These were then gradually "polished up" into stripes.
Sheldon, Peter R. 2001. Punctuated equilibrium and phyletic gradualism. eLS. DOI:
Evolution - gradualism vs. punctuated equilibrium
Scientists think that species with a shorter evolution evolved mostly by punctuated equilibrium, and those with a longer evolution evolved mostly by gradualism.
N2 - The hypothesis of punctuated equilibrium proposes that most phenotypic evolution occurs in rapid bursts associated with speciation events. Several methods have been developed that can infer punctuated equilibrium from molecular phylogenies in the absence of paleontological data. These methods essentially test whether the variance in phenotypes among extant species is better explained by evolutionary time since common ancestry or by the number of estimated speciation events separating taxa. However, apparent "punctuational" trait change can be recovered on molecular phylogenies if the rate of phenotypic evolution is correlated with the rate of speciation. Strong support for punctuational models can arise even if the underlying mode of trait evolution is strictly gradual, so long as rates of speciation and trait evolution covary across the branches of phylogenetic trees, and provided that lineages vary in their rate of speciation. Species selection for accelerated rates of ecological or phenotypic divergence can potentially lead to the perception that most trait divergence occurs in association with speciation events.
• Explain the components of the punctuated equilibrium hypothesis.
Whether the fossil record truly displays a predominant pattern of stasis continues to be an active area of paleontological research. Punctuated equilibrium is most readily tested when geological strata are well resolved temporally with abundant fossil preservation. Empirical studies have resulted in mixed appraisals, with a roughly equal spread among those supporting phyletic gradualism, punctuated equilibrium, or a third "hybrid" process that may be described as punctuated gradualism (Erwin and Anstey 1995). Probably the most tangible contribution of the punctuated equilibrium controversy has been the widespread acceptance of Gould and Eldredge’s claim that "stasis is data." Lack of morphological change is an evolutionary pattern that warrants an explanation.
Furthermore, in debates over punctuated equilibrium, the term gradualism has been used in at least two different senses. Eldredge and Gould’s phyletic gradualism concerns the tempo of evolution, entailing evolutionary trajectories that are geologically slow, constant, and unidi-rectional—a concept pointedly contrasted with the rapid speciation described by punctuated equilibrium. For Darwin, however, gradual has little to do with rate ( 1993, pp. 312-317). Rather, it means that evolution by natural selection advances in small "grades" that are dependent on a population’s normal genetic variation. Morphological change therefore can be genetically gradual and geologically rapid. While phyletic gradualism may have been a widespread evolutionary assumption in the twentieth century, it cannot be pinned on Darwin himself. Thus, given the significant overlap between the tenets of punctuationism and Darwin’s views, punctuated equilibrium is resolutely "Darwinian."
A Medley of Potpourri: Punctuated equilibrium
The hypothesis of punctuated equilibrium proposes that most phenotypic evolution occurs in rapid bursts associated with speciation events. Several methods have been developed that can infer punctuated equilibrium from molecular phylogenies in the absence of paleontological data. These methods essentially test whether the variance in phenotypes among extant species is better explained by evolutionary time since common ancestry or by the number of estimated speciation events separating taxa. However, apparent "punctuational" trait change can be recovered on molecular phylogenies if the rate of phenotypic evolution is correlated with the rate of speciation. Strong support for punctuational models can arise even if the underlying mode of trait evolution is strictly gradual, so long as rates of speciation and trait evolution covary across the branches of phylogenetic trees, and provided that lineages vary in their rate of speciation. Species selection for accelerated rates of ecological or phenotypic divergence can potentially lead to the perception that most trait divergence occurs in association with speciation events.
The idea of gradualism is one of the key facets of macroevolution, and mostly concerns lineages of interrelated populations of organisms at or near the species level. In modern biology, gradualism, or “phyletic gradualism,” refers primarily to a pattern of sustained, directional, and incremental evolutionary change over a long period during the history of a species. With the introduction of the theory of punctuated equilibrium, gradualism has been contrasted with stasis (species’ relative morphological invariance over significant time intervals), and less frequently with lineage patterns that cannot be distinguished from random walks. The idea of gradualism is hardly new. Notions of continuity and transitional intermediate forms between related species extend back well before biology became a recognized science, and these notions have influenced thinking about evolution from prior to Darwin until today. How lineage gradualism is recognized as an evolutionary pattern depends in part upon how species are discriminated and which species concepts are used, which properties are measured, and which methods of analysis are used. The dynamics and causes of macroevolution are crucial yet still unsettled problems in evolutionary biology, either invoking or questioning the extrapolation of microevolutionary mechanisms to explain protracted temporal patterns such as gradualism.
Gradualism and Punctuated Equilibrium are seen almost ..
Gradualism or punctuated equilibrium? | Yahoo Answers
Gradualism - Wikipedia
20/02/2009 · Gradualism or punctuated equilibrium
Evolution of speciation ideas, from gradualism to punctuated equilibrium. Source: Wikimedia Commons; author: wooptoo.
CONCEPTS OF PHYLETIC GRADUALISM AND PUNCTUATED EQUILIBRIUM.
23/06/2017 · Punctuated equilibrium and phyletic gradualism
and I developed the theory of punctuated equilibrium
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Punctuated Equilibrium Theory
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How useful and realistic are these theories?
The Punctuated equilibrium hypothesis states 3 …
The assortment of mechanisms that Gould and Eldredge have proposed to explain stasis and rapid specia-tion is the most contentious aspect of the punctuated equilibrium debate (Coyne and Charlesworth 1997). Eldredge and Gould initially implied that punctuated equilibrium is explained adequately by Mayr’s mainstream theory of allopatric speciation. However, they successively suggested numerous additional "non-Darwinian" mechanisms, including saltational mutations and species selection (Gould 1980, 2002), none of which have been broadly accepted. Emphasizing an antireductionist pluralism, Eldredge and Gould further claimed that each of these speculative mechanisms was significant for its potential to decouple lower-level genetic processes from upper-level macroevolutionary trends. Perhaps ironically, the punctuationist paradigm has been adopted more recently by molecular biologists, who have found within in vitro evolution experiments analogous patterns of stasis interleaved with rapid genetic change (Elena et al. 1996). Thus, punctuated equilibrium may ultimately find its raison d’etre in the very reductionist realm so vigorously opposed by Gould: molecular evolution via the "selfish gene."
Gradualism - Evolutionary Biology - Oxford Bibliographies
In other words, the phenomena associated with “punctuated equilibria” are regionally ecosystem-wide, and involve many different, unrelated species — species whose patterns of evolution, persistence, and extinction occur in near simultaneous fashion. This, perhaps the dominant signal in the evolutionary history of life, is thus profoundly “cross-genealogical” — meaning that such turnover events have causal roots that are deeply ecological — and arise, at base, from large-scale changes in the physical environment. Here, in other words, we finally understand how the physical environment, via ecological systems, impinges on the processes of speciation and extinction.
Punctuated Equilibrium; Gradualism as Null Hypothesis ..
Notes that species-level patterns of gradualism, stasis, and random trends with fluctuating reversals have been demonstrated in the fossil record. Describes the “plus ça change” model, which predicts that environments where gradualism is more likely to prevail are less well represented in the fossil record and thus documented less frequently.
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