Fifty years ago, American invertebrate paleontologists Niles Eldredge (pictured above) and his colleague, the late Harvard University paleontologist and author Stephen Jay Gould (this year marks the 20th anniversary of his death) published a joint paper Punctuated Equilibria: An Alternative to Phyletic Gradualism in the volume Models in Paleobiology, edited by T.J.M. Schopf. The new theory challenged Charles Darwin's premise that evolution occurs gradually (phyletic gradualism), but rather asserts that evolution occurs in spurts of speciation in isolation, interspersed with long periods of stasis (stability). In other words, once a species appears in the fossil record, the population will become stable, showing little evolutionary change for most of its geological history.
By Curtis Abraham
The city of Alpena, Michigan overlooks Lake Huron’s stunningly beautiful Thunder Bay. During the turn of the Twentieth century Alpena, perhaps from the substantial proceeds from being one of North America’s most lucrative lumber harvesting hubs and the prestige that comes with it, installed its first telephone three years after Scotts inventor Alexander Graham Bell received a patent for the telephone, while electricity arrived two years after Thomas Edison invented the light bulb.
Alpena’s ability to adopt new innovations didn’t hinder its willingness to embrace the past, especially if that past proved profitable in the present (the city is also the site to one of the world’s largest shale quarries). The city’s geological past is also scientifically profitable to paleontologist. In addition to its limestone industry is the Alpena Formation, a geologic structure chock-full of fossils dating back to the Devonian period, about 420 million years ago. Although the Devonian has been hailed as the ‘Age of Fishes’ due to the great proliferation of fish species, its ancient marine life also included ammonites, a subclass of mollusk that grew as large as car wheels and have been found on Mt. Everest, great coral reefs and trilobites, mollusk-like brachiopods whose modern relatives includes the horseshoe crab.
In 1966/6 7, while on a field excavation, the American paleontologist Niles Eldredge was busily washing his dirty field clothes in a local laundromat in Alpena, Michigan . Eldredge reached into his pocket and removed what he describes as a “gorgeous specimen” that had fossilized in a rolled-up position (trilobites had the ability to roll themselves into a ball like armadillos or pill bugs).
A native New Yorker and former Chairman and Curator of the Division of Paleontology at the American Museum of Natural History in New York City, Eldredge is also one of the world’s foremost experts on evolutionary theory and trilobites, sea creatures that perished in a mass extinction event way before the rise of the dinosaurs (about 251 million years ago during the Permian period).
But the fossil’s beauty seemed to mock Eldredge’s frustration.
“The thing that started bothering me [was that] I was finding that I couldn’t tell the difference between the specimens I was getting in New York and the ones in Michigan,” Eldredge recalls in the American Museum of Natural History YouTube video: ‘Niles Eldredge: Trilobites and Punctuated Equilibria’.
Eldredge began studying trilobites, distant relatives of horseshoe crabs, believing that he would have a better chance of spotting evolutionary change in these ancient marine creatures (he initially studied fossilized snail shells but trilobites had a more complex morphology and thus having a better chance of spotting evolutionary change). Paleontologists (and paleoanthropologists) frequently bemoan the lack of fossil specimens in the field. But in this case there was certainly no shortage.
“People don’t seem to realize that, particularly on the east coast [of the United States] but basically all over the world, the ancient sea-ways used to flood the interiors leaving a huge dense fossil record with millions of specimens, not just single specimens, and they were preserved rather well,” says Eldredge.
During the Paleozoic era 380 million years ago, North America was largely submerged under no more than 200 feet (about 61 meters) of tropical, briny waters. Circulation of fresh oceanic waters would have been confined to areas closer to the peripheries of the continent than to its central regions such as what later became the American mid-West.
Evaporation rates were high, so high that large quantities of salt precipitated directly from the seas. The basin that is today Michigan was encircled by dead coral reefs, which effectively isolated central Michigan and cutting it off from the main seaway. Salt-laden ocean water repeatedly poured into the basin while constant evaporation and reduced rainfall over millions of years led to the accumulation of miles of salt beds in the center of Lower Michigan. Unsurprisingly, today the area is an inexhaustible source of rock, table and other salts.
The most common arthropod at the time were trilobites (lobsters, crabs and shrimps are arthropods- spineless creatures with exoskeletons, segmented bodies and paired limbs).Over 20,000 species of trilobites are known from the fossil record, but one of the most common and easily collected is Phacops rana, a species that had bulging frog-like eyes atop its head (rana is the Greek word for frog).
Eldredge soon learned that this species had a very wide geographic distribution: from Ontario to the north and all the way down the Appalachian Mountains to the south, and from New York State to the east and as far west as Iowa. Re-examining the fossil, Eldredge came to the realization that the specimen that had caused him such angst in the Alpena laundromat wasn’t what he thought it was.
“Turns out, it was a different species entirely. It wasn’t even Phacops rana [but] Phacops iowensis,” Eldredge remembers. “I did get this sinking feeling that there wasn’t a lot of change going on either geographically or through time.”
(Years later when asked about the genesis of punctuated equilibrium, Eldredge replied “despair and panic” as the investigation into trilobite evolution was part of his doctoral research.)
Charles Darwin, the 19th century English naturalist and co-discoverer of the theory of evolution via natural selection, faced a similar problem searching for evidence of evolution in the fossil record.
“Why then is not every geological formation and every stratum full of such intermediate links?” Darwin pondered. “Geology assuredly does not reveal any such finely graduated organic chain ; and this, perhaps, is the most obvious and serious objection which can be urged against the theory. The explanation lies, as I believe, in the extreme imperfection of the geological record.”
Direct fossil evidence of all the intermediate forms connecting an ancestor to its modern descendants would have provided undeniable evidence in favor of his theory. His contemporaries repeatedly criticized him for advocating a theory that was unsupported by the geological record. Darwin responded to these criticisms by attributing the absence of transitional forms to the fact that the geological record is incomplete
Trilobite (Phacops rana)
But evolution does not unfold in this manner. As Eldredge and Gould wrote in their landmark paper in 1972:
“The theory of allopatric (or geographic) speciation suggests a different interpretation of paleontological data. If new species arise very rapidly in small, peripherally isolated local populations, then the great expectation of insensibly graded fossil sequences is a chimera. A new species does not evolve in the area of its ancestors; it does not arise from the slow transformation of all its forbears. Many breaks in the fossil record are real.”
Meanwhile, at the University of Edinburgh in Scotland, paleontological research was taking place that was to have direct impact on Eldredge’s development of Punctuated Equilibrium (Punctuated Equilibria).
Euan Clarkson is a prolific writer of scientific publications and author of ‘Invertebrate Paleontology and Evolution’, one of his discipline’s most basic texts for undergraduate students. Like Eldredge, the now 85-year old Scottish paleontologist is also an expert on trilobites but from the Silurian Period in England, about 425 million years ago.
“Niles Eldredge invited me to New York in 1971. He told me that he had read my papers, and had an evident interest in phacopid eyes,” remembers Clarkson via email. “We got on very well, though our interests soon diverged.
He added:
“I was much more interested in the biology of trilobites and local geology. I think I wasn't clever enough to understand cladistics, which interested him greatly but I retained a great respect for him, and for Steve Gould, whom I met later, and who remained a good friend.”
Unlike Eldredge, however, Clarkson specialized on the visual systems of these extinct marine arthropods (these specimens had well preserved anatomy). Clarkson noticed that the eyes of his specimens, composed of visible bulging lenses, were arranged in vertical rows, so he counted them row by row.
Eldredge adopted Clarkson’s technique and began counting the number of lenses of his own trilobite specimens.
“Then I had another kind of eureka eventually,” says Eldredge. I saw that specimens from some localities typically had seventeen columns of lenses, but some [others], for instance the famous ones from the silica shale in Ohio, had eighteen.
(The Silica Shale near the Ohio-Michigan border by Toledo, OH, contains a spectacular array of fauna. This includes 40 cnidarian species, 40 species of bryozoans, 63 species of brachiopods, 21 species of bivalves, 19 species of crinoids, and many, many other organisms.)
So I started plotting the numbers on maps and it soon became stunningly obvious that the seventeen column of lenses Phacops rana was in New York all the way down the Appalachians. But in the mid-west there was something different, very similar but it had eighteen columns of lenses.”
What happened was that the trilobite with eighteen columns of lenses (technically called the 18-ventral-file-form) that ended up in the Midwest and surviving was also present in New York, but for a very brief time. In geographic isolation it evolved quickly into the species with seventeen columns of lenses. For two or three million years in the Midwest, a slightly different species with eighteen columns of lenses whereas the eastern muddier environments with had the ones with seventeen columns of lenses.
According to Eldredge:
“What happened was the seas disappeared for a period of time in the American Midwest evidently driving to extinction that earlier form. What really was going on was [when the seventeen column lens species] showed up after the eighteen column lens [species] ceased to exist, it was not an evolutionary change, it was a migration when the seas came back.”
He added:
“That’s what Punctuated Equilibria is; it’s a combination of two things. It’s a combination of the notion of geographic isolation as being important in evolution, and the realization that once a new species appears, if it survives, it tends to do so largely unchanged. There might be some variation within populations and between populations geographically but that species really tends to hang on for, in the case of marine invertebrates, five to ten million years.”
Eldredge and Gould’s ideas were built on the foundations of the theory of geographic speciation, which was developed further in the 1940s and 50s by two architects of the Modern Synthesis (the biological, genetic and fossil evidence that verified Darwin and Wallace’s theory of evolution via natural selection), Harvard evolutionary biologist and ornithologist Ernst Mayr and the Ukrainian-American geneticist Theodosius Dobzahansky. Their investigations had resolved what was called the ‘Species Problem’- the long-standing failure of biologists to agree on how species should be identified and how scientists should define the word 'species'.
(The pair also used geneticist and evolutionary biologist I. Michael Lerner’s term ‘homeostasis’ to frame an initial explanation for the stability [‘stasis’] of species as a main practical component of punctuated equilibrium).
Mayr approached the problem with a new definition for species. In his book Systematics and the Origin of Species (1942) he wrote that a species is not just a group of morphologically similar individuals, but a group that can breed only among themselves, excluding all others.
(Writing in the volume “The Dynamics of Evolution”, Mayr stated that evolutionary stability had been “unexpected by most evolutionary biologists” and that punctuated equilibrium “had a major impact on paleontology and evolutionary biology.”)
Although the sudden appearance of species and its relationship to speciation was proposed and identified by Mayr in 1954, historians of science generally recognize the 1972 Eldredge and Gould’s paper as the basis of the new paleo-biological research program.
“[Puncuated Equalibrium] showed that the fossil record has real data and real patterns in the history of life that need to be explained by evolutionary theory,” Eldredge tells me, “and that the traditional gradualistic picture left behind by Charles Darwin was in fact not the most accurate depiction of what evolution looks like.”
The theory was, of course, not without its critics. The spats were best reflected by the thinly veiled insults that represented those who believed in phyletic gradualism (‘evolution by creeps’) and those who jumped on the punctuated equilibrium bandwagon (‘evolution by jerks’).
The British evolutionary biologist and author Richard Dawkins was a particularly vocal critic at the time. Dawkins is a prominent proponent and popularizer of the gene-centered view of biological evolution, which argues that since heritable information is passed from generation to generation almost exclusively by DNA, natural selection and evolution are best considered from the perspective of genes.
In 1996, Dawkins published The Blind Watchmaker where, twenty-four years after the publication of Eldredge and Gould’s landmark paper, he emphasizes that punctuated equilibrium has been “oversold by some journalists”, but partly due to Eldredge and Gould's “later writings”. Dawkins contends that the hypothesis “does not deserve a particularly large measure of publicity”. It is a “minor gloss,” an “interesting but minor wrinkle on the surface of neo-Darwinian theory,” and “lies firmly within the neo-Darwinian synthesis”.
But the academic quarrels went far beyond the lively discussions of punctuated equilibrium and went to the heart of the nature of what species are.
“I don’t think Richard criticized punk eek per se,” says Eldredge. “The fights were more generalized—[for example,] we thinking that species have something to do with evolution, and not just genes, [for example].[But] we demonstrated that species have a real existence in nature, with births, histories and deaths. They can be construed as ‘packages of genetic information’—but they themselves are individuals living in the natural world. There was a political element as well. It was, in essence, a turf war—as these things nearly always turn out to be.”
[Dr. Eldredge's books can be purchased here: (https://www.thriftbooks.com/a/niles-eldredge/319796/]