Punctuated Equilibrium
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[edit] Definition
The hypothesis of punctuated equilibrium, put forward by Stephen Jay Gould and Niles Eldredge, is that most species remain stable for long periods of time, and that evolution takes place in "rapid" bursts of speciation, i.e. over mere thousands or tens of thousands of years.
[edit] Discussion
The idea of punctuated equilibrium offers to answer a puzzle about the fossil record. While intermediate forms are abundant between the higher taxa, there are surprisingly few examples of intermediate forms between species. Such forms do exist, where the fossil record has been exceptionally well preserved; however, what we usually see is the sudden appearance of a new species (not, we emphasise again, of higher taxa); millions of years of stasis, and then extinction. This seems contrary to the notion many people have that evolution always involves gradual changes over millions of years.
It is no good blaming the imperfection of the fossil record for the relative scarcity of intermediate forms between species; because there is no reason why the imperfections of the fossil record should conceal millions of years of change, but allow us to see millions of years of stasis.
Instead, Gould and Eldredge pointed out that we should not expect to see millions of years of change. We should expect a lineage to undergo adaptive evolution in response to colonisation of a new habitat, or a change in the conditions of an existing habitat. Once adaptation to the new conditions has taken place, there is nothing to drive evolution, and natural selection becomes a conservative force.
Now, we know that adaptation to the point of speciation can take place very rapidly by the standards of geology. Indeed, all the speciation we've observed has been very rapid indeed by those standards, since we've only been looking out for it for the past hundred years or so. Such a rapid process is highly unlikely to be preserved in the geological record, given the rarity of fossilization.
Hence, Gould and Eldredge argue, the theory of evolution predicts that this "punctuated equilibrium" in the fossil record is exactly what we should see:
- I want to argue that the "sudden" appearance of species in the fossil record and our failure to note subsequent evolutionary change within them is the proper prediction of evolutionary theory as we understand it. Evolution usually proceeds by speciation -- the splitting of one lineage from a parental stock -- not by the slow and steady transformation of these large parental stocks. (Stephen Jay Gould, Ever Since Darwin.)
[edit] Creationist Arguments
A number of efforts have been made by creationists trying to shoe-horn the claims of Gould and Eldredge into fitting one or the other of their dogmas. As usual with creationist blunders, the one thing they all have in common is that they bear no relation to the facts (in this case, what Gould and Eldredge wrote); and as usual, this disconnection from the facts is the reason why their blunders on the subject are so disparate and contradictory: in order to agree on a single story, they'd have to agree on what Gould and Eldredge meant: which would involve reading what they actually wrote, rather than merely distorting, quote-mining, and just plain lying about their views.
[edit] "No Intermediate Forms" revisited
One popular creationist myth is that punctuated equilibrium was thought up to explain the absence of intermediate forms in the fossil record. There are, of course, an abundance of such forms: and if they were absent, the claim of Gould and Eldredge that most evolution takes place during speciation would not explain their absence. Punctuated equilibrium purports to explain the relative scarcity (not complete absence) of intermediate forms at the species level only.
To quote Gould:
- 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. Yet a pamphlet entitled "Harvard Scientists Agree Evolution Is a Hoax" states: "The facts of punctuated equilibrium which Gould and Eldredge…are forcing Darwinists to swallow fit the picture that Bryan insisted on, and which God has revealed to us in the Bible. (Stephen Jay Gould, Evolution As Fact And Theory [1])
We too cannot say whether creationists are acting through "design or stupidity" when they summarize Gould's work as "Harvard Scientists Agree Evolution Is a Hoax"; it is true that the amount of stupidity requisite would be enormous, but not necessarily beyond the abilities of a creationist.
[edit] "No New Species" revisited
Another take on punctuated equilibrium is given by the Creationwiki:
- The chief problem with this theory is its unfalsifiability, as these episodes of rapid evolution are, rather conveniently, supposed to be too rare to be seen happening in the present, and too rapid to leave evidences in the fossil record. [2]
But of course these episodes of rapid evolution are seen in the present. (For more information, see our article on the creationist claim that "No New Species Have Been Observed"). Elsewhere in the same wiki we read that:
- Speciation, or the formation of a new species, does occur with some regularity. [3]
This they attribute, without evidence, to Intelligent Design. However they choose to explain speciation away, we marvel at the inconsistency with which they admit or deny its existence at whim.
[edit] "Hopeful Monsters" revisited
Still other creationists have misrepresented punctuated equilibrium as a revival of "saltationism": the notion that evolution might move in sudden jumps from one generation to the next. But Gould and Eldredge are discussing changes which take place over thousands of years:
- Small isolated populations are the source of new species, and the process of speciation takes thousands or tens of thousands of years. This amount of time, so long when measured against our lives, is a geological microsecond. (Stephen Jay Gould, Evolution As Fact And Theory [4])
Or again:
- Species form rapidly in geological perspective (thousands of years) and tend to remain highly stable for millions of years thereafter. (Gould, The Flamingo's Smile)
Speciation, then, is "rapid" only by the standard of paleontologists. Creationist misrepresentation of this point has been particularly blatant. In Gould's words:
- Continuing the distortion, several creationists have equated the theory of punctuated equilibrium with a caricature of the beliefs of Richard Goldschmidt, a great early geneticist. Goldschmidt argued, in a famous book published in 1940, that new groups can arise all at once through major mutations. He referred to these suddenly transformed creatures as "hopeful monsters." (I am attracted to some aspects of the non-caricatured version, but Goldschmidt's theory still has nothing to do with punctuated equilibrium—see essays in section 3 and my explicit essay on Goldschmidt in The Pandas Thumb.) Creationist Luther Sunderland talks of the "punctuated equilibrium hopeful monster theory" and tells his hopeful readers that "it amounts to tacit admission that anti-evolutionists are correct in asserting there is no fossil evidence supporting the theory that all life is connected to a common ancestor." Duane Gish writes, "According to Goldschmidt, and now apparently according to Gould, a reptile laid an egg from which the first bird, feathers and all, was produced." Any evolutionists who believed such nonsense would rightly be laughed off the intellectual stage. (Stephen Jay Gould, Evolution As Fact And Theory [5])
[edit] "Theory Of Evolution Proved Wrong" revisited
Some creationists have tried to make out that punctuated equilibrium is opposed to the theory of evolution. As we have made clear, punctuated equilibrium is put forward as a consequence of the theory of evolution.
In Gould's words:
- We argued that two outstanding facts of the fossil record—geologically "sudden" origin of new species and failure to change thereafter (stasis)— reflect the predictions of evolutionary theory, not the imperfections of the fossil record. (Stephen Jay Gould, Evolution As Fact And Theory [6], emphasis ours.)
And again:
- I want to argue that the "sudden" appearance of species in the fossil record and our failure to note subsequent evolutionary change within them is the proper prediction of evolutionary theory as we understand it. Evolution usually proceeds by speciation -- the splitting of one lineage from a parental stock -- not by the slow and steady transformation of these large parental stocks. (Stephen Jay Gould, Ever Since Darwin, emphasis ours.)
And again:
- In fact, the operation of Darwinian processes should yield exactly what we see in the fossil record. (Gould, S. J. 1977. Evolution's Erratic Pace in Natural History 86(5):12-16.)
It takes a real, if perverse, talent for a creationist to miss a point this obvious and this frequently made.
[edit] "Darwin Was Wrong" revisited
For this mistake, Gould and Eldredge are somewhat to blame. Although Gould writes, as quoted above, that "Darwinian processes" are responsible for punctuated equilibrium, he and Eldredge characterised the opposing point of view as "Darwinian gradualism". This is hardly just to Darwin, considering that he himself put forward punctuated equilibrium as a possibility --- indeed, as "probable":
- 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. (Charles Darwin, On The Origin Of Species)
