Punctuated equilibrium

Punctuated equilibrium is a theory in evolutionary biology which proposes that most sexually reproducing species will experience little evolutionary change for most of their geological history (in an extended state called stasis). When evolution occurs, it is localized in rare, rapid events of branching speciation (called cladogenesis). Cladogenesis is simply the process by which species split into two distinct species, rather than one species gradually transforming into another.

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 theory and called it punctuated equilibria. 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 commonly attributed to 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 much of the basic workings of the theory were proposed and identified by Mayr in 1954, historians of science generally recognize Niles Eldredge and Stephen Jay Gould's 1972 paper as the foundational document of the new paleobiological 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. Mayr later complimented Eldredge and Gould's paper stating that evolutionary stasis had been "unexpected by most evolutionary biologists" and that punctuated equilibrium "had a major impact on paleontology and evolutionary biology."

Prior to the Eldredge and Gould paper, Niles Eldredge published a 1971 paper in the journal Evolution suggesting that gradual evolution was seldom seen in the fossil record and argued that Ernst Mayr's preferred mechanism might suggest a possible resolution.

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." 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."

Theoretical mechanisms
When Eldredge and Gould published their 1972 paper, allopatric speciation held the position as the "standard" theory of speciation. The theory was popularized by Ernst Mayr in his 1954 paper "Change of genetic environment and evolution," and his classic volume Animal Species and Evolution (1963).

Allopatric speciation suggests that species which are comprised of large central populations are stabilized by their large volume and the genetic process of gene flow. New and even beneficial mutations are diluted by the population's size and are unable to reach fixation due to factors such as constantly changing environments. If this is the case, then the transformation of whole lineages should be rare, as the fossil record indicates. Smaller populations on the other hand, which are isolated from the parental stock, are decoupled from the homogenizing effects of gene flow. In addition, pressure from natural selection is especially intense, as peripheral isolated populations exist at the outer edges of ecological tolerance. If most evolution happens in these rare instances of allopatric speciation then evidence of gradual evolution should be rare. This stimulating hypothesis was alluded upon by Mayr in the closing paragraph of his 1954 paper. Ernst Mayr often emphasized the use of population thinking to describe species, rather than models which evoked a typological approach. In order to contrast the differences in population structures he coined the terms ecotypic and typostrophic variation. Ecotypic variation describes the genetics of large central populations which are stabilized by the homogenizing effects of gene flow. Small 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."

As time went on Gould moved away from wedding punctuated equilibrium with allopatric speciation, particularly as other modes of speciation began to gain evidence. He also moved away from wedding stasis to internal constraints. Gould was particularly attracted to Douglas Futuyma's work on the importance of reproductive isolating mechanisms.

Punctuated equilibrium also made contributions to the theory that species are Darwinian individuals, and not just classes, thereby providing a stronger framework for a hierarchical theory of evolution.

Other biologists have applied punctuated equilibrium to non-sexual species, even the evolution of viruses.

Common misconceptions
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&mdash;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.

Criticism
Richard Dawkins believes that the apparent gaps represented in the fossil record document migrational events rather than evolutionary events. According to Dawkins evolution certainly occurred, though "probably gradually" elsewhere. However, the punctuational model may still be inferred from both the observance of stasis, and documented examples of rapid and episodic speciation events documented in the fossil record.

Dawkins also emphasizes that punctuated equilibrium has been "oversold by some journalists," but partly due to Eldredge and Gould's "later writings." Dawkins contends that the theory "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."

Criticism of punctuated equilibrium has even appeared from non-scientists. Philosopher Daniel Dennett is especially 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 the theory, and that each time Gould made a revolutionary statement&mdash;or appeared to do so&mdash;he was criticized, and thus 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.

Heidi Scott, a PhD student of English Literature, argued that Gould's use of analogy and metaphor constitutes a non-scientific discourse serving to validate a scientific theory. Scott accuses Gould—particularly in his popular essays—of using 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). Gould responded that critics often made the mistake of confusing the context of discovery with the context of justification. While Gould is celebrated for the color and energy of his prose, as well as his massive interdisciplinary knowledge, critics such as Scott have concerns that the theory has gained undeserved credence among non-scientists because of Gould's rhetorical skills.

Randy Allen Harris, Professor of Rhetoric at the University of Waterloo, is more positive, stating that "re-analysis of existing fossil data has shown, to the increasing satisfaction of the paleontological community, that Eldredge and Gould were correct in identifying periods of evolutionary stasis which are interrupted by much shorter periods of evolutionary change."

Relation to Darwin's theories
The sudden appearance and lack of substantial gradual change of most species in the geologic record&mdash;from their initial appearance until their extinction&mdash;has long been noted, including by Charles Darwin 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 even the first edition of On the Origin of Species states that "Species of different genera and classes have not changed at the same rate, or in the same degree. In the oldest tertiary beds a few living shells may still be found in the midst of a multitude of extinct forms... The Silurian Lingula differs but little from the living species of this genus". Lingula is among the few brachiopods surviving today but also known from fossils over 500 million years old. In the fifth edition of On 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.

According to early versions 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."

The importance of isolation in forming species had played a significant part in Darwin's early thinking, as shown in his Essay of 1844. But by the time he wrote the Origin he had downplayed its importance. He explained the reasons for his revised view 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."

- Charles Darwin (1869). On The Origin of Species

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 in social theory
Punctuated equilibrium in social theory is a method of understanding change in complex social systems, particularly how policy change and the evolution of conflicts seem to progress in extended periods of stasis, punctuated by sudden shifts in radical change.