Punctuated equilibrium

2007 Schools Wikipedia Selection. Related subjects: Evolution and reproduction

Punctuated equilibrium (or punctuated equilibria) is a theory in evolutionary biology which states that most sexually reproducing species will show little to no evolutionary change throughout their history. When evolution does occur, it happens sporadically (by splitting) and occurs relatively quickly compared to the species' full duration on earth. For this reason, the theory is sometimes called evolution by jerks. Punctuated equilibrium is commonly contrasted against the theory of phyletic gradualism (“evolution by creeps”), which hypothesizes that most evolution occurs uniformly and by the steady and gradual transformation of whole lineages ( anagenesis).

Punctuated equilibrium's history

Punctuated equilibrium originated as an extension of Ernst Mayr's concept of genetic revolutions by peripatric and allopatric 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 acceptance (by both paleontologists and evolutionists) and as the foundational document of a new and serious paleontological research program ( Mayr 1992: 25-26, Shermer 2001: 102-113). 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 Stephen Jay 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." (Gould 2002: 775) 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." (Gould 1991)

Tempo and mode

Ernst Mayr's paper "Change of genetic environment and evolution" ( 1954) emphasized 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' " (1954:160)

Stephen Jay Gould summarized the theory, and its consequences for punctuated equilibrium, in a 1977 essay for Natural History:

"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." (1980:184)

Common misconceptions

Confusion with other rapid modes of evolution

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 more appropriately understood as a form of gradualism (in the strict and literal sense of biological continuity). This is because even though evolutionary change aggregates "quickly" between geological sediments—relative to the species' full geological existence—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.

Challenge of uniformity of rates

Critics of punctuated equilibrium, such as Richard Dawkins, have argued that the concept of phyletic gradualism was merely a straw man—arguing that a belief in the uniformity of rates was never really held by any serious evolutionist (Dawkins 1986, 223-224, 228). Eldredge and Gould's advocacy of the theory brought punctuated equilibrium much attention, including full page stories in The New York Times and Newsweek. The resulting debate stirred up in evolutionary circles was misrepresented by some creationists to portray Darwinism as a "theory in crisis." The actual differences between the various evolutionary theorists were not as large as they were made to appear (Dawkins 1986, 236). Gould himself later said that the theory did not in fact refute Darwin's gradualism, but just added the ideas of catastrophism and stasis.

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).

Relation to Darwinism

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. Nevertheless, with the influence of catastrophism, Darwin needed to forcefully stress the gradual nature of evolution. It is often incorrectly assumed that he insisted that the rate of change must be constant, or nearly so. In 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." (1872:619) 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." (1859:105-106)

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." (1859:107-108)

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.