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Punctuated equilibrium is a theory of evolutionary biology which states that most sexually reproducing populations experience little change for most of their geological history, and that when phenotypic evolution does occur, it is localized in rare, rapid events of branching speciation (called cladogenesis).
Punctuated equilibrium is commonly contrasted against the theory of phyletic gradualism, which states that evolution generally occurs uniformly and by the steady and gradual transformation of whole lineages ( anagenesis). In this view, evolution is seen as generally smooth and continuous.
In 1972 paleontologists Niles Eldredge and Stephen Jay Gould published a landmark paper developing this idea. Their paper was built upon Ernst Mayr's theory of geographic speciation, I. Michael Lerner's theories of developmental and genetic homeostasis, as well as their own empirical research. Eldredge and Gould proposed that the degree of gradualism championed by Charles Darwin was virtually nonexistent in the fossil record, and that stasis dominates the history of most fossil species.
Punctuated equilibrium's history
Punctuated equilibrium originated as an extension of Ernst Mayr's concept of genetic revolutions by allopatric and especially peripatric speciation. Although the workings of the theory were proposed and specifically identified by Mayr in 1954, most historians of science recognize Niles Eldredge and Stephen Jay Gould's 1972 paper as the principal source of its acquiescence, and as the foundational document of a new and serious paleontological research program. Punctuated equilibrium differed from Mayr simply in that Eldredge and Gould had placed considerably greater emphasis on stasis, whereas Mayr was generally concerned with explaining the morphological discontinuity (or "punctuational patterns") found in the fossil record.
The Eldredge and Gould paper was presented at the Annual Meeting of the Geological Society of America in 1971. The symposium focused its attention on how modern microevolutionary studies could revitalize various aspects of paleontology and macroevolution. Tom Schopf, who organized that year's meeting, assigned Gould the topic of speciation. Gould recalls that "Eldredge's 1971 publication [on Paleozoic trilobites] had presented the only new and interesting ideas on the paleontological implications of the subject—so I asked Schopf if we could present the paper jointly." They did. According to Gould "the ideas came mostly from Niles, with yours truly acting as a sounding board and eventual scribe. I coined the term punctuated equilibrium and wrote most of our 1972 paper, but Niles is the proper first author in our pairing of Eldredge and Gould."
Tempo and mode
In 1954 Ernst Mayr published a landmark paper emphasizing the homogenizing effects of gene flow and the stabilizing influence of large interbreeding populations. These populations exemplified " ecotypic variation." Peripherally isolated populations, in contrast, possess " typostrophic variation" which "have the characteristic features of incipient species, but what is more important they often are species or incipient species of an entirely new type. That is, they may have morphological or ecological features that deviate quite strikingly and unexpectedly from the parental 'pattern'"
Gould summarized the theory, and its consequences for punctuated equilibrium, in a 1977 essay for Natural History magazine:
- "A new species can arise when a small segment of the ancestral population is isolated at the periphery of the ancestral range. Large, stable central populations exert a strong homogenizing influence. New and favorable mutations are diluted by the sheer bulk of the population through which they must spread. They may build slowly in frequency, but changing environments usually cancel their selective value long before they reach fixation. Thus, phyletic transformation in large populations should be very rare—as the fossil record proclaims. But small, peripherally isolated groups are cut off from their parental stock. They live as tiny populations in geographic corners of the ancestral range. Selective pressures are usually intense because peripheries mark the edge of ecological tolerance for ancestral forms. Favorable variations spread quickly. Small peripheral isolates are a laboratory of evolutionary Change.
- "What should the fossil record include if most evolution occurs by speciation in peripheral isolates? Species should be static through their range because our fossils are the remains of large central populations. In any local area inhabited by ancestors, a descendant species should appear suddenly by migration from the peripheral region in which it evolved. In the peripheral region itself, we might find direct evidence of speciation, but such good fortune would be rare indeed because the event occurs so rapidly in such a small population. Thus, the fossil record is a faithful rendering of what evolutionary theory predicts, not a pitiful vestige of a once bountiful tale."
Punctuated equilibrium is often confused with George Gaylord Simpson's quantum evolution, Richard Goldschmidt's saltationism, pre-Lyellian catastrophism, and the phenomenon of mass extinction. Punctuated equilibrium is therefore mistakenly thought to oppose the concept of gradualism, when it is actually a form of gradualism, in the ecological sense of biological continuity. This is because even though evolutionary change appears instantaneous between geological sediments, change is still occurring incrementally, with no great change from one generation to the next. To this end, Gould later commented that:
- Most of our paleontological colleagues missed this insight because they had not studied evolutionary theory and either did not know about allopatric speciation or had not considered its translation to geological time. Our evolutionary colleagues also failed to grasp the implication, primarily because they did not think at geological scales.
The relationship between punctuationism and gradualism can be better appreciated by considering an example. Suppose the average length of a limb in a particular species grows 50 centimeters (20 inches) over 70,000 years—a large amount in a geologically short period of time. If the average generation is seven years, then our given time span corresponds to 10,000 generations. It is therefore reasonable to conclude that if the limb size in our hypothetical population evolved in the most conservative manner, it need only increase at a rate of 0.005 cm per generation (= 50 cm/10,000), despite its abrupt appearance in the geological record.
Richard Dawkins dedicated a chapter in The Blind Watchmaker to correcting, in his view, the wide confusion surrounding the theory of punctuated equilibrium. His first, and main point, is to argue that phyletic gradualism in the sense of uniformity of rates—what he refers to as "constant speedism"—is a "caricature of Darwinism" and "does not really exist." His second argument, which follows from the first, is that once this caricature is dismissed, we are left with only one logical alternative, which Dawkins calls "variable speedism." Variable speedism may be distinguished in one of two ways: "discrete variable speedism" and "continuously variable speedism." Eldredge and Gould, believing that evolution jumps between stability and relative rapidity, are described as "discrete variable speedists," and "in this respect they are genuinely radical." They believe that evolution generally proceeds in bursts, or not at all. "Continuously variable speedists," on the other hand believe that "evolutionary rates fluctuate continuously from very fast to very slow and stop, with all intermediates. They see no particular reason to emphasize certain speeds more than others. In particular, stasis, to them, is just an extreme case of ultra-slow evolution. To a punctuationist, there is something very special about stasis." Dawkins therefore commits himself here to an empirical claim about the geological record, and it is this particular claim that Eldredge and Gould have aimed to overturn.
Another pervasive misunderstanding of punctuated equilibrium was that it invoked large-scale mutations, the sort invoked by Richard Goldschmidt in The Material Basis of Evolution. According to Dawkins, punctuated equilibrium "has no connection with macromutation and true saltation, but rather "followed from long accepted conventional Darwinism," namely Mayrian allopatric speciation. But unlike Eldredge and Gould, Dawkins believes that the apparent gaps represented in the fossil record document migrational events, not evolutionary events. According to Dawkins evolution certainly occurred, though "probably gradually" elsewhere.
It is generally difficult to infer whether Dawkins is criticizing Eldredge and Gould or just popular misreadings of their work. Dawkins is rarely specific on this point, but emphasizes that it has been "oversold by some journalists," but partly due to their "later writings." In the end, Dawkins contends, punctuated equilibrium "does not deserve a particularly large measure of publicity." It is a "minor gloss," an "interesting but minor wrinkle on the surface of neo-Darwinian theory," and "lies firmly within the neo-Darwinian synthesis."
Daniel Dennett is also critical of Gould's presentation of punctuated equilibrium. In his book Darwin's Dangerous Idea, Dennett argues that Gould alternated between revolutionary and conservative claims about punctuated equilibrium, and that each time Gould made a revolutionary claim—or appeared to do so—it was criticized, and Gould retreated to a traditional neo-Darwinian position. Gould responded to Dennett's claims in The New York Review of Books, and in his technical volume The Structure of Evolutionary Theory.
Some critics have pointed out that Gould's use of analogy and metaphor to argue for the validity of punctuated equilibrium constitutes a non-scientific discourse serving to validate scientific theory. Particularly in his popular essays, Gould uses a variety of strategies from literature, political science, and personal anecdotes to substantiate the general pattern of Punctuated Equilibrium (long periods of stasis interrupted by rapid, catastrophic change). While Gould is celebrated among non-scientists for the colour and energy of his prose and his massive interdisciplinary knowledge, his critics have concerns that the theory has gained undeserved credence among non-scientists because of Gould's rhetorical skills.
Relation to Darwin's Theories
The sudden appearance and lack of substantial gradual change of most species in the geologic record—from their initial appearance until their extinction—has long been noted, including by Charles Darwin (1859:301, 1871:119-120) who appealed to the imperfection of the record as the favored explanation. When presenting his ideas against the prevailing influence of catastrophism put forward by Georges Cuvier which envisaged species being supernaturally created at intervals, Darwin needed to forcefully stress the gradual nature of evolution in accordance with the gradualism promoted by his friend Charles Lyell. He privately expressed concern, noting in the margin of his 1844 Essay “Better begin with this: If species really, after catastrophes, created in showers world over, my theory false.” It is often incorrectly assumed that he insisted that the rate of change must be constant, or nearly so, but in the fifth edition of The Origin of Species Darwin wrote that "the periods during which species have undergone modification, though long as measured in years, have probably been short in comparison with the periods during which they retain the same form." Thus punctuationism in general is consistent with Darwin's conception of evolution, and with the independent proposals of natural selection by William Charles Wells, Patrick Matthew, and Alfred Russel Wallace.
According to the theory of punctuated equilibrium, "peripheral isolates" are considered to be of critical importance for speciation. However, Darwin wrote, "I can by no means agree…that immigration and isolation are necessary elements. . . . Although isolation is of great importance in the production of new species, on the whole I am inclined to believe that largeness of area is still more important, especially for the production of species which shall prove capable of enduring for a long period, and of spreading widely."
Darwin explained the reasons for this belief as follows:
- "Throughout a great and open area, not only will there be a greater chance of favourable variations, arising from the large number of individuals of the same species there supported, but the conditions of life are much more complex from the large number of already existing species; and if some of these species become modified and improved, others will have to be improved in a corresponding degree, or they will be exterminated. Each new form, also, as soon as it has been improved, will be able to spread over the open and continuous area, and will thus come into competition with many other forms ... the new forms produced on large areas, which have already been victorious over many competitors, will be those that will spread most widely, and will give rise to the greatest number of new varieties and species. They will thus play a more important role in the changing history of the organic world."
Thus punctuated equilibrium contradicts some of Darwin's ideas regarding the specific mechanisms of evolution, but generally accords with Darwin's theory of evolution by natural selection.
Supplemental modes of rapid evolution
Recent work in developmental biology has identified dynamical and physical mechanisms of tissue morphogenesis that may underlie abrupt morphological transitions during evolution. Consequently, consideration of mechanisms of phylogenetic change that are actually (not just apparently) non-gradual is increasingly common in the field of evolutionary developmental biology, particularly in studies of the origin of morphological novelty. A description of such mechanisms can be found in the multi-authored volume Origination of Organismal Form (MIT Press; 2003).
Punctuated Equilibrium has also played a role in social and political theory, particularly in policy studies, as one of many cross-overs of evolutionary theory into social theory. The punctuated equilibrium model of policy change was first presented by Frank Baumgartner and Bryan Jones in 1993, and has subsequently been examined in many policy contexts and has increasingly received attention in the field. The model states that policy generally changes only incrementally due to several restraints, namely lack of institutional change and bounded rationality of individual decision-making. Policy change will thus be punctuated by changes in these conditions, especially change in party control of government or changes in public opinion. Thus, policy is characterized by long periods of stability, punctuated by large, but rare, changes due to large shifts in society or government. This has been shown to be particularly evident in current trends of environmental policy and energy policy. Recently, in conjunction with historical findings of sharp and punctuated policy change, newer findings in gun control and U.S. state tobacco policy have found largely symbolic punctuated changes. For instance, a recent study by Michael Givel found that despite a significant mobilization to change state tobacco policy, U.S. state tobacco policymaking from 1990 to 2003 was characterized by limited and symbolic punctuation that favored the pro-tobacco advocacy coalition’s policy agenda.
In addition, Connie Gerskick studied five models of change from different domains and found similar patterns between the way that change is thought to occur in biological species according to the theory of punctuated equilibrium and the ways adults, groups, organizations and scientific fields develop. In general, the original formulation of theory has been used to explain patterns of change in groups and organizations where periods of "stasis" are punctuated by brief and intense periods of "radical" change. Two widely known applications of the theory of punctuated equilibrium in the social sciences are in organizational theory (e.g., ) and in the study of small work groups (e.g., ). As some researchers have noted, these applications of the original theory have shifted its focus of attention from "a theory about change in populations to a theory about change within entities" .