Punctuated Equilibria

I am almost certainly projecting my own ignorance onto others, but when I was an undergraduate the concept of punctuated equilibria (or equilibrium) was occasionally discussed (in non-paleontological circles), but without anyone really understanding what it meant. At least it wasn't clear in my mind. Then again, given the confusion that has surrounded PE over the years--including among professionals--my sense may have been accurate. My recent casual reading has led me to revisit PE, so I will summarize what I have found below and provide some links for reading.


The idea was put forth by paleontologists Niles Eldredge and Stephen J. Gould in 1972. Although on a popular level Gould's name is probably more closely associated with it, the idea was really Eldredge's. Punctuated equilibria was the paleontological extension of Ernst Mayr's hypothesis of allopatric speciation.

Let me pause and explain. For convenience here is Wikipedia on allopatric speciation:

Allopatric speciation, also known as geographic speciation, is the phenomenon whereby biological populations are physically isolated by an extrinsic barrier and evolve intrinsic (genetic) reproductive isolation, such that if the barrier breaks down, individuals of the populations can no longer interbreed. Evolutionary biologists agree that allopatry is a common method by which new species arise.

In other words, some kind of barrier prevents two populations of a single species from interbreeding. Over time, changes occur such that, even if the barrier is removed, the two populations do not interbreed. Thus, one species has become two. Peripatric speciation is basically the same, except there is more asymmetry (i.e. a small population branches off of a larger one and becomes a new species).

Gould has explained that invertebrate paleontologists were trained in geology departments and often did not get much training from biologists. They therefore did not think of their work in connection with concepts that came out of the modern evolutionary synthesis. Their vision of evolution was anagenesis--i.e. that the whole species gradually changes to become different enough to warrant designation as a new species.

Niles Eldredge and Stephen J. Gould applied allopatric speciation to paleontology. They pointed out that fossil species seemed to appear suddenly (geologically), live for millions of years largely unchanged, and then disappear. In other words, they did not see smooth transitions from one species into another, as was often assumed to occur. At the time, most paleontologists attributed this to imperfections in the fossil record. Eldredge and Gould argued that the pattern of the fossil record was not just a result of imperfection, but that it actually said something about how evolution proceeded. New species were formed as an isolated minority over a few tens of thousands of years (which is why the ongoing change was often not reflected in the fossil record), after which they persisted relatively unchanged until they went extinct. They did not claim that anagenesis never occurs, only that the pattern of PE was more frequent.

So to sum it up, PE holds that the formation of new species occurs in isolated populations in brief geological time periods (i.e. tens of thousands of years), and that the species remains relatively unchanged until extinction several million years later. This is in contrast to anagenesis, where the whole species gradually becomes a new species.

PE launched some esoteric debates that I am not qualified to get into. (As if I am qualified on the rest.) Now I turn to common myths.

Myth #1: Punctuated Equilibria is saltationism, where a new species is formed suddenly by a discontinuous jump. Apparently many people held this misconception, but Gould denied it again and again.

The saltationist canard has persisted as our incubus. The charge could never be supported by proper documentation, for we never made the link or claim. All attempts collapse upon close examination.

Myth #2: PE is not falsifiable because it relies on the absence of fossils as its evidence. There are two parts to PE: the formation of species, and the stasis once formed. Certainly the stasis part is testable because it does rely on fossils. But what about the formation of species; if the evolution of new species occurs too quickly to be recorded in the fossil record, isn't that just an appeal to missing evidence? Eldredge and Gould have discussed tests for PE. Although they are rare, one of those includes sediments that capture a finer time scale so that the relatively rapid evolution can be seen. In fact, in some cases the pattern of PE has been observed alongside anagenesis; in other words, both patterns have been seen in the same series of sediments.

Myth #3: PE was proposed in order to explain the Cambrian Explosion. I certainly would not characterize my reading on the topic as extensive, but I have not seen anything to substantiate this notion. The original paper describing PE makes no such claim, and anyway PE is about the formation of species, not higher taxa.

Myth #4: PE is just a naturalistic justification for lack of transitional fossils, which pattern really supports divine creation. First it should be kept in mind that the lack of transitional fossils is in reference to the formation of species, not major groups. Failure to keep that straight results in distortion of PE. Second, I think it might be helpful to take a step back and remember what kind of fossils we are dealing with. It is easy to think that we are talking about some kind of mouse-like creature and then (BAM!), there's something like a horse, with no hint of transition between. Niles Eldredge recently reviewed how he came up with the idea in the first place. His work involved different species of trilobites, which he distinguished by a subtle difference in the structure of the eye--i.e. the number and arrangement of lenses. Gould's original contribution to PE was a group of Burmudian land snails. Essentially we are talking about subtle changes between species, the likes of which a lay person would probably not notice, and that even young-earth creationists are willing to cede to evolution "within kinds." It is therefore contradictory to allow evolution "within kinds" while trying to turn PE into evidence for divine creation. However, if you think that PE is just naturalistic rationalizing against God creating 17,000 species of trilobites, you are welcome to it.

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Further Reading:

1. Niles Eldredge explains how he came up with the idea.

2. Evolution 101: This is a very simple explanation and illustration of PE.

3. The Unofficial Stephen J. Gould Archive has a number of resources on PE. In particular I recommend the following:

4. Punctuated Equilibria: An Alternative to Phyletic
Gradualism: This is the original paper that coined the term punctuated equilibria. Don't let it scare you, most of it is pretty readable and not too technical.

5. Punctuated Equilibrium at Twenty (pdf): In 1992 paleontologist Donald Prothero provided a non-technical overview of the history and effect of PE.

6. If you are really brave, you can consult Gould's The Structure of Evolutionary Theory, or the chapter that was published as a separate book, Punctuated Equilibrium.

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