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Niles Eldredge
I came to evolution in a roundabout way. Sure, as a kid I had seen
the dinosaurs at the American Museum of Natural History—and had heard
a bit about evolution in high school. But I was intent on studying
Latin and maybe going to law school.
But evolution got in the way. I was dating my now wife, and through
her getting to know members of the Columbia anthropology faculty. At
the time (early 1960s), anthropology to me meant Louis Leakey and his
adventures collecting human fossils at Olduvai Gorge—rather than,
say, Margaret Mead and her adventures studying cultures of the South
Pacific. A summer spent asking embarrassing personal questions in my
halting Portuguese in a small village in northeastern Brazil ended my
quest to study evolution through anthropology. I was far more taken
with the Pleistocene fossils embedded in the sandstone that formed
the protective cove for the fishing boats. By summer's end I was
determined to become a paleontologist.
Little did I know that paleontologists (with a few exceptions) had
had virtually nothing to do with the development of evolutionary
biology since Darwin's day. Vertebrate paleontologists, to be sure,
tended to be trained in zoology departments and to have at least a
passing interest in evolution. But the undergraduate courses in
paleontology at Columbia were in the Geology Department. I took my
undergraduate degree in geology at Columbia, staying on for a PhD and
writing my dissertation on the evolutionary career of the Devonian
trilobite Phacops rana.
Evolution in those days was firmly in the hands of geneticists—who
were at that very moment collectively like deer caught in the
headlights of the onrushing revolution in molecular biology. DNA was
threatening the comfortable world of population genetics—and there
simply was little intellectual time or psychic energy for genetics-
minded biologists to pay any attention to the results of a study on
the evolution of a small cadre of long-dead and all-but-forgotten
trilobites.
Indeed, had I read the introduction to my distinguished predecessor
George Gaylord Simpson's famous 1944 book Tempo and Mode in
Evolution, I might have seen that paleontology was a decidedly rocky
road for walking the evolutionary walk. Simpson had wryly
encapsulated the tension between geneticists and paleontologists when
he wrote:
Not long ago paleontologists felt that a geneticist was a person who
shut himself in a room, pulled down the shades, watched small flies
disporting themselves in bottles, and thought that he was studying
nature. A pursuit so removed from the realities of life, they said,
had no signficance for the true biologist. On the other hand, the
geneticists said that paleontology had no further contributions to
make to biology, that its only point had been the completed
demonstration of the truth of evolution, and that it was a subject
too purely descriptive to merit the name "science." The
paleontologist, they believed, is like a man who undertakes to study
the principles of the internal combustion engine by standing on a
street corner and watching the motor cars whiz by.[1]
And yet it was the evolutionary process—and not just the simple facts
of evolutionary history—that I longed to study. It seemed very much
that paleontology, like anthropology, was the wrong choice if
evolution were to be the focus of all my work.
I had been shocked to find very little change in the 5 million years
or so of history recorded by the main lineage of my Devonian
trilobite. I had been led to believe—along with the rest of the
evolutionary-minded world—that such periods of time would engender
almost as a matter of course some degree of palpable and lasting
evolutionary change. I had pulled the fat out of the fire (one needs
results, after all, to claim a PhD from a dissertation study) by
saying that not only my little group of trilobites but most species
in the history of life showed great stability for most of their
histories; I then said that the idea of geographic speciation—as
championed by the geneticist Theodosius Dobzhansky[2] and the
systematist Ernst Mayr[3] (writing after Dobzhansky but somehow
receiving most of the credit)—could account for the fact that
evolution seems to occur relatively rapidly as new species split off
from their long-stable ancestors.
Published in 1971 with a turgid title in the main journal devoted to
evolutionary biology, my results sank pretty much without a trace.[4]
Repackaged a year later, with additions, in a jointly authored paper
with a former fellow student with a knack for catchy phrases, the
theory of "punctuated equilibria" was born.[5] That colleague, of
course, was Stephen Jay Gould—who would never tolerate the subversion
of paleontology to the interests of any other field. At last, a good
choice!
Our paper seemed to annoy virtually everyone—starting with Tom
Schopf, the editor of the collection of essays in which it appeared.
It just seemed too anti-Darwinian: the denial of natural selection's
inexorably changing entire species through time was too much for
Darwinians—geneticists and even paleontologists—to take. To be sure,
some paleontologists came up to us and, looking furtively around,
admitted sotto voce that their own data showed similar patterns of
stability "punctuated" by sudden events of evolutionary change. But
for the most part we were held to be wrong—and, unkindest cut of all,
traitors to the Darwinian tradition.
The creationists of the day got into the act as well. In a clear
demonstration of how thoroughly political the creationist movement
has always been in the United States, Ronald Reagan told reporters,
after addressing a throng of Christian ministers during the 1980
presidential campaign, that evolution "is a theory, a scientific
theory only, and it has in recent years been challenged in the world
of science and is not yet believed in the scientific community to be
as infallible as it once was believed." The creationist who managed
to get to Reagan's handlers later bragged to me that those scientists
in question were none other than Gould and me. The syllogism ran
something like this: (1) Darwin said that evolution is slow, steady,
and gradual; (2) some scientists say that evolution consists of rapid
bursts of change interrupting vastly longer periods of evolutionary
stagnation; ergo, (3) some scientists don't follow Darwin, meaning
(4) some scientists oppose evolution. Then, as now, at least in the
public domain, "Darwin" is code for "evolution." The two are virtual
synonyms.
I take being called anti-Darwinian very personally. It has always
hurt, for I have always thought of myself as more or less a knee-jerk
neo-Darwinian, someone who thinks the basic mechanism underlying
evolutionary change, including the origin, modification, and
maintenance of adaptations, resides squarely in the domain of natural
selection. And I have always felt that, with one or two major
exceptions, my version of how the evolutionary process works lines up
very well with Darwin's. Take natural selection, for example: I see
natural selection just as Darwin originally did—as the statistical
effect that relative success in the economic sphere (obtaining energy
resources, warding off predators and disease, etc.) has on an
organism's success in reproducing. This conservative view contrasts
strongly with the modern tendency to see natural selection as a
matter of competition among genes to leave copies of themselves to
the next generation—a position I take to be hopelessly teleological,
obfuscating the real interactive dynamics of economic and
reproductive organismic behavior driving the evolutionary process.
But, of course, there are those sticking points: Darwin (or so the
cartoon version of him goes) enjoined us to expect evolution for the
most part to be slow, steady, and gradual—whereas to me the fossil
record screams loudly that such, for the most part, is not the case.
And as I'll discuss a bit later in this essay, there is an even more
dramatic parting of the ways between us which I discovered only last
year in the course of preparing the exhibition "Darwin" (which opened
at the American Museum of Natural History in New York on November 19,
2005) and its companion volume, Darwin: Discovering the Tree of Life
(Norton, 2005).
But I never thought the fact that Darwin—from where I stand as a
paleontologist—got some of his story wrong somehow made me an anti-
Darwinian. For I have admired the man ever since I took my paperback
copy of the sixth edition of On the Origin of Species to read while
waiting for Louis Leakey to show up and give a lecture on human
evolution on the Columbia campus. I had arrived early to get a good
seat, and Louis was late—so I got my first real chance to sample
Darwin's prose. I was fearful of the complexity of the great man's
mind, and of the alien nature of his Victorian prose. But I needn't
have worried, for Darwin proved accessible to the readers of his day—
even lay readers—and he remains so today.
But I am no historian—rather just a simple soldier on the fields of
evolution. Lots of my friends have spent more time reading Darwin and
collecting his books than (until recently!) I have—and of course, as
closely associated with Steve Gould as I have been since the 1960s, I
basically just left the historical part of our discipline up to him.
So I was scared all over again when the offer came along to develop a
major exhibition on the life and works of Charles Robert Darwin. I
jumped at the chance but was quaking in my boots as I embarked on
that particular part of the evolutionary road. There was nothing for
it but to plunge in and become immersed in Darwiniana; getting on
with it is so often the key to conquering one's fears.
The exhibition, only recently opened as I write, is by all measures
and accounts a huge success. The great thing about museum exhibitions
is that they are three-dimensional; you can walk into them, be
surrounded by them, and feel a part of them. And perhaps above all,
exhibitions are about stuff. We are awash in genuine Darwiniana (the
historian David Kohn, a key figure in the exhibition's development,
calls them the "crown jewels"). We have Darwin's Bible and geological
hammer, his specimens and his notebooks, his walking stick and Down
House table, his letters and his manuscripts. And so much more.
The exhibition tells Darwin's story well. Yet it is notorious how
little actual verbiage makes it to the walls of an exhibition—or for
that matter into the typical script of an hour-long sitcom. I decided
to write a companion book primarily because the greater space would
force me to understand Darwin's story in richer detail than could
ever be presented in an exhibition—the better to tell the bare bones
of the story on the exhibition's walls. But the book itself raised
the additional angst of trying to find a fresh take on Darwin's story—
a story that the so-called "Darwin Industry" had analyzed so
assiduously over the past thirty or forty years.
Fortunately—for me, but I think also for anyone looking to probe the
inner workings of a proven creative mind—Darwin left an incredibly
rich (if at times terribly cryptic) paper trail.[6] We can see Darwin
go from a callow, albeit enthusiastic, youth to a hard-working young
man at times almost delirious with the sights, sounds, and smells of
the natural world ("The mind is a chaos of delight," Darwin wrote of
his first encounter with the tropics in Bahia, Brazil, in his
February 28, 1832, diary entry);[7] to an open-minded young
naturalist "letting nature come to him" (in the words of my colleague
Joel Cracraft), noting repeated regularities in the natural phenomena
around him; to an analytic thinker as the patterns built up and began
to seep into his conscious mind as a suspicion (life has evolved!);
to a hypothetico-deductive sharp analyst who saw further patterns as
natural—predicted—outcomes if life had indeed evolved; to a searcher
for an evolutionary mechanism, which (to judge from his first passage
on natural selection in his Notebook D) came to him in a maelstrom of
excited detail, later to be coolly, analytically parsed in three
simple sentences; to a synthesis of his perceptions of patterns with
his later-found theory of evolutionary process (where, according to
me at least, he begins to go a bit astray); to the clever, yet still
thoroughly methodical, deviser of experimental protocols to test the
myriad separate portions of his theory.
The Darwin Industry has done a fine job with much of these phases of
Darwin's scientific sojourn (and I speak here solely of his
evolutionary odyssey; his strictly geological, botanical, and
zoological work, prodigious and generally excellent as it all was, is
nonetheless not what makes someone like me a "Darwinist"). But one
brings to the table what one is already equipped with, and I soon
found, while reading at least the more recent literature on Darwin,
that the focus has been more on mechanism—variation and natural
selection—than on what it was that Darwin saw—and how he saw it—that
led him to the idea of evolution in the first place.
What a briar patch for me! We paleontologists (as George Simpson put
it so well) are consigned to studying the dead hand of history—
the "completed demonstration" that life has evolved. But Simpson's
further point in 1944 was that whatever we say about how evolution
happens, it had better match up with those facts of evolutionary
history. And—better yet—recurrent patterns in the history of life
should suggest to the evolutionary theorist just what processes and
mechanisms—in modern terms, mutation rate, natural selection, the
role of isolation, etc.—combine to give the basic elements of the
evolutionary process. In other words, you can extrapolate from "flies
disporting themselves in bottles," but you had better check to see if
your long-term extrapolations bear at least a passable resemblance to
the facts of history.
That was how I had originally stumbled on what later came to be known
as "punctuated equilibria." Stasis is a common (I would say the
dominant) pattern of anatomical (non)change in the evolutionary
history of species; geographic isolation is a common (again, the
dominant) pattern underlying the development of new species from old.
I know about patterns and what they can tell us as a sort of
resonating feedback against ideas of evolutionary process developed
from and for short-term observations in the wild and in the
laboratory. And so, it turns out, did Charles Robert Darwin.
Darwin tells us in his Autobiography (actually, it wasn't written
for "us," but rather just for his immediate family) that he was as
much a Baconian inductivist as he was "eager to test" his ideas in
what we would these days call good hypothetico-deductive style.[8]
More recent Darwin scholars have overwhelmingly championed Darwin as
one of the earliest exponents of modern hypothetico-deductive science—
tending to treat his protestations of fealty to Bacon's inductivism
as a polite sop to his aged mentors, exponents of a by then discarded
and discredited approach to science.
But Darwin, I am convinced, meant it when he said he was at least in
some measure an inductivist. Though my early training in anthropology
taught me to be leery about taking what "informants" say at face
value, I cannot help but feel that Charles Darwin was an extremely
honest person—above all to himself. He wanted to learn as much as
possible about anything—a fossil, an idea, a breeding procedure. He
looked at everything from as many sides as presented themselves to
him—and devised ways of peering at sides otherwise not immediately
evident. Darwin scholars, like all historians, and anthropologists
with their informants tend to look askance at the recollections of
old folks (Darwin was sixty-seven when he penned his Autobiography in
1876).
And perhaps I am being disingenuous to take Darwin's words so
literally to heart when he tells us what it was that he actually did.
But fortunately there is the contemporary record of his experiences
on the Beagle and the immediate aftermath—when, in a breathtaking
interval of only 2½ years, in his "Red" and "Transmutation"
notebooks, he synthesized the lessons already beginning to dawn on
him inductively on the Beagle, then derived a subsidiary set of
patterns as expected outcomes if evolution were true (purely
deductive) and went on to derive inductively (as a flash of insight),
then parse deductively, the principle of natural selection. We can
see him in action in these notebooks and in his only slightly later
first two essays on evolution.[9] And I do think that my own
experiences grasping patterns almost intuitively allows me to
recognize the very same behavior that first brought Darwin to the
idea of evolution—just as he himself said it did.
Darwin writes briefly of the patterns that first brought him to
evolution in four passages sprinkled fairly evenly through his
lifetime. The first two are in a diary (1837) and a letter (the
famous 1844 letter to Joseph Hooker, just before Darwin announces his
suspicion that species are "not immutable"—and says "it is like
confessing a murder"). These passages, of course, are best known to
Darwin scholars. But the third is hiding out in plain sight in the
opening sentences of On the Origin of Species:
When on board H.M.S. "Beagle," as naturalist, I was much struck with
certain facts in the distribution of the inhabitants of South
America, and in the geological relations of the present to the past
inhabitants of that continent. These facts seemed to me to throw some
light on the origin of species—that mystery of mysteries, as it has
been called by one of our greatest philosophers.[10]
The philosopher in question was John Herschel, who had openly
wondered, in a letter written to Charles Lyell in 1836 and published
in 1838, when a young naturalist would appear who would put forth a
credible, scientific explanation for the replacement of extinct
species by later ones—the "mystery of mysteries." Darwin met with
Herschel in Cape Town in June 1836, after Herschel had written Lyell,
and one is tempted to wonder (along with historian David Kohn in a
footnote of his edition of Notebook E) whether they discussed
the "mystery of mysteries" along with the geological portions of the
letter.
Darwin's topic sentence paints a bare-bones, nearly abstract picture
of his crucial patterns. He does not discuss them further in The
Origin—as these were simply the "facts" that got him started as a
naïve ship's naturalist. Long before 1859, Darwin had come to see
as "my theory" not just the simple idea of evolution, but "evolution
through natural selection." Indeed, by the 1850s, evolution was
spoken about openly in medical schools and other institutions; it was
natural selection that Darwin saw as his special property—and his
major contribution to science. That's why Alfred Russel Wallace's
missive, arriving in 1858, made Darwin so frantic. For Wallace had
discovered natural selection. The morals to that story, of course,
are: if there is any truth to an idea, someone else will surely
discover it; and if you do have a good idea, by all means publish it
before someone else does!
The Wallace event—and the consequent hurried writing of The Origin—
ended Darwin's agonizing twenty-two-year period of keeping his
evolutionary ideas almost completely to himself. Thus freed, by the
time he wrote his Autobiography, he was able to say that his greatest
contribution had been, in fact, the convincing demonstration simply
that life has evolved—rather than the narrower (though monumentally
important) formulation of natural selection. So it is no surprise
that Darwin is far more expansive in his Autobiography than in any
earlier statement when describing exactly what it was that he saw
that took him to evolution in the first place:
During the voyage of the Beagle I had been deeply impressed by
discovering in the Pampean formation great fossil animals covered
with armour like that of the existing armadillos; secondly, by the
manner in which closely allied animals replace one another in
proceeding southwards over the Continent; and thirdly, by the South
American character of most of the productions of the Galapagos
archipelago, and more especially by the manner in which they differ
slightly on each island of the group; none of the islands appearing
to be very old geologically. (Autobiography, pp. 52–53)
The key words here are "closely allied" and "replace"; critical too
is the concept of "distribution"—found in the preceding quote from
The Origin but also implied here.
Darwin had three patterns in mind—all of them involving the
replacement of one species by a similar one. The word "allied"
virtually begs the question of evolutionary relationship—but
Linnaeus, long before Darwin's birth, had been recognizing "natural"
groups without the internal evolutionary connections so offensive to
the creationist mind-set. Edentate mammals (present-day armadillos
and sloths) were already known by the time Darwin reached Argentina
and began finding the fossilized bones of large mammals. The common
denominator in all three sets of Darwin's original patterns was the
replacement—in space or in time—of similar species of organisms known
only in that part of the world—meaning South America plus the
Galápagos archipelago lying 600 miles to the west of Ecuador.
Modern Darwin scholarship tends to emphasize the importance of
taxonomic experts back in England, whose analyses of Darwin's
specimens (including ones he sent home while still on the voyage) for
the most part were rendered after Darwin returned. The classic
example is, of course, "Darwin's finches": it was the ornithologist
John Gould who figured out that there are thirteen species of a
single related group of little brown, greenish, and black birds
displaying an interesting array of beak sizes and shapes that, taken
with their distribution patterns on the various islands, make a
compelling case for evolution. This came long after Darwin sailed
away from the Galápagos, having paid these birds hardly any heed.
Indeed, he learned of Gould's results only when he reached home.
But all the stress on the importance of homegrown taxonomic expertise
tends to obscure what Darwin actually did see and what he made of it
while he was still on the voyage. Darwin's reference to "great fossil
animals covered with armour like that of the existing armadillos" is
a case in point. It is true that Darwin did not learn of Richard
Owen's conclusions concerning the large extinct armadillo Glyptodon
until his arrival home. And it is also the case that there had been
some speculation, known to Darwin when he was collecting these
fossils in the fall of 1832, that giant ground sloths (already known
to Western science) may have had "coats of armor"—raising questions
about just what form of extinct edentate mammal sported such a bony
coat. Yet Darwin wrote to his mentor John Stevens Henslow, upon
finding "osseous polygonal plates" among his fossils: "Immediately I
saw them I thought they must belong to an enormous Armadillo, living
species of which genus are so abundant here."
This is not a statement about evolution. But it is a statement about
one extinct form succeeded by other, still living species:
replacement in time.
Darwin soon thereafter encountered another pattern of replacement—
spatial, or geographic—embodied most tellingly in the existence of
two distinct, if basically similar, species of the ostrich-like
rheas. Rheas, like armadillos and sloths, are endemic to South
America—so here again he is struck by replacement patterns among
similar species found nowhere else on earth. The two species in
question occupy separate territories—the larger ("greater") rhea
living on the Argentine pampas, the somewhat smaller ("lesser," for a
while known as "Darwin's") rhea living to the south in Patagonia (and
curving up northward as far as Peru in the Andes). Late in the
voyage, Darwin wrote of these species replacing one another—the main
(though not the sole) source of his comment about allied species
replacing one another "in proceeding southwards over the continent."
Again, this is simply replacement—no explicit mention of evolution.
But by the time Darwin had written up the rheas in
his "Ornithological Notes," he had already written up his third
replacement pattern—for the first (and apparently only) time on the
voyage seeing, and explicitly commenting on, a possible evolutionary
implication. This is the famous passage, also in the "Ornithological
Notes," on the Galápagos mockingbirds—with the Galápagos tortoises
thrown in for good measure:
These birds are closely allied to the Thenca of Chile. . . . I have
specimens from four of the larger islands. . . . The specimens from
Chatham and Albemarle Isd appear to be the same; but the other two
are different. In each Isld. each kind is exclusively found; habits
of all are indistinguishable. When I recollect, the fact that the
form of the body, shape of scales & general size, the Spaniards can
at once pronounce, from which Island any Tortoise may be brought.
When I see these Islands in sight of each other, & possessed of but a
scanty stock of animals, tenanted by these birds, but slightly
differing in structure and filling the same place in Nature, I must
suspect they are only varieties. The only fact of a similar kind of
which I am aware, is the constant/asserted difference—between the
wolf-like Fox of East and West Falkland Islds.—If there is the
slightest foundation for these remarks to zoology of Archipelagoes—
will be well worth examining; for such facts (would) undermine the
stability of Species. . . .[11]
Here is replacement on a more microgeographic scale ("Islands in
sight of each other"), once again involving closely similar
forms "closely allied" with mainland South American mockingbirds—the
group as a whole being confined, once again, to the Americas.
Darwin did not need John Gould to tell him there were distinctly
different mockingbirds on some of the islands (the older ones, in the
southeastern part of the chain). Gould did later describe the
distinct forms as separate species, thereby resolving Darwin's
indecision on their status. But I do agree with Kohn et al. (2005),
who have convincingly argued that "only varieties" is Darwin's first
statement toward the view that became so central to his later
evolutionary arguments: that varieties, rather than simply reflecting
variation within created kinds, are actually incipient species.[12]
Hence Darwin's ringing conclusion: "for such facts (would) undermine
the stability of species." The verb in this quote is usually
rendered "would undermine"—as "would" is inserted into the sentence
as an afterthought. Others disagree, but even if Darwin wrote "would"
on top of the line only thirty seconds after writing out his
sentence, we nonetheless see his mind at work. For such facts do
undermine the stability of species—and that's what Darwin initially
wrote.
Darwin, as the journey ended, was no longer eager to become a quiet
curate in the English countryside; rather, he was anxious to take his
place among the emerging ranks of scientific professionals, or so he
says in his Autobiography. Coupled with his replacement patterns and
that key passage on the Galápagos mockingbirds and tortoises, it is
difficult for me to believe that he did not have evolution firmly in
mind as the Beagle finally turned from its second trip to the coast
of Brazil—and finally headed for home.
Granted there is no unequivocal Darwin Beagle paper trail to back
this supposition up. Rather, after a few months at home, Darwin
completes his "Red Notebook," begun on the Beagle but containing no
explicit reference to evolution in its initial pages. But then, as he
works on the book, more or less confining his thoughts to the things
he saw and collected while on the voyage, we do find, in disconnected
snippets, Darwin's earliest speculations on evolution. By the time we
pick up on his active mind, he is already convinced of evolution. He
has already let the patterns take him there—and he is already hard at
work imagining how—not if—evolution occurs.
He starts by letting the patterns set the terms and conditions of the
evolutionary process. On page 130, he sees the parallels of
replacement patterns in space and time:
The same kind of relation that common ostrich bears to
Petisse . . . ; extinct Guanaco to recent: in former case position,
in latter time (or changes consequent on lapse) being the relation—As
in first cases distinct species inosculate, so we must believe
ancient ones, therefore not gradual change or degeneration from
circumstances: if one species does change into another it must be per
saltum—or species may perish. This inosculation (representation) of
species important, each its own limit and represented.
Well! I had grown up believing that Darwin was the quintessential
gradualist—so I was flabbergasted to read in his very first
evolutionary thoughts he was a "saltationist." Darwin let the
patterns of abrupt replacement in time and space speak for
themselves. Small wonder that when we resurrected stasis as a pattern
for all to contemplate, Gould and I were tarnished with that same
saltationist brush—this time hurled as an insult. With Darwin, it was
something more pure and innocent—he was letting nature's patterns
that led him to evolution guide him too in his initial explorations
of how the evolutionary process actually works. His initial ideas
along these lines saw species—much like individual organisms—to have
finite lifespans, a sort of built-in survival limit. New species had
to be created to keep life going. But just how new species were
created from others in one blow Darwin never really answered.
Darwin knew about stasis—and was troubled by it from the outset of
his fervent evolutionary explorations in 1837–39. He opened
his "Transmutation Notebooks" (Notebooks B–E devoted to evolution)
not long after filling the "Red Notebook." The "Red Notebook" had
been looking primarily backward to his Beagle experiences—melded now
with the results of study of his specimens by the likes of John Gould
and Richard Owen. He was truly an honest man, not least to himself—
and he wanted to explore not only all avenues that corroborated his
evolutionary views, but also those that seemed opposed. On stasis, he
wrote (Notebook E, 1838):
My very theory requires each form to have lasted for its time: but we
ought in same bed if very thick to find some change in upper & lower
layers.—good objection to my theory: a modern bed at present might be
very thick & yet have same fossils. (p. 6)
Darwin, by this time, had discovered natural selection—and even
before that had abandoned saltationism in favor of a more Lyellian-
infused gradualism. But he was still honest enough not simply to deny
stasis, but actually to acknowledge it as a "good objection to my
theory." Indeed, at one point, Darwin seemed to have the
reconciliation of stasis with his emerging notions of evolutionary
process in hand—only to let it go. Instead he writes (Notebook E):
Species not being observed to change is very great difficulty in
thick strata, can only be explained, by such strata being merely
leaf, if one river did pour sediment in one spot, for many epochs—
such changes would be observed. (p. 18)
Darwin went on to expand this argument—that the fossil record is too
incomplete to allow us to trust its patterns of apparent change—and
especially nonchange. He developed this argument in increasing detail
in his 1842 "Sketch" and 1844 "Essay"—and in The Origin devoted an
entire chapter to the "imperfections of the geological record." How
ironic that Darwin felt himself forced to turn his back on one of the
main signals in nature that took him to evolution in the first place!
But in the main, in his four "Transmutation Notebooks," Darwin had
other fish to fry. Instead of patiently compiling case after case
from the literature—inductively piling on further examples of the
three sorts of patterns that struck him on the Beagle and which
formed the basis of most of his theoretical explorations in the "Red
Notebook"—Darwin pursued his work on two main fronts: addition of
other patterns pointing to evolution and a search through what was
known of the principles of inheritance, in large part through the
work of breeders, for a mechanism of evolutionary change.
There was a distinct mental shift in Darwin's search for additional
evolutionary patterns to bolster his argument. For in a flash of
insight (itself intuitive and therefore inductive) he realized that
Linnaeus's pattern of nested sets of taxa linking up all forms of
life is precisely what you would expect if all species had indeed
descended from a single common ancestor. He wrote "I think" on page
36 of Notebook B and proceeded to draw the first evolutionary tree—a
diagram I feel is the equivalent of Einstein's E=MC2 (our
exhibition "Einstein," including the manuscript of this most famous
of equations, was still on display at the American Museum when I
first accepted the challenge of mounting the Darwin exhibition).
Thus in a trice Darwin brought the very idea of evolution into the
realm of hypothetico-deductive science. Neatly hierarchically
structured classification schemes are what you would expect—i.e.,
predict—if evolution had occurred. The physical resemblances that led
Linnaeus in the pre-evolutionary era to link humans with the great
apes would, now, in an evolutionary context, lead one to predict
greater genetic resemblances between humans and apes than, say,
humans and chickens; and we would predict that the human fossil
record, when explored down deep enough, would reveal our ancestors to
have had smaller, ape-sized brains and not to have been, at the
outset, fully upright and bipedal. That's what we find.
Darwin (in the Autobiography) took special delight in claiming that
it was he who had first pointed out that the earlier stages of the
embryos of "allied" forms would resemble each other more closely than
the adult stages—again, what you would expect if these species had
descended from a common ancestor. And he also saw that the "unity of
type"—the close structural similarity of, say, the arms of bats,
birds, horses, and humans—is also a natural expectation if these
species shared an evolutionary ancestor in the remote geological past.
As to mechanism—natural selection—Darwin seemed close to finding it
throughout the notebooks. He had long since known that evolution must
be a matter of change in the hereditary materials—whatever those
might be. After all, that's essentially how breeders achieve their
results—knowing that organisms tend to resemble their parents. The
variation in the barnyard lets the breeder select just those animals
(or plants in the greenhouse) that have the best development of the
traits to be enhanced in the lineage.
But it was that famous encounter with Malthus's 1798 Essay on the
Principle of Population in which Darwin learned that more organisms
are produced each generation than can possibly survive and themselves
reproduce—else the world would be "standing room only" (as he put it
in The Origin) in a single species. As David Kohn points out,[13]
Darwin, in an impassioned passage full of imagery but little explicit
detail (Notebook D, p. 134), records his initial—and again largely
intuitive—impression of natural selection as "a force of a hundred
thousand wedges" shaping the adaptations of species. It is only after
some time goes by that a clear-eyed Darwin analytically and
economically parses natural selection into three simple "principles"
that "will account for all":
Grandchildren like grandfathers
Tendency to small variation . . . especially with physical change
Great fertility in proportion to support of parents
(Notebook E, p. 58)
In other words, inheritance, variation, and overproduction of
offspring—then as now the main ingredients of the idea of natural
selection.
But then Darwin takes a further, fateful step: he cannot resist
rethinking his original patterns as outcomes, now, of evolution
through natural selection. The results are interesting—as they have
dictated in large measure the subsequent history of evolutionary
biology. On page 118 of Notebook E, Darwin writes:
Has nature any process analogous—if so she can produce great ends—But
how.—Make the difficulty apparent by cross-questioning—even if placed
on Island—if &c &c.—Then give my theory—excellently true theory.
Darwin was coaching himself on the structure of the argument to be
made when he wrote his theory up in essay form (not to happen until
1842). "Process analogous" meant natural selection (analogous to
artificial selection, the subject of the preceding sentences). He was
telling himself to pose hypothetical situations, such as species on
an island, and then produce an explanation of that case based on his
idea of natural selection.
Indeed, this was to become his argumentative structure throughout all
his expositions of evolution: start with chapters on variation,
inheritance, and natural selection; then go on to explain various
patterns in nature as expected outcomes of the evolutionary process.
Darwin had already dismissed the incongruity between his vision of
gradual evolution through natural selection with the typical,
recalcitrant refusal of species to demonstrate much if any change
even through "thick strata." That left evolution on island
archipelagos and the evolution of new species from older ones on
continents to reconsider.
And here is a further irony, considering the importance of the
Galápagos patterns in bringing Darwin to evolution in the first
place: Darwin in the end downplays the significance of islands as a
general model of evolutionary change. Island archipelagos afford the
clearest examples of the importance of physical isolation in
promoting evolutionary change—and isolation in general played an
important role in Darwin's thinking clear through his
(unpublished) "Essay" of 1844. But in The Origin Darwin writes:
Although I do not doubt that isolation is of considerable importance
in the production of new species, on the whole I am inclined to
believe that largeness of area is of more importance, more especially
in the production of species, which will prove capable of enduring
for a long period, and of spreading widely. (p. 105)
Probably just because there are far more species living on continents
than on island archipelagoes, in search of a truly general
evolutionary theory, Darwin looked to continental patterns: the rheas
over the mockingbirds. And though he had discussed isolation on
continents in his notebooks and earlier manuscripts, by the time he
wrote The Origin, Darwin was having a difficult time imagining the
action of physical isolation disrupting species and promoting the
evolution of new ones on continents.
Thus the picture of evolution that I grew up with—the one Steve Gould
and I reprinted in our original paper on punctuated equilibria. The
original shows the gradual divergence of two lineages of snails
(redrawn here as scallops); what we are really looking at, though, is
Darwin's decision regarding which was the most important of his three
patterns—and a version, at least, of his explanation of it through
natural selection. In truth, these snails and scallops are all, at
base, Darwin's rheas.
But it is nonetheless true that some of the most talked-about issues
in the years since the 1859 publication of The Origin have in fact
been hard-won restitutions of some of the very patterns that Darwin
saw but later abandoned. I have already mentioned two. The first is
the reemergence of the importance of geographic isolation in the
production of new species in a movement led by Dobzhansky and Mayr in
the 1930s and 1940s. Isolation had always had its champions, but the
theme was definitely muted in the decades between Darwin's On the
Origin of Species and Dobzhansky's Genetics and the Origin of
Species, published nearly eighty years later.
With the importance of geographic isolation reestablished, my
rediscovery of stasis (the second restitution), combined with the
theory of geographic speciation, led to punctuated equilibria.
Paleontologists had occasionally mentioned stasis over the
intervening years, but most were content to accept Darwin's
condemnation of the fossil record as too faulty to be trusted. Stasis
is now seen to be common—and the expected outcome of the fates of
geographically disjunct populations within a species adapted to
different local ecological conditions: the probability of natural
selection being able to modify the features of all the populations of
a species spread out over wide areas in a single direction is now
seen to be rather small.
Which brings us to the most dramatic and superficially un-Darwinian
pattern of them all. It is a pattern I had looked for in vain as I
reread The Origin a few years back, and I supposed Darwin knew
nothing of it—or at least thought so little of it that it didn't
warrant discussion. It is a pattern that goes back to the great
paleontologist and anatomist Baron Georges Cuvier, who wrote about it
in 1812—three years after Darwin's birth.[14] And it is the pattern
that, if it gets the attention and survives the debate it merits,
promises to produce a significant change in our picture of the
evolution of life on Earth.
It turns out Darwin did know this pattern—and I will let him say what
it is in his own words. These words were never published in his
lifetime. I found them as a footnote in tiny type at the bottom of a
page of Darwin's son Francis Darwin's 1909 transcription of his
father's 1844 "Essay." Francis found a note, in Darwin's hand, in the
fair copy of this manuscript. Darwin wrote: "Better begin with this:
If species really, after catastrophes, created in showers world over,
my theory false." Wow! Darwin, of course, never began with this—he
never mentioned mass extinction anywhere in his published works. It
was just too far beyond the pale—too far outside his expectations of
what evolution under natural selection should look like in geological
time.
But Cuvier was right. There are numerous events that paleontologists
call "turnovers"—episodes of extinction followed by many examples of
the same sort of replacement that Darwin saw with his extinct and
modern armadillos. Cuvier was given short shrift because his
explanation of those replacements—separate creations by the deity—
hardly solved the problem of the "mystery of mysteries." And, of
course, given a concatenation of many separate "punctuated
equilibria" events all happening more or less at the same time,
Darwin, armed with his particular conception of evolution through
natural selection, must have felt totally helpless in the face of
such a monstrous pattern. He could blame stasis on a poor geological
record, but faced with paleontologists talking up Cuvierian patterns,
Darwin simply cut and ran. It's the only time I have ever found him
doing that.
Turnovers have been actively discussed in evolutionary terms by
paleontologists for over twenty years now—ever since the Alvarez
group, beginning in 1980, restored mass extinctions to enough
respectability to be taken seriously.[15] A few years ago I developed
a general theory I called the "sloshing bucket"—an attempt to meld
Cuvier with modern work on turnovers, folding in Darwin's evolution
through natural selection, speciation, and the aperçus of punctuated
equilibria. It is a view of evolution that sees the physical
environment as the fundamental driving force—a theme Darwin would not
throw out of bed a priori, even though he himself preferred notions
of interspecies interactions to explain such important phenomena as
extinction. It says that the degree of evolutionary innovation is
roughly proportional to the degree of severity of extinction.
The Sloshing Bucket theory is really very simple. It says that we
know the details of two separate kinds of phenomena that turn out to
be extremes of a spectrum. At the lower level—short-term, localized—
we have the environmental degradation of local ecosystems: wildfires,
volcanic eruptions, and suchlike damage, and sometimes obliterate,
local ecosystems. But their effects are not so widespread, typically,
as to cause the actual extinction of entire species. Or at least not
many species (some species have very limited ranges—more typically in
the tropics than in the higher latitudes, but this scenario is
possible anywhere). In time, when the cause of the degradation has
died down, the local ecosystem is usually rebuilt to something that
looks very like what it had been: ecological succession may start
with a few hardy "pioneer" species, but over time recruits from all
the surviving populations of the various species formerly there will
show up—and the ecosystem will be restored more or less to its former
self.
At the other extreme we have mass extinctions—events that have
happened five or six times since the advent of complex life (and a
decent fossil record thereof) some 535 million years ago. Though it
is not always clear what causes these mass extinctions, physical
events are always implicated—such as the collision between the earth
and one or more "extraterrestrial bolides" (read comets or meteors).
Entire groups that had been around for 100 million years, sometimes
longer, disappear. Dinosaurs and ammonites at the end of the Mesozoic
era, or trilobites and rugose corals at the end of the Paleozoic. The
divisions of geological time, largely worked out in the creationist
world of the 1830s and 1840s while Darwin was developing and then
honing his evolutionary ideas, reflect these larger-scale turnovers
in the history of life. That's how geologists could tell time and
order their rocks: the sequence of turnovers great and small in the
rock record. It was Darwin's mentor Adam Sedgwick whose training in
the field, just before the letter inviting him to join the Beagle
arrived, equipped Darwin to do all the "geologising" he did on the
voyage. (Darwin was grateful and wrote Henslow asking him to convey
his thanks to Sedgwick—a letter we have included in the American
Museum exhibition.) And it was Adam Sedgwick who named the Cambrian
System—the lowermost division of the Paleozoic.
Mass extinctions are turnovers on the grandest scale. Mammals had
evolved when the dinosaurs first appeared, back in the Triassic. They
stayed, ecologically speaking, pretty much in the deep background
until the dinosaurs finally disappeared, 65 million years or so ago,
in the mass extinction that drew the Mesozoic to a close. Then, after
what seems to be a customary lag, mammals rapidly differentiated into
a panoply of large and small herbivores, carnivores, and scavengers.
They were then cut back in a succession of smaller turnovers, which
pretty soon produced the main groups of mammals—your carnivores,
perissodactyls, artiodactyls, etc.—familiar to us (albeit in earlier,
more primitive guise) today.
Well, if there are small-scale effects leading to ecological
reconstitution rather than palpable evolution, and if there are
global mass extinction events, in which large-scale groups become
extinct, to be replaced by other large-scale groups—wouldn't we
predict ("expect," Darwin would have said) that there would be
intermediate-scale events sufficiently large to knock out entire
species, but smaller in scale nonetheless than truly horrendous mass
extinction events on a global scale? We would—and that is what we
find. They are the basis of all the smaller-scale, finer subdivisions
of geological time empirically deciphered by geologists and
paleontologists since the 1840s. These turnovers tend not to match up
too well across entire continents—or for that matter between
continents—because they are more localized, less extreme versions of
the far less numerous global mass extinction events.
There have been at least hundreds—and probably a few thousand—of
these events since complex life appeared on earth just over half a
billion years ago. These events are where the vast majority of
speciation has occurred. That means most evolution in the history of
life is tied up in such episodes. And that is a very different
picture of evolution than the cartoon version derived from Darwin's
thinking later in his life.
Yet this new picture is not un-Darwinian, let alone anti-Darwinian.
Selection is there—keeping species stable instead of inexorably
changing them. And selection is there as the opportunity for change
comes—for the most part, if not exclusively, when the physical
environment changes the conditions of life.
Darwin would have been fascinated by all the new discoveries in
genetics, which have essentially left his theory of evolution through
natural selection unscathed. But he would also, I feel sure, welcome
with relief the validation of natural patterns he felt compelled to
reject because they seemed to present incongruities with his vision
of how evolution through natural selection should play out through
geological time.
It takes a true Darwinist to tweak the great man's vision, bringing
it into line with the facts of the matter. I confess that I am a true
Darwinist.
FOOTNOTES:
[1] G. G. Simpson, Tempo and Mode in Evolution (New York: Columbia
University Press, 1944), xv.
[2] T. Dobzhansky, Genetics and the Origin of Species (New York:
Columbia University Press, 1937).
[3] E. Mayr, Systematics and the Origin of Species (New York:
Columbia University Press, 1942).
[4] N. Eldredge, "The allopatric model and phylogeny in Paleozoic
invertebrates." Evolution 25 (1971): 156–167.
[5] N. Eldredge and S. J. Gould, "Punctuated Equilibria: An
Alternative to Phyletic Gradualism," in Models in Paleobiology, ed.
T. J. M. Schopf (San Francisco: Freeman, Cooper and Co., 1972), 82–
115.
[6] Many of Darwin's writings are to be found online at the Darwin
Digital Library (D. Kohn, editor): http://darwinlibrary.amnh.org.
[7] C. Darwin, The Beagle Diaries, ed. R. Keynes (Cambridge:
Cambridge University Press, 2001).
[8] C. Darwin, Autobiography, ed. F. Darwin (1876; New York: Schuman,
1950).
[9] C. Darwin, "Red Notebook," ed. S. Herbert, and "Transmutation
Notebooks," ed. D. Kohn, in Charles Darwin's Notebooks, 1836–1844,
ed. P. H. Barrett et al. (Ithaca: Cornell University Press,
1987); "Sketch" (1842) and "Essay" (1844), in The Foundations of the
Origin of Species, ed. F. Darwin (Cambridge: Cambridge University
Press, 1909).
[10] C. Darwin, On the Origin of Species by Means of Natural
Selection; or, The Preservation of Favoured Races in the Struggle for
Life (London: John Murray, 1859), 1.
[11] C. Darwin, "Ornithological Notes," ed. N. Barlow, Bulletin of
the British Museum (Natural History) 2, no. 7 (1963): Galapagos Ms.
73.
[12] D. Kohn et al., "What Henslow Taught Darwin," Nature 436 (2005):
643–45.
[13] D. Kohn, "The Aesthetic Construction of Darwin's Theory," in The
Elusive Synthesis: Aesthetics and Science, ed. A. I. Tauber (Boston:
Kluwer, 1996), 13–48.
[14] G. Cuvier, Discours sur les Révolutions de la Surface du Globe
(Paris, 1812).
[15] L. W. Alvarez et al., "Extraterrestrial Cause for the Cretaceous-
Tertiary Extinction," Science 208 (1980): 1095–1108.
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