All you need to know about Punctuated Equilibrium (almost)

Common misconceptions concerning the hypothesis of Punctuated Equilibrium

Copyright © 2001-2010 by Douglas Theobald

Much confusion has surrounded the concept of Punctuated Equilibrium (PE) as proposed by Niles Eldredge and Stephen Jay Gould in 1972. This essay addresses a few of the erroneous views held by many creationists and even some evolutionary biologists concerning PE. In this article I make the following main points:

1. There are two common uses of "gradualism," one of which is more traditional, the other of which is equivalent to Eldredge and Gould's "phyletic gradualism."
2. Darwin was not a "phyletic gradualist," contrary to the claims of Eldredge and Gould.
3. PE is not anti-Darwinian; in fact, the scientific basis and conclusions of PE originated with Charles Darwin.
4. PE does not require any unique explanatory mechanism (e.g. macromutation or saltation).
5. Eldredge and Gould's PE is founded on positive evidence, and does not "explain away" negative evidence (e.g. a purported lack of transitional fossils).

I will not attempt to give an overview of PE here. For background concerning the details of the PE hypothesis I refer you to the first two major PE papers (Eldredge and Gould 1972; Gould and Eldredge 1977) and to Wesley Elsberry's online PE FAQ .

Gradualism

In the years since the PE debate first hit the evolutionary scene, evolutionary biologists have come to the realization that the term "gradualism" has been used in two very different senses.

The first sense of "gradualism" is the one which Darwin himself originated - evolution proceeds in small 'grades.' The opposite of this is saltation. Darwinian "gradualness" has little to do with the rate or tempo of evolution; it is a mode of genetic change that is dependent on population phenomena. Gradualness concerns genetically probable organismic changes between two consecutive generations, i.e. those changes that are within the range of normal variation observed within modern populations. Morphological change may appear fast geologically speaking, yet still be gradual (Darwin 1872, pp. 312-317 ; Dawkins 1996, p.241; Mayr 1991, pp. 42-47; Rhodes 1983).

The second sense of "gradualism" is that evolutionary rates are geologically slow, constant, and usually orthogenetic. The opposite of this could be "quantum" evolution (fast morphological change on a geological scale). This second usage is equivalent to Eldredge and Gould's "phyletic gradualism" (PG for short) (Eldredge and Gould 1972). This is not the way that Darwin used the term gradualism, although some biologists since Darwin have.

The first sense deals with the mode of evolution; the second deals with the tempo of evolution. This difference has been pointed out by several prominent scientists. Ernst Mayr's work on peripatric speciation is what provided the foundation for Eldredge and Gould's PE. In his book "One Long Argument" (1997, p. 47), Mayr states, "Understanding the independence of gradualness and evolutionary rate is important for evaluating the theory of punctuated equilibria put forth in 1972 by Steven Jay Gould and Niles Eldredge."

In their first PE paper, E&G first described the concept they called "phyletic gradualism." Phyletic gradualism was then contrasted with their PE hypothesis. E&G characterized phyletic gradualism thus:

"To Darwin, therefore, speciation entailed the same expectation as phyletic evolution: a long and insensibly graded chain of intermediate forms.

[a few paragraphs later]

In this Darwinian perspective, paleontology formulated its picture for the origin of new taxa. This picture, though rarely articulated, is familiar to all of us. We refer to it here as 'phyletic gradualism' and identify the following as its tenets:

(1) New species arise by the transformation of an ancestral population into its modified descendants.

(2) The transformation is even and slow.

(3) The transformation involves large numbers, usually the entire ancestral population.

(4) The transformation occurs over all or a large part of the ancestral species' geographic range.

These statements imply several consequences, two of which seem especially important to paleontologists:

(1) Ideally, the fossil record for the origin of a new species should consist of a long sequence of continuous, insensibly graded intermediate forms linking ancestor and descendant.

(2) Morphological breaks in a postulated phyletic sequence are due to imperfections in the geological record." (Eldredge and Gould 1972)

Darwin was not a "Phyletic Gradualist"

Contrary to the claims of E&G, Charles Darwin was not a phyletic gradualist. The first tenet of phyletic gradualism (PG) is common to any and all theories of speciation, is fundamental to modern evolutionary theory, and is not unique to PG. The following quotes show that phyletic gradualism has been wrongly attributed to Darwin, since he did not hold the last three tenets of PG:

Contradicts PG tenet #2

But I must here remark that I do not suppose that the process ever goes on so regularly as is represented in the diagram, though in itself made somewhat irregular, nor that it goes on continuously; it is far more probable that each form remains for long periods unaltered, and then again undergoes modification. (Darwin, Ch. 4, "Natural Selection," pp. 152)

Contradicts PG tenet #2

"It is a more important consideration ... that the period during which each species underwent modification, though long as measured by years, was probably short in comparison with that during which it remained without undergoing any change." (Darwin, Ch. 10, "On the imperfection of the geological record," p. 428)

Contradicts PG tenet #2, #3, and #4

"... natural selection will generally act very slowly, only at long intervals of time, and only on a few of the inhabitants of the same region. I further believe that these slow, intermittent results accord well with what geology tells us of the rate and manner at which the inhabitants of the world have changed." (Darwin, Ch. 4, "Natural Selection," pp. 140-141)

Darwin did not believe speciation to be even (tenet #2 of PG), since he describes natural selection as "intermittent" and "irregular," and since he emphasizes that the evolutionary history of a species is characterized by stasis punctuated by change. Evolution does not "[go] on continuously," since each species remains for long periods unaltered." Neither did he think that speciation involves the entire population (tenet #3 of PG) over a large geographic range (tenet #4 of PG), as he says "only on a few of the inhabitants of the same region." These are not isolated or exceptional quotes from Darwin; they are characteristic of his views on evolution (see the additional quotes given below). Thus, Darwin is not the originator of PG. In fact, as I will further prove below, Darwin's views were in direct opposition to PG, as he did not believe the last two consequences of phyletic gradualism listed by Eldredge and Gould.

Darwin of course believed that evolution from common ancestors produces sequences of gradual intermediate forms. In contrast, he did not believe that the fossil record "should consist of a long sequence of continuous, insensibly graded intermediate forms linking ancestor and descendant," as E&G falsely claim in "Consequence 1" of PG. Close after the above quote, Darwin wrote emphatically and with notable frustration:

Contradicts PG consequence #1

It has been asserted over and over again, by writers who believe in the immutability of species, that geology yields no linking forms. This assertion, as we shall see in the next chapter, is certainly erroneous. As Sir J. Lubbock has remarked, "Every species is a link between other allied forms." If we take a genus having a score of species, recent and extinct, and destroy fourfifths of them, no one doubts that the remainder will stand much more distinct from each other. If the extreme forms in the genus happen to have been thus destroyed, the genus itself will stand more distinct from other allied genera. What geological research has not revealed, is the former existence of infinitely numerous gradations, as fine as existing varieties, connecting together nearly all existing and extinct species. But this ought not to be expected; yet this has been repeatedly advanced as a most serious objection against my views.

Contradicts PG consequence #1

... we have no right to expect to find, in our geological formations, an infinite number of those fine transitional forms, which, on our theory, have connected all the past and present species of the same group into one long and branching chain of life. We ought only to look for a few links, and such assuredly we do find--some more distantly, some more closely, related to each other; and these links, let them be ever so close, if found in different stages of the same formation, would, by many palaeontologists, be ranked as distinct species. (Darwin, Ch. 10, "On the imperfection of the geological record," p. 428, emphasis added)

In fact, over 100 years later, E&G are still repeating this objection to Darwin's views, even though Darwin strongly argued that we should not find "infinitely numerous gradations" in the geological record. Additionally, Darwin did not think that "Morphological breaks in a postulated phyletic sequence are due to imperfections in the geological record" only (E&G's "Consequence 2" of PG). Darwin gave one example of what we should expect in the fossil record with a thought experiment about how biota of the Malay Archipelago would fossilize. Half of what Darwin wrote concerned the imperfect geological record; the other half concerns probable evolutionary population dynamics:

Contradicts PG consequence #2

It is also probable that each great period of subsidence would be interrupted by oscillations of level, and that slight climatical changes would intervene during such lengthy periods; and in these cases the inhabitants of the archipelago would migrate, and no closely consecutive record of their modifications could be preserved in any one formation.

Very many of the marine inhabitants of the archipelago now range thousands of miles beyond its confines; and analogy plainly leads to the belief that it would be chiefly these far-ranging species, though only some of them, which would oftenest produce new varieties; and the varieties would at first be local or confined to one place, but if possessed of any decided advantage, or when further modified and improved, they would slowly spread and supplant their parent-forms. When such varieties returned to their ancient homes, as they would differ from their former state in a nearly uniform, though perhaps extremely slight degree, and as they would be found embedded in slightly different sub-stages of the same formation, they would, according to the principles followed by many palaeontologists, be ranked as new and distinct species.

PE is not anti-Darwinian

Eldredge and Gould based their theory of PE on the neontological theory of allopatric speciation (especially the allopatric theory of Ernst Mayr, which he called "peripatric" speciation) (Mayr 1963, 1971). For instance, in the same paper under the section "Implications of allopatric speciation for the fossil record," they state,

"We wish to consider an alternate picture to phyletic gradualism [i.e., PE]; it is based on a theory of speciation that arises from the behavior, ecology, and distribution of modern biospecies. ... the allopatric theory has grown in popularity to become, for the vast majority of biologists, the theory of speciation." (Eldredge and Gould 1972)

Further down, Eldredge and Gould continue:

"In summary, we contrast the tenets and predictions of allopatric speciation with the corresponding statements of phyletic gradualism previously given:

(1) New species arise by the splitting of lineages.

(2) New species develop rapidly.

(3) A small sub-population of the ancestral form gives rise to the new species.

(4) The new species originates in a very small part of the ancestral species' geographic extent - in an isolated area at the periphery of the range.

These four statements again entail two important consequences:

(1) In any local section containing ancestral species, the fossil record for the descendant's origin should consist of a sharp morphological break between the two forms. .... we will rarely discover the actual event in the fossil record.

(2) Many breaks in the fossil record are real; they express the way in which evolution occurs, not the fragments of an imperfect record." (Eldredge and Gould 1972)

These tenets and conclusions of their "new" theory of PE are quite interesting when compared to these quotes from Origin of Species:

PE tenet #1 and #2

"If, however, the modified offspring of a species get into some distinct country, or become quickly adapted to some quite new station, in which child and parent do not come into competition, both may continue to exist." (Darwin 1872, Ch. 4, "Natural Selection," p. 155)

This quote shows that Darwin understood tenet 1 above in the description of allopatric speciation. It also indicates that Darwin realized that species could develop rapidly (tenet 2 of PE), as he says they may "become quickly adapted." Also,

PE tenet #1,#2 #3, #4, and PE consequences #1 and #2

"... the periods during which species have undergone modification, though long as measured by years, have probably been short in comparison with the periods during which they retained the same form. It is the dominant and widely ranging species which vary most frequently and vary most, and varieties are often at first local—both causes rendering the discovery of intermediate links in any one formation less likely. Local varieties will not spread into other and distant regions until they are considerably modified and improved; and when they have spread, and are discovered in a geological formation, they appear as if suddenly created there, and will be simply classed as new species." (Darwin 1872, Ch. 15, "Recapitulation and Conclusion", p. 619)

This quote is remarkable in that it nearly states every tenet and prediction that Eldredge and Gould listed for their paleontological treatment of allopatric speciation. Darwin agreed with the other tenets of allopatric speciation, including tenets 3 and 4 - "varieties are often at first local" and "will not spread ... until they are considerably modified." Furthermore, it is obvious that Darwin agreed with the two predictions of the allopatric speciation model concerning what will be found in the paleontological record.

Here I present quotes from The Origin of Species , Chapter 10, "On the imperfection of the geological record," that basically sum up the conclusions of PE:

"When we see a species first appearing in the middle of any formation, it would be rash in the extreme to infer that it had not elsewhere previously existed. So again, when we find a species disappearing before the last layers have been deposited, it would be equally rash to suppose that it then became extinct. We forget how small the area of Europe is compared with the rest of the world ... when we see a species first appearing in any formation, the probability is that it only then first immigrated into that area." (p. 423)

"... varieties are generally at first local; and that such local varieties do not spread widely and supplant their parent-form until they have been modified and perfected in some considerable degree. According to this view, the chance of discovering in a formation in any one country all the early stages of transition between any two forms is small, for the successive changes are supposed to have been local or confined to some one spot." (pp. 427-428)

"... it might require a long succession of ages to adapt an organism to some new and peculiar line of life, for instance, to fly through the air; and consequently that the transitional forms would often long remain confined to some one region; but that, when this adaptation had once been effected, and a few species had thus acquired a great advantage over other organisms, a comparatively short time would be necessary to produce many divergent forms, which would spread rapidly and widely throughout the world." (p. 433)

It is obvious from all of these quotes that Darwin did not think the "gaps" between fossil species were only due to geological processes, but that they are a direct consequence of natural speciation processes. Phyletic gradualism is a strawman when attributed to Darwin, and this is one of the reasons why so many evolutionary biologists reacted strongly to the initial presentation of the hypothesis of PE. Furthermore, it is erroneous even to claim PE as an original concept, since all of the tenets of allopatric speciation and the conclusions labelled as PE were stated by Charles Darwin over 100 years before Eldredge and Gould proposed their "novel" hypothesis.

PE neither requires nor proposes novel evolutionary mechanisms

As should be rather clear from the Eldredge and Gould quotations I have given, PE was proposed as a logical consequence of the allopatric speciation model. The allopatric speciation model was not formed by paleontologists, but was based on modern species distributions, observations of ecological interactions, and evidence from laboratory population studies (Mayr 1963, 1971). Eldredge and Gould thought of PE as being adequately explained by, and being a direct consequence of, allopatric speciation theory, which in their eyes was the dominant mainstream theory of speciation in current evolutionary biology. Thus, there is no reason to assume that PE requires a mechanistic explanation that is outside of standard population genetics and synthetic theory. In their second paper discussing the subject in 1977, Gould and Eldredge stated clearly,

"It [PE] represents no departure from Darwinian mechanisms." (Gould and Eldredge 1977, Section IV, "PE as the basis for a Theory of Macroevolution", page 139)

In fact, they considered paleontology alone as incapable of producing new mechanistic hypotheses, as they say,

"No theory of evolutionary mechanisms can be generated directly from paleontological data ... We can apply and test, but we cannot generate new mechanisms." (Gould and Eldredge, p. 93)

Gould and Eldredge have never formally tied macromutations and saltation with PE. No mechanisms outside of the normal genetic explanations for allopatric speciation were given in their seminal paper on PE. I have yet to see a reference where Gould and Eldredge suggest macromutation or saltation as a mechanism for PE. Gould's paper, "The Return of Hopeful Monsters," has been used as evidence that Gould was proposing saltation as a mechanism for PE. However, the theory of PE is not even mentioned or alluded to in this paper. Rather, the paper is about viewing Goldshmidt's work in a more objective light, and it should be thought of as independent from the PE hypothesis.

PE is founded on positive evidence

Because they feel that PE was concocted to "explain away" a purported lack of transitional fossils, some critics have maintained that Eldredge and Gould based their theory of PE on negative evidence. This is a strange argument, however, since the paper where they first proposed PE was completely based on two independent paleontological studies (on pulmonate gastropods and on Phacopsid trilobites), which they described in detail with extremely good temporal resolution. Using positive evidence, these studies showed stasis and rapid evolution that supports the PE model. Furthermore, Gould and Eldredge's second PE paper also extensively analyzed well-resolved paleontological evidence in support of their hypothesis.

That said, PE indeed was hypothesized to explain paleontological discontinuities, but specifically discontinuities between species only, not major taxa. Even young earth creationists hold that "microevolutionary" processes result in new species from closely related species, like the various gastropods, trilobites, and equids considered in the two PE papers. As the authors clearly state,

"PE is a model for discontinuous tempos of change at one biological level only: the process of speciation and the deployment of species in geological time." (Gould and Eldredge 1977, p. 145)

Gould has made their position regarding transitional fossils unambiguously clear time and again. From one of his books of collected essays, Gould says,

"Since we proposed punctuated equilibria to explain trends, it is infuriating to be quoted again and again by creationists - whether through design or stupidity, I do not know - as admitting that the fossil record includes no transitional forms. Transitional forms are generally lacking at the species level, but they are abundant between larger groups." (Gould 1983)

Conclusion

Eldredge and Gould made three conclusions from their theory that most evolutionary biologists found rather controversial. These are (1) most species experience stasis for most of their existence, (2) all adaptive change usually corresponds with speciation, and (3) natural selection at the species level has important macroevolutionary implications (this is now called "species selection"). These last two points were the most controversial at the time, and many scientists were unconvinced that any of these three conclusions were necessary deductions from PE. Gould and Eldredge explicitly admitted that they are biased in drawing these conclusions. Since their proposal, species selection has been shown to be relatively uncommon (as opposed to individual and genic selection), even though it does seem to contribute in rare cases. The evidence is still mixed as to whether adaptive change usually accompanies speciation; there are clearly some cases where it does not.

Punctuated equilibrium is a valid scientific hypothesis, and when geological strata are complete with good temporal resolution and the fossil record is well-represented, the hypothesis is testable. PE, as construed by Eldredge and Gould, is founded upon the modern allopatric speciation model which lies well within mainstream population genetics. However, PE is not novel, and in large part PE originated with Charles Darwin in The Origin of Species (Darwin credits British paleontologist Hugh Falconer with first proposing that stasis is more predominant in the fossil record than periods of morphological change). Thus, in any meaningful sense of the word, the theory of Punctuated Equilibrium is resolutely "Darwinian."