Tempo and Mode in Evolution Genetics and Paleontology 50 Years After Simpson (1995) / Chapter Skim
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Disparate Rates, Differing Fates: Tempo and Mode of Evolution Changed from the Precambrian to the Phanerozoic
Disparate Rates, Differing Fates: Tempo and Mode of Evolution Changed from the Precambrian to the Phanerozoic
Pages 41-62
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From page 41... ...
Thus, Simpson's views of the evolutionary process were based necessarily on Phanerozoic lift the familiar progression from seaweeds to flowering plants, from trilobites to humans—a history of relatively rapidly evolving, sexually reproducing plants and animals successful because of their specialized organ systems (flowers, leaves, teeth, limbs) used to partition and exploit particular environments.
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From page 42... ...
Biochemical-lntracellular Metabolic FIGURE 1 Comparison of the Phanerozoic and Precambrian histories of life. of having specialized organ systems for exploitation of specific ecologic niches, members of the most successful group of these early-evolving microorganisms- photoautotrophic cyanobacteria were ecologic generalists, able to withstand the rigors of a wide range of environments.
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From page 43... ...
a thorough comparison of the early history of life with that of later geologic time is not yet possible. Therefore, as a first approximation, the approach used here is to analyze the known fossil record of Precambrian cyanobacteria: well-studied, widespread, abundant, commonly distinctive, and evidently dominant members of the early prokaryotic biota.
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From page 44... ...
, early biotic history is as yet very incompletely documented. In comparison with the vastly better documented record of Phanerozoic organisms—and even in geologic units of the Proterozoic (2500-550 Ma in age)
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From page 45... ...
Disparate Rates, Differing Fates 145 TABLE 1 Precambrian gener~c namesakes, coined by var~ous authors to suggest similar~ty to modern cyanobacter~al genera (Mendelson and Schopf, 1992a) Precambrian genus; No.
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From page 46... ...
to the cyanobacteria can be quite uncertain, a problem that applies especially to minute morphologically simple forms (atypically small-diameter oscillatoriaceans and chroococcaceans, for example) , which in the fossil state are essentially indistinguishable from various noncyanobacterial prokaryotes (Schopf, 1992d)
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From page 47... ...
provide a powerful argument for the plausibility of cyanobacterial hypobradytely. Plausibility of the concept is similarly indicated by quantitative studies recently carried out on large assemblages of modern and Precambrian microbes.
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From page 48... ...
.. 1 loom FIGURE 2 Comparison of living and Precambrian cyanobacteria.
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From page 49... ...
About half of the ~2000 fossil occurrences included in this morphometric study were reported from cherty carbonate stromatolites, with the remainder from elastic shales and siltstones. Data regarding the specific environmental settings represented by these lithologies are available for very few of the >300 fossiliferous geologic units considered.
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From page 50... ...
Proterozoic tubular sheaths (64 taxa) I, :~ 40 60 0 20 80 100 120 Diameter, Am FIGURE 3 Comparison of patterns and ranges of size distribution of cylindrical sheath-like Precambrian fossils with those of the tubular sheaths of living oscillatoriacean cyanobacteria (Schopf, 1992d)
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From page 51... ...
, but also that both taxa form microtexturally similar stromatolitic structures in comparable intertidal to shallow marine environmental settings, that they undergo similar postmortem degradation sequences, and that they occur in microbial communities that are comparable in both species composition and biological diversity. In a subsequent detailed study, Knoll and Golubic compared the morphology, cell division patterns, ecology, and postmortem degradation sequences of a second Precambrian entophysalidacean (~850-Ma-old Eoentophysalis cumulus)
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From page 52... ...
Cyanobacteria are Hypobradytelic. Thus, numerous workers worldwide have noted and regarded as significant the detailed similarities in cellular morphology between Precambrian and extant cyanobacteria (Table 1; Figure 2~.
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From page 53... ...
Why have cyanobacteria evidently changed so little over their exceedingly long evolutionary history? Survival of the Ecologically Unspecialized To understand the underlying causes of cyanobacterial hypobradytely, it is instructive to review Simpson's thoughtful analysis in Tempo and Mode, for although he was unaware of the Precambrian prokaryotic fossil record, Simpson was much interested in slowly evolving (bradytelic)
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From page 55... ...
Thus, cyanobacteria exhibit notable ecologic flexibility, and even though no single oscillatoriacean or chroococcacean species is known to be capable of tolerating the total range of observed growth conditions (for example, thermophiles dominant in 70°C waters rarely grow below 50°C, and species adapted to highly alkaline lakes do not occur in acid hot springs) , both groups include impressive ecologic generalists, able to thrive in virtually all present-day widespread environments (Table 2~.
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From page 56... ...
, oxygen-producing cyanobacteria would have rapidly supplanted oxygen-sensitive anoxygenic photoautotrophs throughout much of the global photic zone. As a result of the ease of their global dispersal and their success in competing for photosynthetic space, cyanobacteria presumably expanded into a broad range of habitable niches during an early, evidently rapid phase of adaptive radiation (Giovannoni et al., 1988)
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From page 57... ...
It remains to be established whether it, like Simpson's Tempo and Mode, will stand the test of time. SUMMARY Over the past quarter century, detailed genus- and species-level similarities in cellular morphology between described taxa of Precambrian microfossils and extant cyanobacteria have been noted and regarded as biologically and taxonomically significant by numerous workers worldwide.
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From page 58... ...
(1975) The Geologic Setting and Paleobiology of a Late Precambrian Stomatolitic Microflora from South Australia, Ph.D.
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From page 59... ...
(1988) ~icro~ssils Tom oolites and pisolites of the Upper Proterozoic Eleonore Bay Group, central East Greenland, 1.
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From page 60... ...
(1968) Microflora of the Bitter Springs Formation, Late Precambrian, central Australia, I
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From page 61... ...
(1971) New microorganisms from the Bitter Springs Formation (Late Precambrian)
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