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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.
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