Punctuated Equilibria

by

Copyright © 1996-2004
[Last Update: February 4, 1996]

Outline

• 0. Foreword
• 1. Summary of Punctuated Equilibria
• 2. The problem of paleospecies
• 3. Patterns of speciation from neontological study
• 4. Application of neontology to paleontology
• 5. PE vs. Phyletic Gradualism
• 6. Common errors in discussion of PE
• 7. References
• 8. Acknowledgements

0. Foreword

here 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.

1. Summary of Punctuated Equilibria

The essential features that make up Punctuated Equilibria are as follows:

1. Paleontology should be informed by neontology.
2. Most speciation is cladogenesis rather than anagenesis.
3. Most speciation occurs via peripatric speciation.
4. Large, widespread species usually change slowly, if at all, during their time of residence.
5. Daughter species usually develop in a geographically limited region.
6. Daughter species usually develop in a stratigraphically limited extent, which is small in relation to total residence time of the species.
7. Sampling of the fossil record will reveal a pattern of most species in stasis, with abrupt appearance of newly derived species being a consequence of ecological succession and dispersion.
8. Adaptive change in lineages occurs mostly during periods of speciation.
9. Trends in adaptation occur mostly through the mechanism of species selection.

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.

2. The Problem of Paleospecies

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.

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 little debate in the literature.)

The fossil record is incomplete. This incompleteness has many contributing factors. Geological processes may cause to 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. 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.

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.

3. Patterns of speciation from neontological study

Eldredge and Gould's insight into paleontological processes was to derive their understanding of paleospecies from living biological species. In this manner, it can be made clear what PE means for the concept of paleospecies.

First, modern species appear to have derived from cladogenesis, the splitting of a daughter species from an ancestral species rather than transformation of the ancestral species in toto. This is a multiplication of species, and without it, the diversity of the living systems that we see would be impossible.

Second, the mode of speciation most often seen is also identified from modern populations. That mode is allopatric speciation of peripheral isolates, or peripatric speciation in Mayr's terminology. Peripatric speciation states that a population of an ancestral species in a geographically peripheral part of the ancestral range is modified over time until even when the ancestral and daughter populations come into contact, there is reproductive isolation. While saltational speciation by change in ploidy is observed to occur in modern populations, this form of speciation is also known to be rare (except in plants). The incipient speciation of clinal forms has recently become very controversial, and is also likely to be rare in any case. Sympatric speciation, the production of a daughter species within the geographic range of the parent species, likewise is held to be a very rare event seen primarily in insect and parasite lineages.

Third, the frequency with which peripatric speciation occurs in modern lineages can be seen as "rare". This rarity is different from the rarity discussed in the last item. There, we compared how often certain modes of speciation were seen compared to other modes of speciation. Here, we want to know how common it is for a species to produce a daughter species by peripatric speciation. The answer is, "Not very common at all." This rarity means that a species may produce zero, one, or perhaps a few daughter species during its entire time span of existence.

Fourth, the period of transition between parent species and daughter species is short compared to the period of time a species exists as a distinct form. When a small sub-population is isolated from the rest of the population of a species, the particular set of variations in the sub-population is much smaller than that in the remainder of the population. These variations, when in conjunction with suitable features of geographic locale, climate, and resources, can lead to rapid development of reproductive isolation from the ancestral population. The reduction in variation due to small sub-population size is known as the "founder effect".

Fifth, significant adaptations developed or accentuated in the daughter species can lead to the rapid dispersal and establishment of a daughter species throughout the range of the ancestral species, or into new ranges. The ecological processes of dispersal and succession can occur very quickly compared to evolutionary processes of change.

Sixth, the principles of gene flow, genetic homeostasis, and large population size inhibit widespread ancestral populations from much directional (adaptive) change. Any adaptive change found in the ancestral population is likely to be small and unrelated to evolutionary trends.

4. Application of neontology to paleontology

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:

1. Punctuated Equilibria postulates that speciation events comprise most of the evolutionary change seen in adaptation. This is a consequence of the inhibitory effects of gene flow, genetic homeostasis, and large population sizes (6 above). The adaptations of newly speciated daughter populations are forever excluded from the ancestral population because of reproductive isolation (2 above).
   
   
2. PE explains the abrupt appearance of new species in the fossil record. The splitting of lineages (1 above) in the mode of allopatric speciation (2 above) followed by ecological dispersal and succession (5 above) would result in geologically abrupt appearance of the daughter species everywhere except the limited geographic area where the speciation took place. Most speciation takes place as peripatric speciation, which is confined to a limited geographic region, and after which ecological principles argue for relatively rapid reintroduction and spread into new habitats for the daughter species. Since the critical change occurs in such a small region and in such a limited population, the probability of finding specimens which document the transition from ancestral to daughter species is very low. A population which can exploit resources untapped by current populations will grow and spread at somewhere near its theoretical intrinsic rate of increase. The cases of introduced species in modern times (the starling in North America, for example) demonstrate the extreme rapidity in which a species may spread across large geographic extents.
   
   
3. PE explains the relative stasis of most species. A species may produce a few daughter species during its duration (3 above). Large interbreeding populations are unlikely to change much due to genetic homeostasis and gene flow from far-flung parts of the range (6 above) [Eldredge & Gould emphasize homeostatic mechanisms over gene flow.].
   
   
4. PE asserts "species selection" as the way in which major adaptive trends proceed. Closely related species are often likely to overlap in niche space (5 above). Ecological processes may cause the displacement and possible extinction of certain species due to competition with other species. If adaptive change in large populations is largely inhibited (6 above), then each species represents a "hypothesis" that is "tested" in competition. This is one of the more controversial points in PE.
   
   
5. PE also makes a statement concerning the pattern of fossils found. This pattern has both geographic and stratigraphic components. If peripatric speciation is the mode of speciation, then the place where transitional fossils between a parent and daughter species will be found will be limited in geographic region (2 above). Because the time needed for transition from parent to daughter species is short compared to the total residence time of either, the stratigraphic extent of transitional fossil sequences will be very brief (4 above).

5. PE vs. Phyletic Gradualism

Punctuated Equilibria could have been advanced simply upon the basis of features of geology, taphonomy, geography, 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 PE was to be preferred on several points.

The essential features of "phyletic gradualism" are described by Eldredge and Gould.

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.

[E&G 1972]

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, unfortunately, just so much hokum.

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. [Charles Darwin, Origin of Species 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.

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. [Charles Darwin, Origin of Species 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. [Charles Darwin, Origin of Species 1st Edition 1859, p.439]

The above quote comes from the famous section on the imperfection of the goelogical 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.

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. However, I think the onus is upon Eldredge and Gould to demonstrate that phyletic gradualism actually exists outside of their description.

There have been persons that 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.

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.

6. Common errors in discussion of PE

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).

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.

7. References

8. Acknowledgements

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 e-text of Origin of Species, first edition. This makes looking up interesting passages much easier than in the paper versions.

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