I replied that I didn't know anything useful about speciation and
would rather do "morphology" or "rates of change." He told me that he had
already assigned these topics and that I had better "proceed" (euphemism) with
speciation "or get off the pot." (I loved Tom, but he had a peremptory streak.)
So I was stuck with speciation.
I wracked my brains, and my thoughts wandered back to
graduate school, six years earlier, at the American Museum of Natural History
(nose to the grindstone, but at least you could read this magazine for free
each month). Niles Eldredge and I, as fellow students, had talked endlessly
about potential reforms that a proper integration of evolutionary theory could
bring to paleontology. Our most promising insight centered upon speciation, the origin of new and distinct biological
populations. The prevailing opinion of paleontologists was mired in the deepest
bias of Darwin's world view. Darwin, following Lyell's lead in geology, preferred
to interpret substantial change as the insensibly gradual, incremental building
of adaptation, tiny piece by tiny piece, generation by generation. Darwin used
a striking metaphor to express his conviction that results of vast scale arise
from minor inputs summed over geological immensity; the hero of evolution is
time.
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 insensibly working….We see nothing of these slow changes in
progress until the hand of time has marked the long lapse of ages
[Origin of Species,
1859].
Beguiled by this vision, most paleontologists envisioned new
species as arising by the insensibly slow and steady change of entire
populations over long stretches of time, even by geological standards—a
notion known as gradualism. Under this model, "the species problem in
paleontology"—I put the phrase in quotes because it then resounded through
our literature as a catechism—centered upon the difficulty of stating
where ancestral species A ended and descendant species B began in such a
continuously graded transition (the problem, so formulated, has no objective
answer, only an arbitrary one).
And yet, while thus stating the issue in general writings,
all paleontologists knew that the practical world of fossil collecting rarely
imposed such a dilemma. The oldest truth of paleontology proclaimed that the
vast majority of species appear fully formed in the fossil record and do not
change substantially during the long period of their later existence (average
durations for marine invertebrate species may be as high as 5 to 10 million
years). In other words, geologically abrupt appearance followed by subsequent
stability.
But how could traditional paleontology live with such a
striking discordance between a theoretical expectation of gradual transition
and the practical knowledge of stability and geologically abrupt appearance as
the recorded history of most species? Our colleagues resolved their
schizophrenia by taking refuge in a traditional argument, advanced with
special ardor by Darwin himself—the gross imperfection of the fossil
record. If true history is continuous and gradational, but only one step in a
thousand is preserved as geological evidence, then a truly gradual sequence
becomes a series of abrupt transitions. Darwin staked his whole argument on
this proposition:
The geological record [is] extremely imperfect,
and will to a large extent explain why we do not find interminable varieties,
connecting together all the extinct and existing forms of life by the finest
graduated steps. He who rejects these views on the nature of the geological
record, will rightly reject my whole theory
[Origin of Species,
1859].
This resolution worked in some logical sense, but it filled
Niles and me with frustration and sadness. We were young, ambitious,
enthusiastic, and in love with our subject. We had trained ourselves in
evolutionary theory, particularly in the application of statistical methods to
the measurement of evolutionary change, and we longed to get our hands dirty
with practical applications. Our colleagues had virtually defined evolution as
gradual change and had then eviscerated the subject as a paleontological topic
by citing the imperfection of the fossil record to explain why we never (or so
very rarely) saw direct evidence for the process that supposedly made life's
history. This argument did resolve a contradiction (theoretical gradualism
with overt punctuation), but at a crushing price for any practicing scientist,
for if evolution meant gradual change, we could not discern the very
phenomenon we most wished to study.
If this argument were sound, then so be it. Catch-22 rears
its ugly head in many variations; sometimes you have to admit the intractable
and move on to something else. But Niles and I realized that our evolutionary
training, then rare for paleontologists, suggested an alternative reading full
of fascination for its theoretical implications and promising as an honorable
exit from the chill that Darwin's "argument from imperfection" had imposed
upon evolutionary studies in the fossil record. (Before I forget, let me
record that the ideas came mostly from Niles, with yours truly acting as a
sounding board and eventual scribe. I coined the term punctuated
equilibrium and wrote most of our
1972 paper,
but Niles is the proper first author in our pairing of Eldredge and Gould.)
The idea that we eventually called punctuated equilibrium
had two sources and one overriding purpose—to provide an exit from the
"disabling rescue" of Darwin's argument on imperfection. First, a statement
about mode of change: Most new species do not arise by transformation of
entire ancestral populations but by the splitting (branching) of a lineage
into two populations. Niles and I had learned the standard evolutionary
version of speciation by branching—a notion popularized by
Ernst Mayr and called by him the allopatric theory.
Allopatric means "in another place," and the theory argues that new
species may arise when a small population becomes isolated at the periphery
of the parental geographic range. Isolation can occur by a variety of geological
and geographic contingencies—mountains rising, rivers changing course,
islands forming. Without geographic isolation, favorable variants will not
accumulate in local populations, for breeding with parental forms is a remarkably
efficient way to blur and dilute any change that might otherwise become
substantial enough to constitute a new species. Most peripherally isolated
populations never become new species; they die out or rejoin the larger
parental mass. But as species may have no other common means of origin, even
a tiny fraction of isolated populations provide more than enough "raw material"
for the genesis of evolutionary novelty.
Second, a statement about rate of change. The simple claim
that species arise by splitting, and not by transformation of entire ancestral
populations, does not guarantee punctuated equilibrium. Suppose that most
splitting events divide large populations into two units of roughly equal
size, which then change at the conventional gradualistic rate. Splitting
events, in this scenario, would yield two examples of gradualism—and the
case for punctuated equilibrium would be compromised, not strengthened.
Punctuated equilibrium gains its rationale from the ideal also a standard
component of the allopatric speciation theory, that most peripherally isolated
populations are relatively small and undergo their characteristic changes at a
rate that translates into geological time as an instant.
For a variety of reasons, small isolated populations have
unusual potential for effective change: for example, favorable genes can
quickly spread throughout the population, while the interaction of random
change (rarely important in large populations) with natural selection provides
another effective pathway for substantial evolution. Even with these
possibilities for accelerated change, the formation of a new species from a
peripherally isolated population would be glacially slow by the usual standard
of our lifetimes. Suppose the process took five to ten thousand years. We
might stand in the midst of this peripheral isolate for all our earthly days
and see nothing in the way of major change.
But now we come to the nub of punctuated equilibrium. Five
to ten thousand years may be an eternity in human time, but such an interval
represents an earthly instant in almost any geological situation—a single
bedding plane (not a gradual sequence through meters of strata). Moreover,
peripheral isolates are small in geographic extent and not located in the
larger area where parents are living, dying, and contributing their skeletons
to the fossil record.
What then is the expected geological expression of
speciation in a peripherally isolated population? The answer is, and must be,
punctuated equilibrium. The speciation event occurs in a geological instant
and in a region of limited extent at some distance from the parental
population. In other words, punctuated equilibrium—and not
gradualism—is the expected geological translation for the standard
account of speciation in evolutionary theory. Species arise in a geological
moment—the punctuation (slow by our standards, abrupt by the planet's).
They then persist as large and stable populations on substantial geological
watches, usually changing little (if at all) and in an aimless fashion about
an unaltered average—the equilibrium.
Most of our paleontological colleagues missed this insight
because they had not studied evolutionary theory and either did not know about
allopatric speciation or had not considered its translation to geological
time. Our evolutionary colleagues also failed to grasp the implication,
primarily because they did not think at geological scales. But whatever the
theoretical meaning of punctuated equilibrium, Niles and I were most pleased
by its practical and heuristic value. We had reinterpreted the fossil record
as an accurate reflection of evolution, rather than an embarrassment that
made reality (read gradualism) invisible by its imperfections. We gave
paleontologists something to do, a way to get hands dirty. Evolution can be
studied directly; change by the ordinary route of allopatric speciation is
palpable in geological evidence. Stop apologizing for natural imperfections
and get to work.
I showed our initial article to my father. He said, "This
is terrific; it will really shake things up." I replied, "Nobody will read it,
and no one will pay any attention." He was right. He usually was.
Punctuated equilibrium provoked a major brouhaha, still
continuing, but now in much more productive directions. With my vicennial
perspective, I can identify both bad and good in the extensive debate. Three
points stand out on the negative side. First, simple misunderstanding of
basic content was distressingly common, even among professional evolutionists.
Many colleagues thought that we had raised the old anti-Darwinian specter of
macromutationism, or truly sudden speciation in a single generation by a
large and incredibly lucky mutation. I do not know why this happened; I think
that all our articles and public statements were clear in separating human
from geological rapidity. The theory, after all, is rooted in this
distinction—for punctuated equilibrium is the recognition that gradualism
on our mortal measuring rod of three score years and ten translates to
suddenness at the planet's temporal scale. Many less than adequate press
reports conveyed this disabling confusion (others were
very
accurate). Some colleagues probably read no further. For others, the very
word "sudden" raised such hackles that anger displaced critical thought, and
they never probed the key distinction.
Second, the theory became an issue (quite coincidentally)
just when creationism reached its acme of thankfully temporary influence.
Creationists, with their usual skill in the art of phony rhetoric, cynically
distorted punctuated equilibrium for their own ends, claiming that we had
virtually thrown in the towel and admitted that the fossil record contains no
intermediate forms. (Punctuated equilibrium, on the other hand, is a different
theory of intermediacy for evolutionary trends—pushing a ball up an
inclined plane for gradualism, climbing a staircase for punctuated
equilibrium.) Some of our colleagues, in an all too common and literally
perverse reaction, blamed
us for this mayhem upon our theory. At least we were able to fight back
effectively. Most of
my
testimony at the Arkansas creationism trial in 1980 centered upon the
creationists' distortion of punctuated equilibrium.
Third, and this is harder to say but cannot be ignored, a
few colleagues allowed personal jealousy to cloud their judgment—for
their vitriol simply cannot be understood as a response to intellectual
issues. Punctuated equilibrium got a good dollop of publicity—including
editorials in the Times of India and articles in the Beijing
People's Daily. Some colleagues assumed that we had orchestrated all
this with the help of press agents and largely for personal glory. Why not
consider the more honorable altemative—that we had raised interesting
and important issues, and that people (as basically intelligent entities)
responded?
But the good has far outweighed these frustrations. No
scholar can ask for more than a serious consideration of his ideas, and no
scientist can hope for more than the conversion of his concepts into
fruitful research (whatever the fate of particular nuances and centralities).
The great joy of punctuated equilibrium has been its extension (largely by
others) into areas and implications
that we never even conceptualized at the inception. I cringe now when I read
our original paper
of 1972. Both sides of the theory have been useful to our profession.
On stasis (equilibrium). Niles and I, with some
grammatical (but no intellectual) doubt, soon took as the motto of punctuated
equilibrium: "Stasis is data." We see the world in the light of theories and
ideas; as Peter Medawar said, "Innocent, unbiased observation is a myth." My
greatest pride in punctuated equilibrium lies in its role in turning the basic
fact of paleontology from an unstated embarrassment into a subject of active
and burgeoning research. When most of our colleagues defined evolution as
gradual change, the stability of species counted as no data—that is, as
absence of evolution. All paleontologists recognized the stability of species,
but the subject never entered active research. At most, the fact of
stability might be noted in the midst of a taxonomic description. Punctuated
equilibrium has changed the context. Stasis has become interesting as a
central prediction of our theory. No one, twenty years ago, would have dreamed
of publishing a paper about the lack of change in some particular brachiopod
during umpteen million years in Michigan. But is it not intrinsically
fascinating, in a world of change and a history of life crafted by evolution,
that species don't alter over such extended stretches of time? Don't we want
to know why so many species don't change for so long? Stasis is a
puzzle, not a negativity. Paleontologists now routinely document stasis; many
studies are complexly quantitative and meticulously elegant. I am proud that
punctuated equilibrium served as the midwife for this fruitful work.
Moreover, given the invisibility of stasis in older
paleontological literature, many students of modern organisms simply didn't
know about this primary fact of the fossil record; they assumed that gradual
change was the norm for most species most of the time. Stasis is now
generally recognized as an intriguing puzzle by evolutionists. No definitive
resolution is in sight, but geneticists and embryologists have offered their
counsel, and I am tickled that our much maligned profession (dull,
descriptive paleontology) has provided such a puzzle to kings of the
theoretical mountain.
On punctuation. Punctuated equilibrium has provided
a new context for the most important phenomenon of
paleontology—evolutionary trends (larger size in horses, more complex
sutures in ammonites, bigger brains in
humans). Under gradualism, trends arise because natural selection favors some
traits over others, and a genealogical continuum builds these features further
and further along the path of advantage. Species are arbitrary segments of the
resulting continuum—a largely artificial consequence of change.
Punctuated equilibrium cleanly reverses this perspective. Species are real
units, arising by branching in the first moments of a long and stable
existence. A trend arises by the differential success of certain kinds of
species. (if large-bodied horses either arise more frequently or live longer
than small-bodied horses, then a trend to increased size will permeate the
equine bush.) Speciation is the real cause of change, not an arbitrary
consequence of artificial division of a continuum. Since the causes of
branching are so different from those of continuous transformation, trends
must receive a new explanatory apparatus under punctuated equilibrium.
Enough puffery for consequences of the theory. What about a
more basic question? Is punctuated equilibrium true? I suppose I'm the worst
person to ask. What parent would hand his child to the executioner, whatever
the kid's shortcomings? Nature is a domain of relative frequencies, not
absolutes. Niles and I never denied that cases of gradualism would be found,
just as the most ardent Darwinian selectionist does not claim exclusivity for
his favored mechanism of change, but only a presence overwhelming enough to
shape the major patterns of life's history. On this proper ground, I am
confident that punctuated equilibrium does prevail as the primary molder of
pattern in the fossil record. Others would disagree on the totality, but all
would concur that punctuated equilibrium is a real and important phenomenon,
and that many elegant studies of its operation have been published in the
past two decades—Cheetham on bryozoans, Ager on brachiopods, Stanley and
Yang on clams, Williamson on snails, Prothero and Shubin on horses, to name
just a few. The current "out of Africa" versus "candelabra" debate on human
evolution represents a claim for speciation and punctuated equilibrium in our
own origin.
But my greatest pleasure has been the passage of punctuated
equilibrium from a much debated theory to an ordinary instrument of active
research. To cite just one example, a prominent criticism of punctuated
equilibrium has held that the morphologically stable "entities" documented in
the fossil record might not be true biological species by the proper
definition of a population reproductively isolated from all others (breeding
only among its members and not with others—a criterion of ultimate and
permanent evolutionary independence). Perhaps, the critics say, many of these
morphological "packages" hide several species of virtually identical body
form, but differing in properties not preserved in the fossil record (color,
behavior, and so on). Such so-called cryptic species are quite common in some
groups. On the other hand, some packages might be only parts of highly
variable species, with other geographically distant populations as portions of
the same unit by proper reproductive criteria.
The best treatment of this objection must be sought in
studies of living species with good fossil records—where direct surveys
can be made for correspondence of a morphological package with a true
biological species, and the origin and history of the same package can then
be traced in the fossil record and assessed for punctuated equilibrium. I am
delighted to report that two such pioneering studies have been published in
the past few years, and both support punctuated equilibrium.
New Zealand biologist B. Michaux did a morphological and
genetic survey of four species in the snail genus Amalda. He found no
cryptic populations; each morphologically defined package corresponds
perfectly with a biological species. Three of these species extend back in
the New Zealand fossil record for several million years. In an elegant,
multivariate study of morphological pattern, Michaux demonstrated stasis
throughout the ranges of all species. He concludes (in the Biological
Journal of the Linnaean Society of London, vol. 38, 1989):
This study demonstrates that fossil members of
three biologically distinct species fall within the range of variation that
is exhibited by extant members of these species. The phenotypic trajectory of
each species is shown to oscillate around the modern mean through the time
period under consideration. This pattern demonstrates oscillatory change in
phenotype [our jargon for overt morphological appearance as contrasted with
underlying genetics, or genotype] within prescribed limits, that is,
phenotypic stasis.
In a second study (Science, vol. 248, 1990), Jeremy
Jackson and Alan Cheetham studied eight species in three genera of bryozoans.
These colonial marine organisms are so highly variable, and supposedly so
subject to immediate shaping by nongenetic forces (temperature, crowding, and
so on), that many biologists have doubted any correspondence between
morphological package and true biological species and have even doubted that
morphological packages could be specified at all. But Jackson and Cheetham,
in a series of carefully controlled experiments, first determined that they
could find no cryptic species within any of the packages (coincidence of
morphological and biological species). They then grew two generations of the
species in a common environment to see if the morphological distinctions were
truly inherited or merely a transient result of growing in certain places
under certain conditions. Each package checked out as genetically distinct
and morphologically unique and stable. They conclude:
Our results show that the identity of
quantitatively defined morphospecies of cheilostome bryozoans is both
heritable and unambiguously distinct genetically. . . . Thus, cheilostome
morphospecies appear to be good biological species.
Since Cheetham had presented the most elegant and
persuasive of all cases of punctuated equilibrium in his studies of the
cheilostome bryozoan Metrarabdotos (in Paleobiology, 1986 and 1987),
these findings increase our confidence in his results. Jackson and
Cheetham end their paper with these words:
The consistency of our results across three
distantly related cheilostome genera suggests that previously documented
patterns of morphologic stasis punctuated by relatively sudden appearances
of new morphospecies in the cheilostome Metrarabdotos do indeed reflect
patterns of evolution at the species level. This is consistent with the
punctuated equilibrium model.
As ordinary human beings with egos and arrogances,
scientists love to be right. But we would, I think, all say that to be
useful is more important, that is, to propose an idea that gets people
excited and suggests fruitful strategies for potential confirmation and
disproof. The jury is still out on the relative frequency of punctuated
equilibrium (twenty years is a short case in biology), but utility has
already been proved in the pudding of practice.
On the subject of things that come in twenties and then
lead on to greater fruitfulness, I can only quote some lines of Robert
Herrick, with their delicious final rhyme:
Give'me a kiss, and to that kiss
a score;
Then to that twenty, add
a hundred more:
A thousand to the hundred:
so kiss on,
To make that thousand up
a million.
Now we're getting to proper geological scales!
[ Stephen Jay Gould, "Opus 200," Natural History
100 (August 1991): 12-18. Reprinted here with permission. ]
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