|Peripatus Home Page Evolution >> Punctuated Equilibrium||Updated: 03-Feb-2017|
In 1972, Niles Eldridge and Stephen Jay Gould promoted a concept they termed "punctuated equilibrium." Their ideas have generated much subsequent debate. In the following essay the concept is critiqued and dismissed as trivial.
Keywords: anagenesis, evolution, phyletic gradualism, punctuated equilibrium, speciation
The original Eldredge & Gould paper (1972) presents three principal ideas. The first – that even ostensibly ‘objective’ observations are influenced by the observers’ preconceptions, not least in deciding what to observe – is so obvious, and delivered with such an offensive sanctimony, that it deserves no further attention and will certainly receive none from me.
The remaining theses are (1) that lengthy periods of evolutionary stasis within lineages are genuine and important phenomena, and (2) that speciation events usually appear in the fossil record as more or less instantaneous events. The combination of these two observations leads to a step-wise evolutionary pattern, rather than steady, insensible ‘gradualism.’
These ideas are considered below.
There is no doubt that periods of morphological stasis (for who can say what is happening to the genotypes?) are a common feature of fossil lineages. Common and useful indeed: these intervals are the very basis of biostratigraphy, of Oppel zones, recognized and utilised by the founding fathers of stratigraphy – Lapworth, Murchson, Sedgewick, and Lyell himself – since the 1830s, at least. One might even stretch a point and moot that the first person to attribute a Linnaean binomial to a fossil species was implicitly recognising stasis.
The point I am making here is not that Eldredge & Gould’s observation of stasis was mistaken – of course it isn’t; it is simply commonplace, and had surely been already understood by professional biostratigrapher and thoughtful layman, alike, for almost 150 years prior to 1972.
It is banal.
Drs. Eldredge and Gould are to be scoffed at, loudly and insultingly, for making any kind of intellectual claim upon this idea:
“Putting together the philosophical insight of ineluctable theoretical bias, with the empirical theme of the tractability of stasis, we devised a motto: ‘stasis is data’” (Gould & Eldredge 1993, p. 223).
Their implication that stratigraphers had failed to fully appreciate that stasis is data prior to 1972 is simply bizarre. It is the life-blood of biostratigraphy and soft-rock field mapping.
The ‘classic’ speciation model which, despite the assertions to the contrary in Eldredge & Gould 1972, would have been familiar to both Charles Darwin and Alfred Wallace though neither used the modern jargon, is today known as allopatric speciation.
(Sympatric speciation is important too, especially for planktonic organisms which underpin by far the majority of modern biostratigraphic efforts, but is not much discussed in Eldredge & Gould 1972. Generally the plankton abounds with ‘gradualist’ bioseries and provides rather poor evidence for punctuated equilibrium, so their preference for an allopatric model is understandable.)
Speciation is said to be allopatric when the actively “evolving” members of a population are reproductively isolated – usually by geography – from those “in stasis.” The evolving group is usually conceived as a numerically small peripheral population having limited gene flow with the larger group, which somehow becomes cut off altogether. An example Darwin might have offered is that of a peninsular population finding itself stranded upon an island by rising sea levels (see, for example, pp. 105-106 of Origin).
These days, the rhythmic ebb and flow of the Milankovic cycles, and a clearer understanding of ecological constraints, reveals that millions of these peripheral niches are being created and reabsorbed all the time.
Consider now the series of diagrams (fig. 1) starting from the bottom (fig. 1A). Even before the (relatively) tiny subgroup is isolated from the main population, it is already significantly different from the norm insofar as large portions of the total gene pool will almost certainly be absent. (Think of an aeroplane full of humans; any plane, anywhere, any time. Even if the plane does just so happen to be a UN charter, the majority of racial variation exhibited by humans will surely be missing, just for starters.) As time passes, the bulk of the population will tend to oscillate within a fixed range of morphologies (’stasis’) whereas, in fits and starts, the isolated peripheral group will change – perhaps directionally, in response to some selection pressure, or possibly just at random (fig. 1B). Either way, something different emerges (fig. 1C). Finally, as shown in the top diagram their isolation ends and the new group invades the ancestral territory (fig. 1D), perhaps displacing the ancestral taxon, perhaps not. The diagram illustrates a partial displacement only so as to suggest both possibilities.
Now consider the fossil record which might be available at each of the four stratigraphic sections labelled I through IV. The unfortunate paleontologist working up column I has missed all the action (”stasis is data” notwithstanding!) They see no change, no new taxa, no useful biostratigraphic events. The paleontologist at IV sees phyletic gradualism: a more or less insensible evolution from one thing into another: a process called anagenesis. Does that mean that the rate of change is exactly constant all the way through? Of course not. There may even be intervals of no observable change – ‘stasis’ – but nevertheless anyone with one eye and half a brain will recognise this for what it is: gradual evolutionary change.
Sections II and III show sudden appearances which I guess we might call punctuations if we are hell-bent on inventing obscurantist jargon. However, these are not evolutionary events, but invasions from another area.
Now none of this explanation contradicts Eldredge & Gould 1972. Nor is it sophisticated or original. It is old hat. Darwin may have used different words but he’d surely have recognised the model. My explanation given here introduces nothing exciting or new, but nor – and this is the point – does that of Eldredge & Gould. Indeed, their explanation presented in the 1972 paper is just a version of the foregoing, give or take an obfuscation or two. Once again, we find Eldredge & Gould 1972 to offer us only the commonplace.
Fig. 1: Four hypothetical stratigraphic sections through an interval in which allopatric speciation has given rise to a daughter species, B, from the parent population of A.
Other than reiterating the obvious – and Eldredge & Gould 1972 was not intentionally a review – it is difficult to understand the purpose of the paper, or T.M. Schopf’s motive for accepting it into the overall volume (Schopf 1972). Yet it has engendered reaction, commentary, and serious attention from professionals: What is wrong here?
For better or for worse, I am just one of those odd folks who likes to wander off into the hills and flail away at the rocks with a funny pointy hammer: I have no explanations for human behaviour. I leave you to guess.
Eldredge, N.; Gould, S.J. 1972: Punctuated Equilibria: An Alternative to Phyletic Gradualism. In Schopf, T.J.M. (ed.) 1972: Models of Paleobiology. Freeman, Cooper and Co., San Francisco. : 82-115.
Gould, S.J.; Eldredge, N. 1993: Punctuated Equilibrium Comes of Age. Nature 366: 223-227.
Schopf, T.J.M. (ed.) 1972: Models of Paleobiology. Freeman, Cooper and Co., San Francisco.
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