Punctuated balance

The punctuated balance is a theory of the evolution suggested by two American paleontologists, Stephen Jay Gould and Niles Eldredge. She postulates that the evolution includes/understands punctuated long periods of balance of short periods of material changes like the Spéciation.

History

The theory of punctuated balances was for the first time presented by Niles Eldredge and Stephen Jay Gould in 1972, in an article entitled: Punctuated equilibrium: year alternative to phyletic gradualism in the collective work Models in Paleobiology . The theory will be very criticized but will end up carrying the adhesion of a majority of paleontologists.

Contents of the theory

Gould and Eldredge presented their theory like an alternative to the “phyletic Gradualisme”, one of the postulates of the theory of the evolution then in force (known under the name of “Théorie Synthetic of the Evolution” and resulting from the Neo-Darwinism). According to this theory, the morphological evolution of the Espèce S would occur by very slow and continuous modifications of the same population during time by the play of the changes and the Natural selection.

The partisans of the theory of punctuated balances affirm that this gradualism is in contradiction with the contents of the files Fossile S: according to them, those would contain in fact only seldom the intermediate forms linking a species with the other. Actually, the species would be characterized at the same time by an abrupt appearance in the fossil register (what can mean that it extended on a few thousands from years, a time geologically runs compared to the million years postulated by the phyletic gradualism) and a great stability following their appearance. This period of morphological stagnation of the species (being accompanied at most by some minor and reversible modifications) is called damning up by the ponctualists. Gould will go until writing that “the extreme scarcity of the intermediate forms is the professional secrecy of paleontology”.

According to Gould and Eldredge, the theory of punctuated balances does nothing but solve one contradiction inherent in the Synthetic Theory: indeed, celebrates it biologist Ernst Mayr, who had forged this theory, had supported a model of Spéciation (i.e. of formation of new species) according to which the new species appear when small a Population of a given species is isolated far from its species from origin. This one then knows a fast series of morphological changes and genetics before becoming a new species with whole share: it is the model of peripatric speciation . The ponctualists support that the theory of punctuated balances does nothing but apply this model, established by the biologists by the observation of the modern organizations, at the fossil organizations: the species are formed quickly, starting from small isolated populations (what explains the scarcity of the intermediaries), then stagnate morphologiquement once established. Thus would be explained the observation of punctuated balances.

Criticisms of the theory

Importance of the theory

Certain authors, such as Richard Dawkins affirmed that, even if it were exact, it was not revolutionary which its founders lent to him. According to Dawkins, the theory of punctuated balances would be “only one minor wave on the ocean of the neodarwinism”.

Frequency of the examples

Examples of phyletic gradualism exist and were documented in the fossil register, in particular at the Campagnol S and some Trilobite S. One reproached Gould and Eldredge to have underestimated the frequency of these examples or not to have taken into account the fact that the absence of intermediaries can be rather due to a lack of research or the lacunar character of the fossil files that to their real scarcity.

One as could affirm as the paleontologists arbitrarily named species which they cut out within a continuum of organizations changing gradually with the length of geological times, creating thus artificially the discontinuities then recovered like evidence by the ponctualists.

The ponctualists answered the first objection by studies of relative Fréquence covering a broad number of species, but the conclusion of these studies there was too variable according to the cases, some being favorable to the ponctualists and others with the gradualists: it would seem in fact that the two modes of evolution exist, according to the groups, even according to the characters considered.

They also answered the third by studies evaluating in a quantitative way the morphological modifications observed in the fossils, without taking into account classification in cash: they thus could show objectively, at least in certain cases, the reality of the evolution by punctuated balances.

Errors on the theory of punctuated balances

The theory of punctuated balances could be on several occasions caricatured by its adversaries or, more simply, badly included/understood. The most frequent errors are the following ones.

Negative data

A first objection affirms that the theory of punctuated balances is based only on one absence of transitory fossils; however, an absence of evidence proves anything, neither in a direction nor in the other. The absence of evidence is not the proof of the absence, and the quality of the fossil files being very random, one cannot in no case to affirm that such gaps have really a significance as for the evolution of the species.

If this objection is logical, it is false, since the theory of punctuated balances is not based only on negative data (like the absence of transitory fossils) but also on positive data, namely the existence of the periods of damning up , these long lives when a species does not know any major morphological change. According to a famous aphorism of Stephen Jay Gould, “stasis is dated” ( the damning up is a data ): if the paleontologists often tended to neglect to study the simple absence of change when they observed it, it does not constitute of it less one real data.

Intermediate forms

Another bad interpretation would like that the theory of punctuated balances affirms that there does not exist, or little, of intermediate fossils of forms.

Actually, the theory of punctuated balances relates to only the formation of new species, therefore the absence of intermediate forms between different species. The species is one of the levels low of the classification of the alive world: the intermediate species between vaster groups (for example between Mammalian Reptiles and or between Dinosaur S and Birds) are on the contrary numerous. “The intermediate forms usually miss on the level of the species, but they abound between vaster groups”, written Gould.

Promising monster

According to some, the theory of punctuated balances is a new version of the theory of the promising monster.

The theory of the promising monster is a Théorie of the evolution elaborate in the years 1930 by the geneticist Richard Goldschmidt, according to whom only one change of great width could give rise to a new species, even with a new “great group” of living beings by giving birth in only one stage to a new plan of organization.

However, the theory of punctuated balances affirms just that the formation of new species is instantaneous geologically speaking: that can imply one real duration of a few thousands of years, and a good number of small changes rather than only one change of great width. In addition, the theory of the promising monster especially sought to explain the birth of great groups characterized by radical evolutionary innovations, whereas the theory of punctuated balances relates to only the phenomenon of Spéciation, much more common.

Their only common point (rather surface) is to be theories of the fast evolutionary change. Part of confusion can come owing to the fact that Gould was sometimes declared favorable to a moderate form of the theory of the promising monster (even if it changed opinion on the question during its career), but these declarations were deprived of any relationship with the theory of punctuated balances.

Works and bonds

Works

Richard Dawkins, the clock and watch maker plugs , Robert Laffont, 1989
Niles Eldredge, The pattern off evolution , Freeman, 2000
Stephen Jay Gould, the inch of the panda , Grasset, 1982
Stephen Jay Gould, Pigs might fly , Fayard, 1984
Stephen Jay Gould, The structure off evolutionary theory , Belknap, 2002
Stephen Jay Gould, the structure of the theory of the evolution , Gallimard, 2006
Michael givel, Failure to Change through Multiple Policy Come Instruments and the Tobacco Industry Policy Subsystem in the States from 1990 to 2003 , University off Oklahoma, 2006

Bonds

Punctuated equilibria

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