Evolution Through Punctuated Equilibrium: History Of An Idea

Palaeontologist Niles Eldredge explains how one of the most famous papers on paleontology and evolution came to be published: 

Steve was determined to be a part of Tom’s plan to do a GSA symposium and publish a book of essays on this new-fangled concept of “paleobiology.” Tom had a list of topics and was shopping around for speakers to be assigned to each one. When Steve saw the list, he told me that he had first wanted “morphology”—but that was already assigned to Dave Raup. So he opted instead for “phylogeny”—but that had been grabbed up by Mike Ghiselin. That left only “speciation,” the last of the evolutionarily imbued topics on Tom’s list, as yet unassigned. Steve called me up, explained the situation, and said he had settled for speciation—but could not think of anything much to say about it beyond the manuscript I had written and recently submitted to Evolution—there of course being no Paleobiology as yet. “The Allopatric Model and Phylogeny in Paleozoic Invertebrates”—a distinctly un-Gouldian, plodding, if accurate, title (Eldredge 1971). Without Ralph Gordon Johnson in the editorial chair of Evolution at that time, I doubt that that early paper would have been accepted. As it was, it was likely to have gone relatively unnoticed—had not Tom come along, Steve grabbing “Speciation”—and Steve asking if we could coauthor the paper along the basic lines of my first effort. He was stuck with “speciation,” and couldn’t think of anything much to say beyond what I had said in the Allopatric Model manuscript.

This passage is from an article by Niles Eldredge titled Reflections On Punctuated Equilibria, published in a recent issue of Paleobiology. The 1972 paper he refers to, coauthored with Stephen Jay Gould, was, Punctuated equilibria: an alternative to phyletic gradualism. It marked the beginnings of a long debate on how to interpret the patterns of morphological change observed in the fossil record. Do species remain in stasis, showing little morphological change through much of their existence as Eldredge and Gould argued? Do periods of rapid morphological change coincide with the origin of new species (speciation)? Supporters hailed it as a revolutionary work. Critics called it 'evolution by jerks', a jibe aimed not just at the patterns of change.

Dr. Eldredge provides a very insightful look at the history of this idea including some fascinating snippets on Darwin's thinking about divergence and species origins. For Darwin, change accumulates incrementally over long passages of time. Divergence via natural selection can give rise to descendant varieties even without geographic isolation of a population. Later thinking has given more importance to exogenous factors like climate change in causing habitat fragmentation and reproductive isolation. Populations gets geographically isolated first, and then diverge from the ancestral species either through natural selection or random genetic drift. Eldredge and Gould applied this idea to the fossil record and emphasized that the sudden appearance of new fossil species is a manifestation of long periods of stability interrupted by episodes of isolation and geologically rapid shifts in morphology (allopatric speciation).

There is a lot to take in and think about the long term patterns of change preserved in the fossil record. But it is enriching reading. The article is open access.

Punctuated Equilibrium Is About Small Subtle Changes

The theory of punctuated equilibrium (PE) put forth by paleontologists Niles Eldredge and Stephen Jay Gould proposes that morphological evolution speeds up during lineage splitting events when new species form. In practice, a paleontologist sampling fossil species in a sedimentary section will find that a species over its lifetime does not show any trend in morphological changes. That species eventually goes extinct at a particular interval. Fossils of the inferred descendant species appear abruptly in the sedimentary bed above. A fossil population showing a mix of traits diagnostic of both species is not generally seen. 

Eldredge and Gould argued that this pattern of change observed in fossil species shows that the history of a species is really characterized by long periods of no change, eventually interrupted by rapid evolution to a new species.

A reassessment of one textbook example in bryozoans suggests no evidence of such a punctuated mode. 

A write up in Phys.org describes the new work. 

Revisiting a Landmark Study System: No Evidence for a Punctuated Mode of Evolution in Metrarabdotos - Kjetil Lysne Voje, Emanuela Di Martino, and Arthur Porto

I am not going into the work itself. There are well documented examples of the punctuated equilibrium mode of evolution while others as in this case may see revision from time to time. 

Instead, I wanted to comment on one of my pet peeves about this topic which is the common use of the phrase "major evolutionary change" to describe the actual evolutionary changes.

This idea which never seems to go way that PE involves big or major changes has caused a lot of misunderstanding about the theory.  As the authors of PE took pains to point out, the amount of morphological difference between ancestor and descendant species is subtle, often taking excruciating examination to recognize. 

Here is Niles Eldredge describing his work on Devonian trilobites.

I measured some 50 different lengths and widths- length of the head, distance between the eyes, height of the eyes, length of the tail and so on -on hundreds of specimens. This was tedious. Each specimen had to be cleaned to at least well enough to make all the anatomical landmarks visible. Each had to be mounted on a block of wood, stuck to a blob of plastilene, with the tops of the eyes (a flat surface) in a horizontal plane, perpendicular to my line of sight down the barrel of the microscope. There was a little scale inside the right eyepiece reticle from which to read off the various measurements. After a day of measuring, and until I got used to the microscope, I would see double from the bus window on my way home. 

 from Time Frames: The Evolution of Punctuated Equilibria.

Trilobites have compound eyes, made up of columns of lenses. All this detailed work led to the recognition that a new species had 17 columns of lenses, instead of 18 in the ancestral species found in slightly older strata!

This point about small changes was missed by many observers. Creationists conflated PE with a theory of macro-mutation or saltation, a sudden origin of big morphological change as for example seen in the transition from reptiles to mammals. They claimed that Darwin with his insistence on gradual accumulation of small changes had been wrong all along! 

Not so. The morphological differences observed involved small changes between the ancestor and descendant species.  It takes an expert to recognize this shift in form. 

The real significance of PE is not about the amount of change, but the pattern. PE proposes that very little morphological divergence takes place during the lifetime of a species. It is only when a sub-population of that species gets isolated that it may experience relatively rapid evolution. This genetic isolation results in the formation of a new species. The ancestral species may persist in the main parts of its range, while one of its peripheral populations has evolved into a new species. Biologists term this process of branching or lineage splitting as cladogenesis. 

PE is about explaining these rhythms in the history of lineages. Long periods of stasis (little or no change in form), punctuated by relatively rapid transition to a new form. This transition, though rapid in geologic time,  may take place over hundred of generations and thousand of years.

Why then isn't this transition to a new form preserved in the rock record as a fossil population with a mix of characters? The answer Eldredge and Gould prefer is that reproductive isolation takes place in outlier regions of a species range. Sediments deposited in these isolated basins will be archiving the incremental evolution of a new form, just as Darwin envisaged,  via a mixed or transitional population. However, these outliers containing transitional populations have less chance of getting preserved in the rock record.  The sedimentary sections that paleontologists examine are generally from the main part of the basin. This is not where the change to a new species has taken place.  The sudden appearance of an inferred descendant species in this strata is really recording the migration of the descendant from an isolated part of the species range where it evolved, into the central range of its extinct ancestor. 

Biologists have named this process of formation of a new species through reproductive isolation in a geographically distant area as allopatric speciation. Eldridge and Gould's theory of punctuated equilibrium invokes allopatric speciation and migration to explain the abrupt appearance of new species in the fossil record. In terms of the mechanisms of change, it is not an alternative to Darwinism as is often portrayed. Both the authors accept that even during the period of 'rapid' evolution, changes accumulate through natural selection or genetic drift incrementally across generations. But the vagaries of preservation means that sediments which record this transition are rarely available for study.

Stephen Jay Gould On Justice Scalia's Misunderstanding About Evolution Science

The United States Supreme Court Justice Antonin Scalia died a couple of days ago. I remembered an  essay on him written by Stephen Jay Gould. The occasion was Justice Scalia 's dissenting opinion in the 1987 case that invalidated the State of Louisiana 's "Balanced Treatment for Creation-Science and Evolution-Science Act." Louisiana was in effect arguing that "Creation Science" be put on the same footing as the theory of evolution in science classrooms.

Here is an excerpt that goes to the core of Justice Scalia's misunderstanding about what the theory of evolution seeks to elucidate:

 Justice Scalia has defined evolution as the search for life's origin--and nothing more. He keeps speaking about "the current state of scientific evidence about the origin of life" when he means to designate evolution. He writes that "the legislature wanted to ensure that students would be free to decide for themselves how life began based upon a far and balanced presentation of the scientific evidence." Never does he even hint that evolution might be the study of how life changes after it originates--the entire panoply of transformation from simple molecules to all modern multi-cellular complexity.

and a wonderful concluding passage-

   Consider also, indeed especially, Scalia's false concept of science. He equates creation and evolution because creationists can't explain life's beginning, while evolutionists can't resolve the ultimate origin of the inorganic components that later aggregated to life. But this inability is the very heart of creationist logic and the central reason why their doctrine is not science, while science's inability to specify the ultimate origin of matter is irrelevant because we are not trying to do any such thing. We know that we can't, and we do not even consider such a question as part of science.

    We understand Hutton's wisdom. We do not search for unattainable ultimates.

We define evolution, using Darwin's phrase, as "descent with modification" from prior living things. Our documentation of life's evolutionary tree records one of science's greatest triumphs, a profoundly liberating discovery on the oldest maxim that truth can make us free. We have made this discovery by recognizing what can be answered and what must be left alone. If Justice Scalia heeded our definitions and our practices, he would understand why creationism cannot qualify as science. He would also, by the way, sense the excitement of evolution and its evidence; no person of substance could be unmoved by something so interesting. Only Aristotle's creator may be so impassive.

    Don Quixote recognized "no limits but the sky," but became thereby the literary embodiment of unattainable reverie. G.K Chesterton understood that any discipline must define its borders of fruitfulness. He spoke for painting, but you may substitute any creative enterprise: "Art is limitation: the essence of every picture is the frame."

Let me add here that although the problem of the origin of life is not addressed directly by the theory of evolution, it does come under the purview of being a scientific problem. When Gould writes "we do not even consider such a question as part of science", he means specifically the theory of evolution.  Creationists would have you think that life began as some miraculous spark, a forever mysterious event that is beyond scientific inquiry. Not so. Basic chemistry is making headway in our understanding of how inanimate matter can aggregate to form self replicating entities. See this fine post by Ashutosh Jogalekar on how chemists ponder the Origin Of Life problem. 

Quote: Stephen Jay Gould On Paleontology

This study of periodicity of mass extinction was published last month-

Periodic impact cratering and extinction events over the last 260 million years - Michael Rampino and Ken Caldiera.
 
The claims of periodicity in impact cratering and biological extinction events are controversial. A newly revised record of dated impact craters has been analyzed for periodicity, and compared with the record of extinctions over the past 260 Myr. A digital circular spectral analysis of 37 crater ages (ranging in age from 15 to 254 Myr ago) yielded evidence for a significant 25.8 ± 0.6 Myr cycle. Using the same method, we found a significant 27.0 ± 0.7 Myr cycle in the dates of the eight recognized marine extinction events over the same period. The cycles detected in impacts and extinctions have a similar phase. The impact crater dataset shows 11 apparent peaks in the last 260 Myr, at least 5 of which correlate closely with significant extinction peaks. These results suggest that the hypothesis of periodic impacts and extinction events is still viable.

This idea is not new and I remembered Stephen Jay Gould's essay "The Cosmic Dance of Siva" (in The Flamingos's Smile) which describes one of the early such hypothesis followed by his trademark meditations of the role of paleontology in understanding the nature of evolution  and the history of life. Gould was always irked by the opinions held by many that paleontology is a dusty sort of a science, where people spend their careers fighting over species names, and that they have nothing important to say about the theoretical aspects of evolution. A large part of his popular science writing effort was devoted to demolishing  this notion. He was very successful in it with essay after essay beautifully demonstrating the utmost importance of paleontology in highlighting life's little oddities as well as its grand patterns.

Paleontology in the early 1970's underwent something of a change in attitude. Decades before "big  data" became a buzzword, researchers led by David Raup, Jack Sepkoski,  Leigh Van Valen and Tom Schopf, to name a few,  began amassing enormous data sets on fossil characteristics and species distributions and subjected them to rigorous statistical analysis in an effort to elucidate distribution of biodiversity and macro-evolutionary trends. Gould's own work was substantial. Ideas such as "punctuated equilibrium" (with Niles Eldridge) which relies on the fossil record to tease out patterns in the mode and tempo of evolution, were met with admiration as well as fierce criticism as was his thinking on the role of contingency and chance and the limits of natural selection and adaptive evolution as explanations for life's historical trajectories. His theoretical forays did make paleontologists think more broadly about their data and what it tells us about evolution but my take is that it didn't cause a revolution in evolutionary theory as was made out by the media and by Gould's  rhetoric. But he did elevate the status of paleontology and that can only be good for science.

He writes:

Most hot  ideas turn out to be  wrong. I can only hope that I will not be remembered as the man who campaigned with a name for the nonexistent (surely worse than a moon for the misbegotten). Some chances are certainly worth taking. If Thalia smiles and Siva exists, think what it all will mean for my beloved science of paleontology. We have labored so long under the onus of boredom and dullness. We are guardians of life's history, but we are often depicted as mindless philatelists of stone; specialists in tiny corners of space, time, and taxonomy; purveyors of such arcane names  as Pharkidonotus percarinatus in extended orgies of irrelevant detail. The editors of Britain's leading scientific journal wrote of 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." 

That impression has changed since to include paleontology as an important contributor to evolutionary theory. For that, the field owes him a debt.