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Usuário:Leonardorejorge/Equilíbrio pontuado

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Equilíbrio pontuado, embaixo, consiste de predominância de estabilidade morfológica com explosões raras de mudança evolutiva.

Equilíbrio pontuado é uma teoria de em evolução que propõe que a maior parte das populações de organismos de reprodução sexuada experimentam puca mudança ao longo do tempo geológico, e quando mudanças evolutivas no fenótipo ocorrem, elas se dão de forma rara e localizada em eventos rápidos de especiação denominados cladogênese.

O equilíbrio pontuado é frequentemente contrastado com a teoria do gradualista, que afirma que a evolução ocorre de maneira uniforme, por mudança contínua e gradual de linhagens inteiras (anagênese). Segundo essa visão, a evolução é vista como um processo suave e contínuo.

Em 1972 os paleontólogos Niles Edredge e Stephen Jay Gould publicaram um trabalho historicamente importante, em que desenvolvem essa idéia. O trabalho se baseava na teoria de especiação alopátrica proposta por Ernst Mayr, nas teorias de homeostase genética e do desenvolvimento de I. Michael Lerner, assim como na pesquisa empírica dos autores. Eldredge e Gould propuseram que o nível de gradualismo considerado por Charles Darwin era praticamente inexistente no registro fóssil, e que a estabilidade dominava a história da maioria das espécies fósseis

História do Equilíbrio pontuado editar

O equilíbrio pontuado surgiu como uma extensão do conceito de revoluções genéticas por especiação alopátrica e especialmente especiação peripátrica, de Ernst Mayr. Apesar desse conceito ter sido proposto e desenvolvido por Mayr em 1954, ela se tornou amplamente conhecida após o trabalho de Niles Eldredge e Stephen Jay Gould de 1972, sendo esse trabalho também considerado o documento fundador de um novo programa de pesquisa sério em paleontologia.[1][2] O Equilíbrio pontuado diferia da proposta de Mayr simplesmente pela ênfase dada por à estase evolutiva por parte de Eldredge e Gould, enquanto Mayr estava mais interessado em explicar a descontinuidade morfológica no registro fóssil.

O trabalho de Eldredge e Gould foi apresentado no encontro anual da Sociedade Geológica da América de 1971[3]. O simpósio focou ua atenção a como estudos recentes em microevolução poderiam revitalizar os campos de pesquisa em palentologia e macroevolução. Tom Schopf, organizador do encontro, atribuiu a Gould o tema de especiação. Segundo Gould, "A publicação de Eldredge de 1971 [sobre trilobitas do paleozóico] apresentava as únicas idéias novas e interessantes sobre as implicações paleontológicas no tema- então eu perguntei a Schopf se nós podíamos apresentar o trabalho em parceria."[4]

Tempo e modo editar

Em 1954 Ernst Mayr publicou um trabalho muito importante, em que enfatizou os efeitos homogeneizadores do fluxo gênico e a influência estabilizadora de grandes populações intercruzantes [5]. Essas populações exemplificavam "variação ecotípica". Populações perifericamente isoladas, por outro lado, possuem "variação tipostrófica", ou seja, "apresentam as propriedades características de espécies incipientes, e o que é mais importante, elas frequentemente representam um tipo totalmente diferente de espécie. Ou seja, elas podem ter traços morfológicos ou ecológicos que desviam muito e de forma inesperada do padrão parental"[6].

Gould resumiu essa teoria, e suas consequências para o equilíbrio pontuado, em um texto de 1977 para a revista Natural History:[7]

"Uma nova espécie pode surgir quando um pequeno segmento da população ancestral é isolado na periferia da distribuição geográfica ancestral. Populações centrais grandes e estáveis exercem um efeito homogeneizador muito forte. Mutações novas e favoráveis são diluídas pela grande quantidade de indivíduos pela qual ela precisa se espalhar. Elas podem aumentar lentamente em frequência, mas mudanças no ambiente acabam cancelando seus efeitos seletivos positivos muito antes que haja fixação da característica. Assim, transformação filética em grandes populações deveria ser muito rara - como o registro fóssil demonstra. Mas grupos pequenos, perifericamente isolados, são separados da população estoque que lhes deu origem. Eles vivem como minúsculas populações em cantos da distribuição ancestral. Pressões seletivas são intensas porque as periferias marcam o limite de tolerância ecológica das formas ancestrais. Variantes favoráveis se espalham rapidamente. Pequenos isolados periféricos são um laboratório para a mudança evolutiva."
"O que o registro fóssil deveria incluir se a maior parte da evolução ocorre por especiação de isolados periféricos? Espécies deveriam ser estáticas ao longo de sua distribuição porque nossos fósseis são o remanescente de grandes populações centrais. Em qualquer área habitada por ancestrais, a espécie descendente deveria aparecer de repente por migração da região periférica em que ela evoluiu. Na região periférica em si, nós deveríamos encontrar evidência direta de especiação, mas tal boa sorte seria muito rara devido à rapidez com que tais eventos ocorrem, numa população muito pequena. Assim, o registro fóssil é um retrato fiel do que a teoria evolutiva prevê, não um vestígio pobre de um conto originalmente detalhado"[8].

Common misconceptions editar

Punctuated equilibrium is often confused with George Gaylord Simpson's quantum evolution,[9] Richard Goldschmidt's saltationism,[10] 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.[3] 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.[11]

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"[12] and "does not really exist."[13] 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."[14] 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."[15] Dawkins therefore commits himself here to an empirical claim about the geological record,[16] 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.[17] According to Dawkins, punctuated equilibrium "has no connection with macromutation and true saltation,[18] but rather "followed from long accepted conventional Darwinism," namely Mayrian allopatric speciation.[19]

It is generally difficult to infer whether Dawkins is criticizing Eldredge and Gould or just popular misreadings of their work, as Dawkins is rarely specific on this point.

Criticism editar

Richard Dawkins, unlike Eldredge and Gould, 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.[20] However, the punctuational model may still be inferred from: (1) stasis, and (2) the brief and episodic nature of speciation events, when they are seen in the fossil record.

Dawkins also emphasizes that punctuated equilibrium has been "oversold by some journalists,"[21] but partly due to their "later writings."[22] In the end, Dawkins contends, punctuated equilibrium "does not deserve a particularly large measure of publicity."[23] 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."[24]

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.[25] Gould responded to Dennett's claims in The New York Review of Books,[26] and in his technical volume The Structure of Evolutionary Theory.[27]

Some critics have argued 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 color 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.[28]

Relation to Darwin's Theories editar

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.”[29] 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."[30] Thus punctuationism in general is consistent with Darwin's conception of evolution,[29] 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."[31]

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."[32]

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.[29]

Supplemental modes of rapid evolution editar

Ver também: Rapid modes of 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).

In social theory editar

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.[33]

In addition, Connie Gerskick [34] 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., [35]) and in the study of small work groups (e.g., [36]). 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" [37].

See also editar

References editar

  1. ↑ Ernst Mayr, 1992. "Speciational Evolution or Punctuated Equilibria" In Albert Somit and Steven Peterson The Dynamics of Evolution. New York: Cornell University Press, pp. pp. 25-26.
  2. ↑ Michael Shermer, 2001. The Borderlands of Science. New York: Oxford University Press.
  3. ↑ a b Niles Eldredge and Stephen Jay Gould, 1972. "Punctuated equilibria: an alternative to phyletic gradualism" In T.J.M. Schopf, ed., Models in Paleobiology. San Francisco: Freeman Cooper. pp. 82-115. Reprinted in N. Eldredge Time frames. Princeton: Princeton Univ. Press. 1985
  4. ↑ Stephen Jay Gould, 2002. The Structure of Evolutionary Theory, Cambridge, Massachusetts: Harvard University Press, pp. 775.
  5. ↑ Ernst Mayr, 1954. "Change of genetic environment and evolution" In J. Huxley, A. C. Hardy and E. B. Ford. Evolution as a Process London: Allen and Unwin. pp. 157-180.
  6. ↑ Ernst Mayr, 1954. "Change of genetic environment and evolution" In J. Huxley, A. C. Hardy and E. B. Ford. Evolution as a Process London: Allen and Unwin. pp. 160
  7. ↑ S. J. Gould, 1977. "Evolution's erratic pace." Natural History 86 (May): 12-16.
  8. ↑ Stephen Jay Gould, 1980. The Panda's Thumb. New York. W. W. Norton. pp. 182-184.
  9. ↑ Francisco Ayala, 2005. "On Stephen Jay Gould's Monumental Masterpiece" Theology and Science 3 (1): 103
  10. ↑ Ernst Mayr, 1982. The Growth of Biological Thought Cambridge MA: Harvard University Press, p. 617. See also S. J. Gould, 2002. Structure pp. 765, 778, 1001, 1005, 1009.
  11. ↑ Stephen Jay Gould, 1991. "Opus 200" Natural History 100 (August): 12-18.
  12. ↑ Richard Dawkins, 1996. The Blind Watchmaker New York: W. W. Norton, p. 227.
  13. ↑ Richard Dawkins, 1996. The Blind Watchmaker New York: W. W. Norton, p. 228. His one exception to this rule is the non-adaptive evolution observed in molecular evolution.
  14. ↑ Richard Dawkins, 1996. The Blind Watchmaker New York: W. W. Norton, p. 245.
  15. ↑ Richard Dawkins, 1996. The Blind Watchmaker New York: W. W. Norton, p. 245-246, emphasis added.
  16. ↑ This is in contrast to his claim that: "The paleontological evidence can be argued about, and I am not qualified to judge it." The Extended Phenotype, 1982, p. 102.
  17. ↑ Richard Dawkins, 1996. The Blind Watchmaker New York: W. W. Norton, p. 230-236.
  18. ↑ Richard Dawkins, 1996. The Blind Watchmaker New York: W. W. Norton, p 236.
  19. ↑ Richard Dawkins, 1996. The Blind Watchmaker New York: W. W. Norton, p. 236, 239, 243.
  20. ↑ Richard Dawkins, 1996. The Blind Watchmaker New York: W. W. Norton, p. 240.
  21. ↑ Richard Dawkins, 1996. The Blind Watchmaker New York: W. W. Norton, p. 250-251.
  22. ↑ Richard Dawkins, 1996. The Blind Watchmaker New York: W. W. Norton, p. 241.
  23. ↑ Richard Dawkins, 1996. The Blind Watchmaker New York: W. W. Norton, p. 250.
  24. ↑ Richard Dawkins, 1996. The Blind Watchmaker New York: W. W. Norton, p. 251.
  25. ↑ Daniel Dennett, 1995. Darwin's Dangerous Idea. New York: Simon & Schuster, pp. 282-299.
  26. ↑ Stephen Jay Gould, 1997. "Darwinian Fundamentalism" The New York Review of Books, June 12, pp. 34-37; And "Evolution: The Pleasures of Pluralism" The New York Review of Books, June 26, pp. 47-52.
  27. ↑ Stephen Jay Gould, 2002. The Structure of Evolutionary Theory, pp. 1006-1021. Online here
  28. ↑ Heidi Scott, 2007: Stephen Jay Gould and the Rhetoric of Evolutionary Theory, Rhetoric Review, 26(2):120-141.
  29. ↑ a b c Niles Eldredge (Spring 2006). «VQR > Confessions of a Darwinist». The Virginia Quarterly Review. Consultado em 4 de setembro de 2007  Verifique data em: |data= (ajuda)
  30. ↑ Charles Darwin, 1869. On the origin of species by means of natural selection London: John Murray. 5th edition, p. 551.
  31. ↑ Charles Darwin, 1869. On the origin of species by means of natural selection London: John Murray. 5th edition, p. 120-121.
  32. ↑ Charles Darwin, 1869. On the origin of species by means of natural selection London: John Murray. 5th edition, p. 121-122.
  33. ↑ Michael Givel (2006). ""Punctuated Equilibrium in Limbo: The Tobacco Lobby and U.S. State Policy Making From 1990 to 2003"". Policy Studies Journal 43 (3): 405-418
  34. ↑ Connie J. G. Gersick (1991). ""Revolutionary Change Theories: A Multilevel Exploration of the Punctuated Equilibrium Paradigm"". The Academy of Management Review 16(1): 10-36
  35. ↑ Tushman, M. L.,&Romanelli, E. (1985). ""Organizational evolution:Ametamorphosis model of convergence and reorientation."" In B. M. Staw & L. L. Cummings (Eds.), Research in organizational behavior, 7, (pp. 171-222). Greenwich, CT: JAI Press.
  36. ↑ Connie J. G. Gersick (1988). ""Time and transition in work teams: Toward a new model of group development."" Academy of Management Journal, 31, 9-41.
  37. ↑ Arrow, H., Poole, M. S., Henry, K. B., Wheelan, S., & Moreland, R. (2004). ""Time, change, and development: The temporal perspective on groups."" Small Group Research, 35 (1), 73-105.

Further reading editar

  • Adler, J. and Carey, J. (1982) "Enigmas of Evolution", Newsweek, March 29, 1982.
  • Brett, C. E., L. C. Ivany, and K. M. Schopf (1996). «Coordinated stasis: An overview». Palaeogeography Palaeoclimatology Palaeoecology. 127 (1-4): 1–20. doi:10.1016/S0031-0182(96)00085-5 
  • Erwin, D. H. and R. L. Anstey (1995) New approaches to speciation in the fossil record. New York : Columbia University Press.
  • Fitch, W. J. and F. J. Ayala (1995) Tempo and mode in evolution : genetics and paleontology 50 years after Simpson. Washington, D.C.: National Academy Press.
  • Gersick, C. J. G. (1991): Revolutionary Change Theories: A Multilevel Exploration of the Punctuated Equilibrium Paradigm. The Academy of Management Review 16 (1), pp. 10-36.
  • Ghiselin, M.T. (1986) "We Are All Contraptions", The New York Times, December 14, 1986.
  • Givel, Michael (2006). «Punctuated Equilibrium in Limbo: The Tobacco Lobby and U.S. State Policy Making From 1990 to 2003». Policy Studies Journal. 43 (3): 405–418. doi:10.1111/j.1541-0072.2006.00182.x 
  • Gould, S. J. (1992) "Punctuated equilibrium in fact and theory." In Albert Somit and Steven Peterson The Dynamics of Evolution. New York: Cornell University Press. pp. 54-84.
  • Gould, S. J. and N. Eldredge (1993). «Punctuated equilibrium comes of age». Nature. 366 (6452): 223–227. doi:10.1038/366223a0 
  • Mayr, E. (1963) Animal Species and Evolution. Cambridge MA: Harvard University Press.
  • Rhodes, R. H. T. (1983). «Gradualism, punctuated equilibrium and the Origin of Species». Nature. 305 (5932): 269–272. doi:10.1038/305269a0 
  • Pharoah, M.C. (online). Looking to systems theory for a reductive explanation of phenomenal experience and evolutionary foundations for higher order thought Retrieved Jan, 15 2008 - An explanation as to how the evolution of consciousness has had a punctuating and periodic influence on physiological adaptations.

External links editar

Predefinição:Speciation

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