Punctuated Equilibrium

From: Jeffrey Martz [martz@holly.ColoState.EDU]

>What was Darwin's theory of evolution (wasn't it natural selection/
>adaptation rather than an actual theory of evolution?)
>and I thought I read that the gradualist model was not proposed by Darwin

By the time Darwin came around, evolution was not a new idea. Darwin's big innovation was proposing natural selection as the MECHANISM of evolution. Probably the most well known (to us today) anyway alternative around before that was Lamarkism. Of course, today we have people suggesting that God controls evolution, apparently replacing natural selection with divine intervention as the main driving force of evolution.

>and I thought I read that the gradualist model was challenged in favor of
>a "punctuated" model in the 19th century as soon as the gradualist model was
>proposed.

Before evolution was a well accepted idea, one of the big ideas to explain the fossil record and its obviously extinct forms, as well as new species popping in apparently out of nowhere, was Catastrophism. According to this theory, history was punctuated by several catastrophes (I can't remember if they were all floods or not) that wiped out existing species, each followed by a brand new creation. Cuvier was a big fan of this theory (as an opponent of evolution). In the 19th century, when Hutton and Lyell came up with uniformitarianism (sp?), geologic history in general and evolution in particular began to be seen as a gradual, uniform process. In the 70s or 80s (can't remember), Steven J. Gould and Niles Eldridge (sp?) came up with the theory of punctuated equilibrium, which says that evolution is mostly stasis, with brief periods of explosive speciation. Today it is largely accepted that uniformitarianism is not always the case, and that things in the geologic past sometimes happened differently and at different rates than they do today. This merging of uniformitarianism and catastrophism (using the term loosely in light of its previous biblical connotations) is called ACTUALISM.

From: Stan Friesen [swf@ElSegundoCA.ATTGIS.COM]

After Darwin published Origins of the Species there were *several* widely supported theories of evolution. One was Darwin's theory that evolution occurs by natural selection. Another was a variant of the old idea of orthogenesis - that evolution is driven by an intrinsic force within living things that drives life "upwards".

Later, after the rediscovery of Mendelian genetics, when it appeared as if Mendelian genetics was inconsistent with Darwin's theory, several researchers proposed a *mutationalist* theory of evolution. This theory proposes that new species arise in a *single* *generation*, by means of macromutations. While many people have confused this saltational theory with punctuated equilibrium - they are NOT even remotely similar. P.E. is a Darwinian, that is a selectionist, theory, saltationalism is not Darwinian at all.

This last point is critical - P.E. in no way denies the importance of natural selection. The mechanism of speciation proposed in P.E. is the peripatric speciation process described by Ernst Mayr, one of the founders of the Modern Synthesis (aka Neodarwinism). The idea here is that the reorganization of the genome in a way that produces reproductive isolation is most likely to occur in small, isolated population near the periphery of the range of the parent species. Since genetic drift and selection can both act rapidly and efficiently in a small population, this process probably occurs in a geologically short period of time.

What Eldridge and Gould realized was that a species that originates in less than 100,000 years in an isolated patch of environment is likely to appear fully formed in the fossil record - when it finally expands out of its initial enclave. Note - this is merely a *likelihood*, not a certainty, and capturing an occasional speciation event in progress is quite possible. This may be what Achelousaurus is. In one of the early books on P.E. E&G suggest that they may have found an instance of speciation in progress in the trilobites that Eldridge specializes in.

From: geojbb@vaxc.hofstra.edu (Bret Bennington)

If one is going to discuss punctuated equilibrium and gradualism without a lot of confusion and misapprehension, then it is very important that the terms be defined as they were originally used by Eldridge and Gould (the authors of the original paper defining p.e. Eldridge, N. and Gould, S. J. 1972. Punctuated equilibrium: an alternative pyletic gradualism. In Schopf, T. J. M., ed. Models in Paleobiology. Freeman, Cooper, and Co.). In this paper, E and G present punctuated equilibrium as a model to describe the pattern of evolutionary change AS SEEN IN THE FOSSIL RECORD. They argue that most well studied stratigraphic sequences of fossils do not show the gradual accumulation of evolutionary change through the temporal span of the species (Phyletic gradualism). In other words, the fossil record does not support the hypothesis that a large population of individuals of a species can accumulate adaptive change over time and slowly transform from one species into a daughter species. Rather, E and G argue that in most cases the fossil record shows change in a species population to be a random walk - sometimes in one direction, later in another - that does not accumulate over time to produce speciation. Instead, the fossil record shows the GEOLOGICALLY rapid appearance of new species with a jump or punctuation in morphological change. In other words, species appear to acquire their particular novelties during the speciation event and henceforth remain relatively unchanged until they go extinct. The speciation event itself is not usually preserved in the fossil record because 1) it probably occurs in a small population inhabiting a restricted geographic range and 2) it occurs over a geologically short period of time - perhaps 1000 to 10,000 years. Both of these factors would greatly diminish the probability of preserving the speciation event relative to the probability of preserving the expanded, established population of the new species over the million years (average for marine invertebrates) of its existence.

From: Tompaleo@aol.com

>Grad. is plagued by that problem and punk just seeks to avoid it. What's
>wrong with "Abrupt appearances." I mean the record just says that. No more,
>no less.

But just because the fossil record "shows" a sudden appearance (or disappearance for that matter) does not necessarily prove that the event was sudden. There are numerous factors that continually bias the fossil record. Many are geologically related such as post depositional diagenesis of the sediment which may destroy the fossil such as metamorphism, volcanism, depositional hiatuses and taphonomic biases just to name a few. Then there's the problem of a particular animal being fossilized at all! Then there is sampling bias both man made and that caused by weathering/erosion. So in essence, the record does not necessarily speak for itself! It would be better to say (IMHO) that both models are partially right. You have a particular biota, in the absence of a major calamity, species evolve and go extinct at a "regular rate". According to Bakker and others this rate is about 2 Million years on the average per species. This is the "background" extinction. Then periodically(?), a calamity strikes that affect large numbers of taxa and then you have a mass extinction. The survivors then move in to the niches left vacant by the mass extinction and soon begin a new period of origination. The above factors that affect fossilization /preservation /recovery tend to bias our opinion in the direction of "sudden appearance" events that were obviously gradual. To me, that is what the geologic record is telling us. INMHO

From: pwillis@ozemail.com.au (Paul Willis)

>One of the biggest debates in evolutionary theory deals with the appropriate
> model for the evolutionary process. The traditional view of gradualism
> states that evolution is a constant process, while the new view of
> punctuated equilibrium states that evolution occurs in short bursts. The
> big question focusing on which model is correct.
>
>What if both views simply describe different aspects of the evolutionary
> process. Punk eq covers the realm of what is usually thought when people
> hear the word "evolution," being the process that results in profound
> changes in a population of animals. Gradualism would describe the realm of
> adaptation; in other words, the process that deals with subtle changes in a
> population. My point being that since animals are constantly adapting to
> their environment, gradualism would be the best model for this process.
>
>Therefore, both models are correct.

This is more or less the way that the issue has been settled. Another way of thinking about it is to consider the scale at which you are looking at evolutionary change. PE is best supported by paleontological evidence and describes the small increments of species-to-species evolution. Gradualism is supported by biochemical data that looks at evolution over a longer period and wider phylogenetic scope. PE is a particularly useful marriage of allopatric speciation models with the temporal aspect of the fossil record. Gradualism is a better model for analyzing genus-to-genus or even higher level evolutionary change.

What does seem apparent is that gradualism is a poor model for species-level evolutionary change and, equally, PE is a poor model for higher-level evolutionary change.

There is, of course, another level in which all evolutionary change is gradual, even within PE. The "sudden" appearance of a new species in the fossil record represents the occupation of an area by a daughter species that evolved gradually in isolation elsewhere.