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.

Natural Selection And Punctuated Equilibrium

A reader asks-

I am still curious as to the mechanism and speed with which evolution by natural selection itself happened, like was it "punctuated equilibrium" or was it a slow and steady (continuous) mode of evolution, or was it a combination of the two, or some altogether third process. Not many programs discuss this unfortunately..

I left a short answer in a comment. Some additional thoughts:

Well.. Natural Selection and Punctuated Equilibrium are not directly connected with each other. Natural selection along with random genetic drift are mechanisms of evolution. Populations change in their genetic character and morphology due to natural selection or random genetic drift or a combination of the two. Punctuated equilibrium on the other hand refers to the tempo and pattern of morphological change seen in the fossil record and its significance. As was originally proposed by Eldridge and Gould it said that morphological change is concentrated in short bursts during cladogenesis i.e. when a new species buds off from an ancestral species. This change in morphology can be driven via natural selection or drift. So Punctuated Equilibrium says nothing about the primacy of any particular mechanism of evolution. Rather the emphasis is on the observed long periods of statis or little directional change in morphology in a lineage interrupted by geologically rapid bursts of change interpreted to be coupled to cladogenesis.

Gould later retracted somewhat from this position. He accepted the explanation for the pattern of Punctuated Equilibrium put forth by evolutionary biologist Douglas Futuyama which was that change could occur at any time during the life of a species but it is only when a small population gets reproductively isolated from its parent population i.e when the two populations stop exchanging genes that any directional change may get fixed or become permanent enough to show up in the fossil record.

Then there are examples of lineages changing in a slow and steady fashion too.. so can't generalize.. Nature has examples of both.

Regarding why programs don't cover this-its true that television programs have to my knowledge not covered this topic. My sense is programs on evolution and fossils stick to popular topics like the discovery of fossils of recognizable creatures like dinosaurs, the first tetrapods, proto-whales or on dramatic events in the history of life such as mass extinctions. A debate that uses detailed morphometric analysis of creatures like trilobites and foraminifers to reveal the pace of evolution (two organisms used extensively to test for patterns of punctuated equilibrium due to their abundance) may be thought of as too arcane to make for great television.

Having said that punctuated equilibrium got its fair share of attention in the print media and mostly for the wrong reasons for it was widely misinterpreted by the media as some kind of alternative to conventional evolution. The biggest mistakes made were in thinking that punctuated equilibrium means that new species form due to large morphological changes that occur suddenly. The primary authors Eldridge and Gould never advocated this, but the garbled version promoted in popular press made it seem so and large changes meant that some unknown genetic mechanism (macromutations?) may be at work. In reality, some of the demonstrated cases of punctuated equilibrium from trilobites showed that the new species differed only slightly from the ancestral species, nothing that could not be explained by well understood processes in an evolving population. So, an interesting theory that sought to explain patterns of appearances of new species in the fossil record as an example of allopatric speciation and migration became sensationalized as an alternative to "Darwin's theory" of evolution.

Creationists loved it,  palaeontologists banged their heads in frustration and much of the reading public have been confused ever since.