by Wesley R. Elsberry
Last updated: 19991224
There are few components of modern evolutionary theory which seem so prone to misinterpretation as Niles Eldredge and Stephen Jay Gould's theory of punctuated equilibria (PE for short). In this matter, the person attempting to come to a better understanding of punctuated equilibria will find that he or she may be hampered by the popular writings of those same authors rather than helped. As in most cases, the primary literature remains the best source of information.
Those who have obtained most or all of their information from popular descriptions almost always come to a mistaken view of what PE is or says. PE really is not that complex a theory, so my personal opinion is that the popular treatments (even those from the originators of the theory) have tended to be misleading, usually by omission.
This revision changes a few things about the FAQ. I have added a glossary for brief expansions upon biological jargon. The section on species and paleontology corrects a possibly misleading use of the term "paleospecies". The original document is now in HTML format, which provides an easy way to get at the glossary. For those reading printouts, a "[gl]" sequence indicates that the preceding term has a glossary entry. Unfortunately, I have not yet had time to expand the FAQ to incorporate a summary of the extended discussion of PE in the biological literature.
Meta-summary: PE is the application of Mayr's theory of allopatric speciation in peripheral isolates to the fossil record.
The features that make up Punctuated Equilibria as described by Eldredge and Gould are as follows:
The theory of Punctuated Equilibria provides paleontologists with an explanation for the patterns which they find in the fossil record. This pattern includes the characteristically abrupt appearance of new species, the relative stability of morphology in widespread species, the distribution of transitional fossils when those are found, the apparent differences in morphology between ancestral and daughter species, and the pattern of extinction of species.
PE relies upon the insights of study of modern species for its principles. These studies indicate the importance of consideration of geography and interspecies interactions upon predictions of the distribution and abundance of transitional specimens. While Eldredge and Gould acknowledge that geological processes contribute to the "gappiness" of the fossil record, they also assert that PE is by far the more important consideration in that regard.
Paleontologists have to recognize species from their fossil remains. The problem of "What is a paleospecies?" led Niles Eldredge and Stephen Jay Gould to propose the theory of punctuated equilibria. The term "paleospecies" makes explicit the distinction between the classification of species from fossil remains and the process of recognizing species in modern populations. This problem involves geology, taphonomy, taxonomy, and -- though often ignored -- geography. Of course, Eldredge and Gould don't then define "paleospecies", but rather identify the search for a definition as an insoluble problem: the essence of species lies in the nondimensional character of the category, but any definition of paleospecies must take time into account.
Mayr's Biological Species Concept uses the criterion of reproductive isolation to distinguish species in modern populations. Paleontologists who pursue taxonomic endeavors (which includes most of them) have to classify their finds generally based upon morphological features. The rareness of preservation of tissues containing DNA, or even of soft tissues, limits the range of diagnostic characters which may be utilized. The paleontologist has no access to such information. (Whether modern biologists really do have access to that information is a matter of some debate in the literature.)
The fossil record is incomplete. This incompleteness has many contributing factors. Broadly, one can divide these into geological and biological factors. Geological processes may cause confusion or error, as sedimentary deposition rates may vary, erosion may erase some strata, compression may turn possible fossils into unrecognizable junk, and various other means by which the local fossil record can be turned into the equivalent of a partially burned book, which is then unbound, pages perhaps shuffled, and from which a few pages are retrieved. Beyond geology, there remains taphonomy -- the study of how organisms come to be preserved as fossils. Here, there are further issues to be addressed. Hard parts of organisms fossilize preferentially. The conditions under which even those parts may become fossilized are fairly specialized. Changes due to permineralization occur over time. All this results in a heavily skewed distribution of even what *parts* of organisms become fossilized, and that affects which features of morphology are available for use in classification. The issue of geography enters into all this, as a consequence of the fact that living lineages occupy ecological niches, and those niches are bound to certain features of geography. Biological issues include the mode of speciation typical for each group, its preferred habitat (an interaction of biological and geographical issues), and behavior which might impact the probability of fossilization.
Paleospecies, then, have to be recognized as species from morphology alone, where the available morphological characters are drawn from a skewed distribution, the pattern of fossilization is skewed, and the geographic correlates of fossilization are limited in extent. Eldredge and Gould contend that what has to be approximated is the concept of a biospecies in the paleontological framework.
Eldredge and Gould's insight into paleontological processes was to derive their understanding from living biological species. In this manner, it can be made clear what PE means for the distribution of speciation events seen in the fossil record.
Now we are ready to apply these concepts from the biology of extant organisms to that of fossil organisms. This proceeds on the simple inference that past life went about its business in much the same way as present life does.
Some of the predictions of Punctuated Equilibria are as follows:
Punctuated Equilibria could have been advanced simply upon the basis of features of geology, taphonomy, geography, biology, and taxonomy. However, that is not how Eldredge and Gould chose to do it. Instead, they codified what they saw as an inaccurate and incorrect "picture" of the fossil record, labelled it as "phyletic gradualism", and demonstrated that their "picture" of "punctuated equilibria" was to be preferred on several points.
The essential features of "phyletic gradualism" are described by Eldredge and Gould.
[End quote -- E&G 1972]
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 lo 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.
While it is nice to have terminology by which obtuse opponents of a theory may be conveniently labelled, there is no actual point to even bringing up "phyletic gradualism" in establishing PE.
Eldredge and Gould quoted from Darwin in their 1972 paper to establish their concept of phyletic gradualism. They claim that Darwin set the task of later workers to search out confirmation of phyletic gradualism. That view is not supported by reference to Darwin's original works.
Nothing can be effected, unless favourable variations occur, and variation itself is apparently always a very slow process. The process will often be greatly retarded by free intercrossing. Many will exclaim that these several causes are amply sufficient wholly to stop the action of natural selection. I do not believe so. On the other hand, I do believe that natural selection will always act very slowly, often only at long intervals of time, and generally on only a very few of the inhabitants of the same region at the same time. I further believe, that this very slow, intermittent action of natural selection accords perfectly well with what geology tells us of the rate and manner at which the inhabitants of this world have changed.[End quote -- Charles Darwin, OoS 1st Edition 1859, p.153]
The above quote from Darwin also shows that Darwin did not embrace three of the four antecedent conditions that Eldredge and Gould specified for phyletic gradualism, and the single one that Darwin did embrace is also a tenet of any theory of speciation. Some people may embrace phyletic gradualism, but it is incorrect to attribute the concept to Charles Darwin. The quote above is noteable on several points. The "free intercrossing" bit is easily recognizable as a forerunner of the concept of gene flow, though Darwin was probably concerned there with blending inheritance. Darwin makes explicit that there is no constancy of rate implied with the comment on intermittent action. Darwin also recognized that change would be more likely to occur in a sub-population. Whether Darwin meant by "of the same region" much the same thing as the modern concept of allopatric speciation is disputable, although passages from his "B" notebook on species transmutation show that Darwin was guided in his thoughts on the importance of isolation to the production of new species in much the same way that Mayr was - by consideration of species found on islands.
Darwin did think that a daughter species arose from a population of the parent species. So do punctuated equilibrists. Darwin did think that the transformation would be slow, but he did not think that it would be "even". Darwin did not think that the transformation would involve large numbers, and certainly not the entire parent population. Darwin did not think that the transformation would occur across the entire ancestral range.
But on the view of all the species of a genus having descended from a single parent, though now distributed to the most remote points of the world, we ought to find, and I believe as a general rule we do find, that some at least of the species range very widely; for it is necessary that the unmodified parent should range widely, undergoing modification during its diffusion, and should place itself under diverse conditions favourable for the conversion of its offspring, firstly into new varieties and ultimately into new species.[End quote -- Charles Darwin, OoS 1st Edition 1859, p.391]
It is difficult to extract meaning from the above without recognition that Darwin was well aware of the importance of geographical distribution in the production of new species.
Only a small portion of the world has been geologically explored. Only organic beings of certain classes can be preserved in a fossil condition, at least in any great number. Widely ranging species vary most, and varieties are often at first local, -- both causes rendering the discovery of intermediate links less likely. Local varieties will not spread into other and distant regions until they are considerably modified and improved; and when they do spread, if discovered in a geological formation, they will appear as if suddenly created there, and will be simply classed as new species.[End quote -- Charles Darwin, OoS 1st Edition 1859, p.439]
The above quote comes from the famous section on the imperfection of the geological record. However, Darwin makes it clear that geographic location makes a difference in the finding of intermediate forms. "Both causes" in the above could not make discovery of intermediate links less likely if Darwin expected the transformation of the entire parent population. It is important to note that Darwin incorporates both geological and biological causes into his discussion.
Gould has said that history cannot be done on the basis of selective quotes and qualifying footnotes, but rather must be a matter of general tenor and understanding. It appears that general tenor supports the view that Darwin was never a proponent of phyletic gradualism, whatever less perspicacious interpreters might have themselves believed Darwin to have meant. That "gradualist" interpretations are not the sole interpretations extant can be verified, ironically, from an anti-evolutionary source. One anti-evolutionary text file which can be found on various SciCre bulletin boards involves quoting Leon Trotsky extolling the punctuational nature of Darwin's theory, with the apparent aim of guilt-by-association.
There have been persons who have advanced positions that could more or less be termed "phyletic gradualism". A feature of Gould and Eldredge 1977 is the discussion of various purported examples of change in the phyletic gradualist mode. Of many examples, G&E only found one to meet their criteria for establishing phyletic gradualism. Most examples were disputed by G&E because the original work ignored the geographical dimension.
In looking over the exchanges early in the history of PE as a theory, it is difficult not to be disturbed by how misapprehension of "phyletic gradualism" generated many responses to the promulgation of PE. It is also clear that the looseness of reference to "phyletic gradualism" adopted by Eldredge, Gould, and others who became known as "punctuationists" may have fostered these misapprehensions. In almost all cases where the word "gradualism" appears in the PE literature, the concept being referenced is actually "phyletic gradualism". But "gradualism" was already long current in the biological literature as the antithesis of "saltationism", and this ambiguity seems to have led to the paleontological equivalent of Abbott and Costello's "Who's on First?" sketch. Because "gradualism" was attacked in PE articles, responses demonstrating non-saltational speciation in the fossil record were advanced. These were dismissed as not meeting the criteria of "phyletic gradualism" by punctuationists. In many cases, the entire issue was caused by a failure to disambiguate.
Richard Dawkins has a chapter in "The Blind Watchmaker" which goes into some detail on how "phyletic gradualism" is in many ways a strawman of Eldredge and Gould's creation. It is a recommended read.
While "phyletic gradualism" is plainly defined and discussed in the writings of Eldredge and Gould, it is less commonly correctly referenced. Eldredge and Gould are among the culprits in this, as they refer to "phyletic gradualism" simply as "gradualism" in an apparent attempt at economy of expression. This is unfortunate and misleading, since "gradualism" already has meaning for biologists, and when Eldredge and Gould inveigh against "gradualism", many mistakenly believe them to be opposed to the other gradualism. That gradualism is expressed by Mayr as non-saltational change in populations.
I have been asked what my opinion is of the significance of Eldredge and Gould's explication of "punctuated equilibrium". While I disagree with the way the idea of PE has been presented and hyped, I feel that PE is indeed a significant theoretical advance. That doesn't mean that I think it is *entirely* novel, nor that others had not presaged various parts of it before Eldredge and Gould published in 1972. However, Eldredge and Gould *deserve* credit for developing the theoretical part fully and for clearly presenting the coherent framework with all its pertinence to paleontology, which had not been done before. They took the various pieces of the puzzle and put them together in a way that has proved fruitful for research, and that is an entirely laudable thing, fully worthy of our admiration. The application of work from neontology to paleontology is not something that has happened with great regularity, nor is it some simple straightforward mapping that can be or is achieved by just anyone. At least, the simplicity and straightforwardness only becomes apparent *after* the work has been completed. At least one early reviewer of my FAQ seemed to be let down by the manner in which PE can be stated and derived logically with just a handful of premises leading to the conclusions. I think that is one of the strengths of how Eldredge and Gould formulated PE, though.
Now, it has also been my continuing contention that Eldredge and Gould unnecessarily excluded Charles Darwin as one of the intellectual forebears whose work held components of PE. This is, I've come to appreciate, a separable concern. It does not really detract from their achievement in framing PE as a theory in paleobiology. However, it does mean that I take somewhat more skeptically various pronouncements that they made concerning who may or may not have had earlier glimpses of the modern theory of PE. Darwin did not have a complete theory for PE in the way that Eldredge and Gould do. Darwin did, however, identify species arising in geographically local variants as a major reason why transitional sequences would be rare. Darwin did not stress stasis, but I have found no evidence to support the contention that he advocated constant rates of change in *each* lineage, either. Those passages that are quoted in that regard all refer to natural selection's actions across all species, not the action of natural selection within a single lineage. It is my opinion that the work of Charles Darwin, when considered in context and in totality, is far closer to expressing a view or "picture" of fossil history in line with PE than it is to PG. This is not to say that others have not concluded otherwise, merely that I disagree with their analysis. I'll point out that one of the quote-bytes floating around on SciCre bulletin boards comes from Leon Trotsky, who wrote notes concerning Darwin's origin of species that eerily express a view consonant with PE, but which Trotsky simply reports as being Darwin's view. (The SciCre'ists use this, of course, as guilt-by-association.)
"The Darwinian theory of the origin of species encompasses the entire span of development of the plant and animal kingdoms. The struggle for survival and the processes of natural and sexual selection proceed continuously and uninterruptedly. But if one could observe these processes with ample time at one's disposal a millennium, say, as the smallest unit of measure one would undoubtedly discover with one's own eyes that there are long ages of relative equilibrium in the world of living things, when the laws of selection operate almost imperceptibly, and the different species remain relatively stable, seeming the very embodiment of Plato's ideal types. But there are also ages when the equilibrium between plants, animals, and their geophysical environment is disrupted, epochs of geobiological crisis, when the laws of natural selection come to the fore in all their ferocity, and evolution passes over the corpses of entire plant and animal species. On this gigantic scale Darwinian theory stands out above all as the theory of critical epochs in plant and animal development." - "Portraits, Personal and Political", by Leon Trotsky. George Breitman and George Saunders, eds. New York : Pathfinder Press, 1977.
[End quote - http://www.nettrade.com.au/kyurhee/00061.htm]
Many errors can be found in discussion of the concept of PE. G&E 1977 point out several of these.
PE is not mutually exclusive of phyletic gradualism. Gould and Eldredge take pains to explicitly point out that PE is an expansive theory, not an exclusive one (1977). Gould and Eldredge explicitly validate Ozawa's 1975 paper on forams as demonstrating an instance of phyletic gradualism.
PE sometimes is claimed to be a theory resting upon the lack of evidence rather than upon evidence. This is a curious, but false claim, since Eldredge and Gould spent a significant portion of their original work examining two separate lines of evidence (one involving pulmonate gastropods, the other one involving Phacopsid trilobites) demonstrating the issues behind PE (1972). Similarly, discussion of actual paleontological evidence consumes a significant proportion of pages in Gould and Eldredge 1977. This also answers those who claimed that E&G said that PE was unverifiable.
PE is essentially and exclusively directed to questions at the level of speciation and processes affecting species. The basis of PE is the neontological theory of peripatric speciation. The criteria by which "punctuations" are recognized by Gould and Eldredge involve temporal issues and geographic issues. PE is not expected to be as useful at lower or higher levels of change.
PE is by no means either synonymous with "saltationism", nor did Gould's essay on Richard Goldschmidt "link" PE with Goldschmidt's "hopeful monster" conjecture. Gould wrote an article that has caused much confusion. "Return of the hopeful monsters" sought to point out that a hatchet job had been done on some of the concepts that Richard Goldschmidt had formulated. The discussion of systemic mutations as mutations which affect rate or timing of development has caused many people to assume that Gould was somehow linking PE to this concept. A close reading of the article shows this to not be the case.
Gould and Eldredge did not specify any particular genetic mechanism for PE. PE does not require large scale mutations.
PE is not a saltational theory of evolution. The emphasis upon applying consequences of peripatric speciation to paleontology shows this critique to be unfounded. PE is no more saltational than peripatric speciation is in study of modern organisms.
The objection is raised that since PE says that phyletic gradualism (PG) is rare, then PE conceptually disqualifies the idea that one can find transitional sequences linking species, where the transitional sequence is fine-grained or as one objector put it, finely graduated. The objection is based upon a misunderstanding of what PE implies.
Eldredge & Gould's brief is not against *finely (graduated | graded) transitional sequences* (FGTS), it is against FGTS which are interpreted as showing anagenetic change without appropriate reference to geography and stratigraphy. This further set of restrictions is important, but overlooked by those who believe that accepting PE means rejecting the idea of transitional sequences. E&G argue against "phyletic gradualism" (PG) as typifying the mode of evolutionary change seen in the fossil record. E&G (and other proponents of PE) often refer to PG confusingly as "gradualism". Gradualism, though, is a separate and distinguishable concept.
Some seem to think that E&G banished phyletic gradualism as existing. E&G did not do that. Gould and Eldredge 1977 takes critics to task for implying that they did so. (The critics may not agree that they were mistaken; it is an arguable point.) G&E 1977 specifically validate an example of phyletic gradualism in the fossil record.
Now, on to the main point: It *is* possible to have finely graded transitional sequences in the PE mode, and it *is* possible that FGTS showing PG may be found. E&G 1972 present two sequences showing PE-type FGTS from the authors' own fieldwork, and G&E 1977 point to several sequences showing PE as well. What distinguishes a PG FGTS from a PE FGTS? A PG FGTS samples will show the changes over the whole range of the parent species, and will show the same rate of change in characters throughout the transition. A mismatch in geographic range seen for the change and the parent species, or an indication that collected specimens had no per-specimen information on geographic siting is enough to disestablish a sequence as showing the *characteristics of* PG. A difference in rate of change of a character during the transition period is enough to disestablish a transitional sequence as showing the *characteristics of* PG. Disestablishing a sequence as showing the characteristics of PG does not necessarily disestablish the sequence as showing the characteristics of descent with modification linking two species. It is a necessary, but not sufficient, condition that a sequence show descent with modification between two species to be considered an example of PG or an example of an observed transition under PE.
In contrast to a PG FGTS, a PE FGTS will be restricted to some relatively small region of the entire range of the parent species, will take place in a small fraction of the total residence time of the parent species, and will result in at least some time of overlap between the parent species and the daughter species to indicate cladogenesis has occurred. As I have noted above, E&G 1972 discusses two fine-grained transitional sequences as examples of transitions in the PE mode on pp.98-108, and G&E 1977 cites other researchers' examples of transitionals in the PE mode approvingly.
What does all this mean for the conceptual status of transitional fossil sequences? First, accepting PE's conclusions means that one views PE as the mode of evolutionary change, and not that it is exclusive of PG. Fossil evidence may be found which comports more closely with PE than PG, or vice versa. The classification takes place on a case by case basis. PE's claim concerns the relative frequency with which one will find examples of PE as compared to PG. Second, any finely graded sequence linking two successive species *is* a transitional sequence; the only question is whether its geographic, morphological, and stratigraphic characteristics argue for *classification* as an example of PE or as an example of PG.
Chris Colby provided a cogent and detailed critique of the first draft, pointing out the "gene flow vs. homeostasis" argument in E&G 1972 among many other points. Many of his points remain to be incorporated, which says more about the extent of my free time rather than the importance of those points.
Chris Nedin suggested several changes incorporated here, including expansion of the discussion of Darwin in relation to phyletic gradualism. I still need to check out "The Beak of the Finch" for examples of (relatively) rapid evolutionary change.
I thank the Oxford Text Archive for providing a free etext of "Origin of Species", first edition. This makes looking up interesting passages much easier than in the paper versions.