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Yes, the Punctuated Equilibrium Model Was Developed to Explain the Lack of Transitional Fossils

Casey Luskin
Photo credit: J.M. Luijt, CC BY-SA 2.5 NL , via Wikimedia Commons.

I’ve been involved in the evolution debate for years — long enough to observe that certain arguments trigger defenders of the evolutionary paradigm. One of those arguments, oddly, is the claim that the punctuated equilibrium model of evolution was intended to explain the lack of transitional forms in the fossil record. The subject came up recently in the comments section of my extended interview for the “Fossils: Mysterious Origins” episode of Science Uprising. I wrote an article discussing this topic years ago at the IDEA Center, but let’s go over it again.

Defining Punctuated Equilibrium

Punctuated equilibrium, sometimes abbreviated as “punc eq,” is a model of evolution that was first developed in the 1970s by paleontologists Stephen Jay Gould and Niles Eldredge. In contrast with Darwinian gradualism, punctuated equilibrium proposes that populations exhibit “stasis” (remaining unchanged for long periods of time), punctuated by short periods of rapid evolutionary change during which new species emerge. According to this theory, most evolution takes place in small populations over relatively short geological time periods. By proposing reduced transitional population sizes and time spans, punc eq drastically limits the expected number of transitional organisms. Since most individuals do not fossilize, the likelihood of finding transitional forms is decreased.

Central to the punc eq model is the concept of allopatric speciation, where a portion of a population becomes geographically isolated. Because the “daughter population” may be small, it could potentially change rapidly in response to new selection pressures in its environment. When first proposing punc eq, Eldredge and Gould explained the implications of this mode of speciation:

(1) The expectations of theory color perception to such a degree that new notions seldom arise from facts collected under the influence of old pictures of the world. New pictures must cast their influence before facts can be seen in different perspective.

(2) Paleontology’s view of speciation has been dominated by the picture of “phyletic gradualism.” It holds that new species arise from the slow and steady transformation of entire populations. Under its influence, we seek unbroken fossil series linking two forms by insensible gradation as the only complete mirror of Darwinian processes; we ascribe all breaks to imperfections in the record.

(3) The theory of allopatric (or geographic) speciation suggests a different interpretation of paleontological data. If new species arise very rapidly in small, peripherally isolated populations, then the expectation of insensibly graded fossils 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. 

(4) The history of life is more adequately represented by a picture of “punctuated equilibria” than by the notion of phyletic gradualism. The history of evolution is not one of stately unfolding, but a story of homeostatic equilibria, disturbed only “rarely” (i.e., rather often in the fullness of time) by rapid and episodic events of speciation.

Eldredge and Gould, 1972, pp. 83-84

So, was punctuated equilibrium developed as a model to explain the abrupt appearance of new species in the fossil record — aka the lack of transitional forms? The answer is yes—seen in what you just read, and also in many quotes you’ll read below.

The passage above comes from the abstract of a chapter by Eldredge and Gould in the 1972 book Models in Paleobiology, edited by Thomas J. M. SchopfThey note there that the standard gradualistic view of evolution led paleontologists to “seek unbroken fossil series linking two forms by insensible gradation as the only complete mirror of Darwinian processes.” But then they suggest “a different interpretation of paleontological data” where “the expectation of insensibly graded fossils is a chimera” and “Many breaks in the fossil record are real.” In other words, Eldredge and Gould tried to justify why paleontologists should not expect to find series of transitional forms, and should instead expect “breaks” in the fossil record where new species arise without leaving fossils of transitional forms.

Their Editor Agreed

The author of the editor’s note at the beginning of their chapter understood the need to explain the lack of transitional forms, and that this was a major intent behind the punctuated equilibrium model. As Schopf put it, “gaps” are said to be “the logical and expected result of the allopatric model of speciation” and “allopatric speciation in small, peripheral populations automatically results in ‘gaps’ in the fossil record” (Schopf, 1972, pp. 83-83). Elsewhere Gould elaborated on exactly why the allopatric model would not result in fossilization of transitional forms:

Speciation, the process of macroevolution, is a process of branching. And this branching … is so rapid in geological translation (thousands of years at most compared with millions for the duration of most fossil species) that its results should generally lie on a bedding plane, not through the thick sedimentary sequence of a long hillslope. 

Gould, 1980

Because, according to Gould, the speciation process takes place in such a geologically short amount of time, it would not lead to fossilized transitional forms. The National Academy of Sciences (1998) explained it this way: “changes in populations might occur too rapidly to leave many transitional fossils.”

Grounded in the Lack of Transitional Forms

Why would Eldredge and Gould propose such a model? Because they knew the data showed that potential transitional fossils are an extreme rarity. Gould and Eldredge said this many times over the years. Here are a few notorious quotes:

  • “The extreme rarity of transitional forms in the fossil record persists as the trade secret of paleontology. The evolutionary trees that adorn our textbooks have data only at the tips and nodes of their branches; the rest is inference, however reasonable, not the evidence of fossils. … In any local area, a species does not arise gradually by the steady transformation of its ancestors; it appears all at once and ‘fully formed.’” (Gould, 1977) 
  • “The absence of fossil evidence for intermediary stages between major transitions in organic design, indeed our inability, even in our imagination, to construct functional intermediates in many cases, has been a persistent and nagging problem for gradualistic accounts of evolution.” (Gould, 1980)
  • “All paleontologists know that the fossil record contains precious little in the way of intermediate forms; transitions between major groups are characteristically abrupt.” (Gould, 1982, p. 189)
  • “No wonder paleontologists shied away from evolution for so long. It never seemed to happen. Assiduous collecting up cliff faces yields … a rate too slow to account for all the prodigious change that has occurred in evolutionary history. When we do see the introduction of evolutionary novelty, it usually shows up with a bang, and often with no firm evidence that the fossils did not evolve elsewhere!” (Eldredge, 1995, p. 95)

Gould and Eldredge readily admitted the commonality of abrupt appearances of new species and the lack of transitional forms in the fossil record. And they admitted this pattern with respect to the fossil record as a whole — not simply when discussing “preservational bias” for or against certain groups or something like that. They recognized the problem for gradualistic accounts of evolution across the board. Their model therefore sought to explain why abrupt change was the dominant pattern in the fossil record. The logic goes like this: We have a problem (abrupt appearance and stasis), and punc eq, in their telling, provides a solution. This alone tells us a major reason they proposed their theory was to explain the lack of transitional forms. 

But was their exact motive for proposing punctuated equilibrium ever stated explicitly? 

Explicitly Stated Motives Behind Punc Eq

While writing this, I searched for a nice passage from Gould explaining the basics of punc eq, and so I turned to his book Punctuated Equilibrium, published posthumously in 2007. Here I found a discussion by Gould of his thinking “in proposing punctuated equilibrium”:

I recount this story at some length, as an introduction to punctuated equilibrium, both because Falconer and Darwin presage in such a striking manner, the main positions of supporters and opponents (respectively) of punctuated equilibrium in our generation, and because the tale itself illustrates the central fact of the fossil record so well — geologically abrupt origin and subsequent extended stasis of most species. Falconer, especially, illustrates the transition from too easy a false resolution under creationist premises, to recognizing a puzzle (and proposing some interesting solutions) within the new world of evolutionary explanation. Most importantly, this tale exemplifies what may be called the cardinal and dominant fact of the fossil record, something that professional paleontologists learned as soon as they developed tools for an adequate stratigraphic tracing of fossils through time: the great majority of species appear with geological abruptness in the fossil record and then persist in stasis until their extinction. Anatomy may fluctuate through time, but the last remnants of a species usually look pretty much like the first representatives. In proposing punctuated equilibrium, Eldredge and I did not discover, or even rediscover, this fundamental fact of the fossil record. Paleontologists have always recognized the longterm stability of most species, but we had become more than a bit ashamed by this strong and literal signal, for the dominant theory of our scientific culture told us to look for the opposite result of gradualism as the primary empirical expression of every biologist’s favorite subject — evolution itself. 

Testimonials to Common Knowledge

The common knowledge of a profession often goes unrecorded in technical literature for two reasons: one need not preach commonplaces to the initiated; and one should not attempt to inform the uninitiated in publications they do not read. The longterm stasis, following a geologically abrupt origin, of most fossil morphospecies, has always been recognized by professional paleontologists.

Gould, 2007, pp. 19-20, emphases added

As you can see, a main point that Gould cites as animating his theory is the “geologically abrupt origin” of species, followed by their stasis (i.e., lack of evolution). 

But a much more direct statement of the motive for proposing punctuated equilibrium came in a 1977 paper by Gould and Eldredge in Paleobiology.

Two other classes of information were explained away or simply ignored: 1) morphological gaps in stratigraphic sequences — which might have suggested a punctuational view of evolution were attributed to imperfections of the fossil record; 2) evolutionary stasis, though recognized by all and used by stratigraphers in the practical work of our profession, was ignored by evolutionists as “no data.” Thus, Trueman rejoiced in Gryphaea(1922) but never mentioned the hundreds of Liassic species that show no temporal change. Rowe (1899) monographed Micraster but spoke not a word about its legion of static colleagues in the English chalk. In fact, the situation in paleontology is far worse than that confronting genetics a decade ago. At least the geneticists were frustrated by an absent technology: they knew what data they needed. Paleontologists allowed a potent, historical bias to direct their inquiry along a single path, though they could have accumulated other data at any time. What’s more, paleontologists accumulated hardly any good examples: the gradualistic idols that were established had feet of clay and rarely survived an intensive restudy. The tale of Gryphaea is dead in Trueman’s formulation (Hallam 1968; Gould 1972). Micraster will soon follow. (Rowe’s data identified three successive species, but he had no stratigraphic control for samples within taxa. Even if his gradualistic tale were true — which it is not — his own limited data could not have established it.) The collapse of classic after classic should have brought these gradualistic biases into question. The alienation of practical stratigraphy from an evolutionary science that required gradualism should have suggested trouble (see Eldredge and Gould, in press): always trust the practitioners.

This sorry situation led us to postulate our alternative model of punctuated equilibria (Eldredge 1971; Eldredge and Gould 1972). We wanted to expand the scope of relevant data by arguing that morphological breaks in the stratigraphic record may be real, and that stasis is data-that each case of stasis has as much meaning for evolutionary theory as each example of change. 

Gould and Eldredge, 1977, emphases added

At the beginning of the second paragraph they say there is a “sorry situation” that “led” them to postulate punc eq. The “sorry situation” is described in their first paragraph: there were two facts about the fossil record that were “ignored by evolutionists”: (1) “morphological gaps” and (2) “stasis.” “What’s more,” they write, “paleontologists accumulated hardly any good examples” of gradual change in the fossil record. They call these rare instances “gradualistic idols.” In other words, the lack of fossil evidence for transitional forms “led” them to postulate punc eq.

As a final example, consider this statement from Gould in his final tome, The Structure of Evolutionary Theory, published soon after his death: 

As I began my professional preparation for a career in paleontology, this vague dissatisfaction coagulated into two operational foci of discontent. First (and with Niles Eldredge, for we worried this subject virtually to death as graduate students), I became deeply troubled by the Darwinian convention that attributed all non-gradualistic literal appearances to imperfections of the geological record. This traditional argument contained no logical holes, but the practical consequences struck me as unacceptable (especially at the outset of a career, full of enthusiasm for empirical work, and trained in statistical techniques that would permit the discernment of small evolutionary changes). For, by the conventional rationale, the study of microevolution became virtually nonoperational in paleontology — as one almost never found this anticipated form of gradual change up geological sections, and one therefore had to interpret the vastly predominant signal of stasis and geologically abrupt appearance as a sign of the record’s imperfection, and therefore as no empirical guide to the nature of evolution. Second, I became increasingly disturbed that, at the higher level of evolutionary trends within clades, the majority of well documented examples (reduction of stipe number in graptolites, increasing symmetry of crinoidal cups, growing complexity of ammonoid sutures, for example) had never been adequately explained in the terms demanded by Darwinian convention-that is, as adaptive improvements of constituent organisms in anagenetic sequences. Most so-called explanations amounted to little more than what Lewontin and I, following Kipling, would later call “just-so stories,” or plausible claims without tested evidence, whereas other prominent trends couldn’t even generate a plausible story in adaptationist terms at all.

As Eldredge and I devised punctuated equilibrium, I did use the theory to resolve these two puzzles to my satisfaction, and each resolution, when finally generalized and further developed, led to my two major critiques of the first two branches of the essential triad of Darwinian central logic — so Oliver Sacks’s identification of punctuated equilibrium as central to my theoretical world holds, although more as a starting point than as a coordinating focus. By accepting the geologically abrupt appearance and subsequent extended stasis of species as a fair description of an evolutionary reality, and not only as a sign of the poverty of paleontological data, we soon recognized that species met all criteria for definition and operation as genuine Darwinian individuals in the higher-level domain of macroevolution-and this insight (by complex routes discussed in Chapter 9) led us to concepts of species selection in particular and, eventually, to the full hierarchical model of selection as an interesting theoretical challenge and contrast to Darwinian convictions about the exclusivity of organismal selection. In this way, punctuated equilibrium led to the reformulation proposed herein for the first branch of essential Darwinian logic.

Gould, 2002, pp. 38-39, emphases added

Gould says there that he “devised punctuated equilibrium” to “resolve … two puzzles.” The first puzzle was why the fossil record contained a “the vastly predominant signal of stasis and geologically abrupt appearance” and “one almost never found this anticipated form of gradual change.” As Gould tells it, he was “deeply troubled” by the typical “Darwinian convention that attributed all non-gradualistic literal appearances to imperfections of the geological record.” He wanted a better explanation for the pattern of abrupt appearances. For Gould, punctuated equilibrium was the answer. 

Imperfect, Yes. But Adequate. 

The final quote above emphasizes that before Gould and Eldredge could make their case for punctuated equilibrium, they had to address a different common explanation for the lack of transitional forms: the imperfection of the fossil record. Darwin (1859) attempted to save his theory of gradual evolution by maintaining that intermediate fossils are not found because of “the extreme imperfection of the geological record.” Even Gould (1977) noted that Darwin’s argument that the fossil record is imperfect “persists as the favored escape of most paleontologists from the embarrassment of a record that seems to show so little of evolution directly.” We still see evolution defenders appealing to the incompleteness of the fossil record to explain the lack of transitional forms. But in the last few decades, this excuse has lost credibility.

Paleontologists today generally recognize that while the fossil record is imperfect, it is still adequate to assess questions about evolution. One study in Nature reported that “if scaled to the … taxonomic level of the family, the past 540 million years of the fossil record provide uniformly good documentation of the life of the past” (Benton et al., 2000). A paper in Paleobiology evaluated our knowledge of the fossil record and concluded that “our view of the history of biological diversity is mature” (Foote, 1997). Paleontologists now frequently recognize that “jumps” between species, without intermediates, are not simply the result of an incomplete record. Eldredge and Tattersall (1982) put it this way: “The record jumps, and all the evidence shows that the record is real: the gaps we see reflect real events in life’s history — not the artifact of a poor fossil record.” This conclusion did not come easily, as one scientist who studied under Gould felt the need to implore his colleagues that “[e]volutionary biologists can no longer ignore the fossil record on the ground that it is imperfect” (Woodruff, 1980). 

Once paleontologists overcame “imperfection” as the favored excuse for the lack of transitional forms, they were able to start taking the fossil record at face value and seek explanations for what they saw. Punctuated equilibrium is one proposed explanation. But is it compelling? 

Analyzing the Trigger

There are many scientific problems with punctuated equilibrium. The biggest is that it requires too much evolutionary change too quickly. My purpose here is not to elaborate those problems — for details please see Luskin (2004)Luskin (2008), and Meyer (2013). The latter source is probably the most comprehensive: Chapter 7 in Stephen Meyer’s book Darwin’s Doubt

Those problems aside, it should be clear now that the model of punctuated equilibrium was developed, at least in large part, to explain the lack of transitional forms in the fossil record. Both the logic of the argument and the direct admissions of the architects of the model attest to this fact. So why would some Darwin defenders get so upset when you suggest just that?

I’m not a mind-reader, but if I had to speculate I’d say it’s because they like to see punc eq as a benign theory that flowed directly from of the logic of allopatric speciation — not as an attempt to explain away inconvenient data. But the literature shows they are wrong. As Gould and Eldredge attest, the first preference of paleontologists historically has been to seek gradual change documented by transitional forms and when those aren’t found they have historically explained the absence by appealing to an incomplete fossils record. They didn’t find gradual change, and it was determined that the record wasn’t so in complete after all. This led paleontologists like Gould and Eldredge to seek other explanations. 

Sometimes, Darwin defenders will answer that Gould was merely concerned with understanding “rates” of evolution. By reframing the debate that way, they attempt to argue (my paraphrase), “Whether evolution took place at a gradual rate or a rapid one, either way evolution still took place!” 

Well, it’s true that Gould was very concerned about rates of evolution — in fact that’s the point: Abrupt appearance reflects a very rapid rate of evolutionary change. So, when someone claims that Gould sought to understand “rates” of evolution, that’s exactly right. And the rates of evolution that left him (in his words) most “deeply troubled” were the rapid rates of evolution: evolution that occurred (apparently) at such a rapid rate that it left no evidence of transitions. 

Given that lack of direct fossil evidence, I would add that we are justified, it would seem, in wondering if evolution really occurred at all.

References

  • Benton, M. J., M. A. Wills, and R. Hitchin (February 3, 2000), “Quality of the fossil record through time,” Nature, 403: 534-536.
  • Darwin, Charles. The Origin of Species (1859), p. 292 (reprint, London: Penguin Group, 1985).
  • Eldredge, Niles and Stephen Jay Gould. “Punctuated Equilibria: An Alternative to Phyletic Gradualism.” In Models in Paleobiology, edited by Thomas J.M. Schopf, 82-115. San Francisco, CA: Freeman, Cooper & Company, 1972.
  • Eldredge, Niles and Ian Tattersall, The Myths of Human Evolution (Columbia University Press, 1982).
  • Eldredge, Niles, Reinventing Darwin (Wiley, 1995).
  • Foote, Mike (Spring, 1997). “Sampling, Taxonomic Description, and Our Evolving Knowledge of Morphological Diversity,” Paleobiology, 23: 181-206.
  • Gould, Stephen Jay. “Evolution’s erratic pace.” Natural History 86 (May, 1977): 12-16.
  • Gould, Stephen Jay. “Is a new and general theory of evolution emerging?” Paleobiology 6(1) (1980): 119-130.
  • Gould, Stephen Jay. The Panda’s Thumb: More Reflections in Natural History (W.W. Norton, 1982).
  • Gould, Stephen Jay. The Structure of Evolutionary Theory. Cambridge, MA: Belknapp Press of Harvard University Press, 2002.
  • Gould, Stephen Jay and Niles Eldredge. “Punctuated Equilibria: The Tempo and Mode of Evolution Reconsidered.” Paleobiology 3 (2) (Spring, 1977): 115-151.
  • Luskin, Casey. “Punctuated Equilibrium and Patterns from the Fossil Record.” Intelligent Design and Evolution Awareness (IDEA) Center (September 18, 2004).
  • Luskin, Casey. “Finding Intelligent Design in Nature,” Intelligent Design 101: Leading Experts Explain the Key Issues. Edited by H. Wayne House, 67-112. Grand Rapids, MI: Kregel, 2008.
  • Meyer, Stephen C. Darwin’s Doubt: The Explosive Origin of Animal Life and the Case for Intelligent Design. New York, NY: HarperOne, 2013.
  • National Academy of Sciences. Teaching About Evolution and the Nature of Science. Washington D.C.: National Academy Press, 1998.
  • Schopf, Thomas J. M. Editorial Introduction to Niles Eldredge and Stephen Jay Gould, “Punctuated Equilibria: An Alternative to Phyletic Gradualism.” In Models in Paleobiology. Edited by Thomas J.M. Schopf. San Francisco, CA: Freeman, Cooper & Company, 1972.
  • Woodruff, David S. (May 16, 1980), “Evolution: The Paleobiological View,” Science, 208: 716-717.