Punctuated Equilibrium

In evolutionary biology, punctuated equilibrium (also called punctuated equilibria) is a theory that proposes that once a species appears in the fossil record, the population will become stable, showing little evolutionary change for most of its geological history.

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This state of little or no morphological change is called ''stasis''. When significant evolutionary change occurs, the theory proposes that it is generally restricted to rare and geologically rapid events of branching speciation called cladogenesis. Cladogenesis is the process by which a species splits into two distinct species, rather than one species gradually transforming into another.

Punctuated equilibrium is commonly contrasted with phyletic gradualism, the idea that evolution generally occurs uniformly by the steady and gradual transformation of whole lineages (anagenesis).

In 1972, paleontologists Niles Eldredge and Stephen Jay Gould published a landmark paper developing their theory and called it ''punctuated equilibria''. Their paper built upon Ernst Mayr's model of geographic speciation,

I. M. Lerner's theories of developmental and genetic homeostasis,
and their own empirical research.

Eldredge and Gould proposed that the degree of gradualism commonly attributed to Charles Darwin

is virtually nonexistent in the fossil record, and that stasis dominates the history of most fossil species.

History

Punctuated equilibrium originated as a logical consequence of Ernst Mayr's concept of genetic revolutions by allopatric and especially peripatric speciation as applied to the fossil record. Although the sudden appearance of species and its relationship to speciation was proposed and identified by Mayr in 1954, historians of science generally recognize the 1972 Eldredge and Gould paper as the basis of the new paleobiological research program. Mayr, Ernst (1992)
"Speciational Evolution or Punctuated Equilibria."
In Albert Somit and Steven Peterson ''The Dynamics of Evolution''. New York: Cornell University Press, pp. 21-48. Punctuated equilibrium differs from Mayr's ideas mainly in that Eldredge and Gould placed considerably greater emphasis on stasis, whereas Mayr was concerned with explaining the morphological discontinuity (or "sudden jumps") 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."

A year before their 1972 Eldredge and Gould paper, Niles Eldredge published a paper in the journal ''Evolution'' which suggested that gradual evolution was seldom seen in the fossil record and argued that Ernst Mayr's standard mechanism of allopatric speciation 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 n_ n_Paleozoic_trilobite">Paleozoic.html"_;"title="n_Paleozoic">n_ n_Paleozoic_trilobite">Paleozoic.html"_;"title="n_Paleozoic">n_Paleozoic_trilobites">Paleozoic_trilobite.html"_;"title="Paleozoic.html"_;"title="n_Paleozoic">n_Paleozoic_trilobite">Paleozoic.html"_;"title="n_Paleozoic">n_Paleozoic_trilobiteshad_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."Stephen_Jay_Gould.html" ;"title="Paleozoic_trilobites.html" ;"title="Paleozoic_trilobite.html" ;"title="Paleozoic.html" ;"title="n Paleozoic">n Paleozoic trilobite">Paleozoic.html" ;"title="n Paleozoic">n Paleozoic trilobites">Paleozoic_trilobite.html" ;"title="Paleozoic.html" ;"title="n Paleozoic">n Paleozoic trilobite">Paleozoic.html" ;"title="n Paleozoic">n Paleozoic trilobiteshad 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."Stephen Jay Gould">S. J. Gould (1991)
"Opus 200"
''Natural History'' 100 (August): 12-18. In his book ''Time Frames'' Eldredge recalls that after much discussion the pair "each wrote roughly half. Some of the parts that would seem obviously the work of one of us were actually first penned by the other—I remember for example, writing the section on Gould's snails. Other parts are harder to reconstruct. Gould edited the entire manuscript for better consistency. We sent it in, and Schopf reacted strongly against it—thus signaling the tenor of the reaction it has engendered, though for shifting reasons, down to the present day."

John Wilkins and Gareth Nelson have argued that French architect Pierre Trémaux proposed an "anticipation of the theory of punctuated equilibrium of Gould and Eldredge."

Evidence from the fossil record

The fossil record includes well documented examples of both phyletic gradualism and punctuational evolution. As such, much debate persists over the prominence of stasis in the fossil record. Before punctuated equilibrium, most evolutionists considered stasis to be rare or unimportant. The paleontologist George Gaylord Simpson, for example, believed that phyletic gradual evolution (called ''horotely'' in his terminology) comprised 90% of evolution. More modern studies,Campbell, N.A. (1990) ''Biology'' p. 450–451, 487–490, 499–501. Redwood City CA: Benjamin Cummings Publishing Company. S. J. Gould, & Eldredge, Niles (1977)
"Punctuated equilibria: the tempo and mode of evolution reconsidered."
''Paleobiology'' 3 (2): 115-151. (p.145)McCarthy, T. & Rubridge, B. (2005) ''The Story of Earth and Life''. Cape Town: Struik Publishers. . including a meta-analysis examining 58 published studies on speciation patterns in the fossil record showed that 71% of species exhibited stasis, and 63% were associated with punctuated patterns of evolutionary change. According to Michael Benton, "it seems clear then that stasis is common, and that had not been predicted from modern genetic studies."Benton, Michael and David Harper (2009) ''Introduction to Paleobiology and the Fossil Record'' New York: John Wiley & Sons
pp. 123-124.
/ref> A paramount example of evolutionary stasis is the fern ''Osmunda claytoniana''. Based on paleontological evidence it has remained unchanged, even at the level of fossilized nuclei and chromosomes, for at least 180 million years.

Theoretical mechanisms

Punctuational change

When Eldredge and Gould published their 1972 paper, allopatric speciation was considered the "standard" model of speciation. This model was popularized by Ernst Mayr in his 1954 paper "Change of genetic environment and evolution," and his classic volume ''Animal Species and Evolution'' (1963). Mayr, Ernst (1963). ''Animal Species and Evolution''. Cambridge, MA: Harvard University Press.

Allopatric speciation suggests that species with large central populations are stabilized by their large volume and the process of gene flow. New and even beneficial mutations are diluted by the population's large size and are unable to reach fixation, due to such factors 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 in the fossil record should be rare. This hypothesis was alluded to by Mayr in the closing paragraph of his 1954 paper:

Although punctuated equilibrium generally applies to sexually reproducing organisms, some biologists have applied the model to non-sexual species like viruses, which cannot be stabilized by conventional gene flow. As time went on biologists like Gould moved away from wedding punctuated equilibrium to allopatric speciation, particularly as evidence accumulated in support of other modes of speciation. Gould, for example, was particularly attracted to Douglas Futuyma's work on the importance of reproductive isolating mechanisms.

Stasis

Many hypotheses have been proposed to explain the putative