Charlotte J. Avers, Process & Pattern in Evolution (New York: Oxford University Press, 1989)
All ellipses need to be filled in by researching the quotes on the internet, NOTE.
- Note that I could particularly use her arguments re: the patterns of speciation (pgs. 303-316) to constructively argue that my revised Peircean model of teleological evolution is compatible with either gradualistic or punctuated evolution.
“The Greeks and Romans had held to the precept that nature and time were cyclical and that change reflected the guidance of a cosmic intelligence. Orderliness was the expected outcome of supernatural causes imposed on the world, and was beyond the physical limits of nature” (3).
The early Christian view “of a single event giving rise to all of existence was at odds with the Greco-Roman tradition of a succession of worlds arising and waning in cyclic fashion but saved from doom and destruction toward the end of each cycle by the intervention of the gods” (4).
As Avers notes, Saint Augustine and Saint Aquinas, however, were “largely responsible for laying the theological groundwork for evolutionary ideas. The biblical Creation story was interpreted to mean that at the time of creation, God infused the Earth with the necessary potency to produce living things by a natural process that represented the unfolding of the divine plan.” (4).
Regarding the middle ages, Avers contends that the discoveries in that time enabled Christian-minded scientists to view the study of nature as a means to better understand the divine. In fact, speculation “concerning the origin of the Earth and its antiquity could not be divorced from theories growing out of astronomical studies… [however,] the new concepts of geologic change and terrestrial antiquity were not recognized as having biological implications until the nineteenth century” (5).
Avers notes that Aristotle proposed the existence of a graded sequence of organisms composed of an indefinite number of links in a chain of being that ranged from the lowest forms to the gods… and the epic heroes and demigods, maintaining this position in every cycle” (5).
“In the sixteenth century, Leonardo da Vinci and a handful of scholars considered fossils to be the remains of organisms from different times and places not associated with the Flood” (5-6). Unfortunately, the worldview of the time excluded the possibility of descent with modification, preferring instead that creatures were fashioned as immutable entities during the original creation. The credo, Avers notes, “remained staunchly in favor of the unbroken chain of being, the immutability of species, and the impossibility of extinction” (6).
However, by the early 19th century, two geologic concepts were embraced by most scholars, laity and clergy alike: uniformitarianism and catastrophism. James Hutton and his followers supported uniformitarianism, whereas Georges Cuvier and his followers supported catastrophism. Uniformitarianism postulated that, given adequate time, the processes of deposition, erosion, and uplifting could account for the past physical changes on Earth, just as they now account for contemporary changes on Earth. Catastrophism posited that cataclysmic events (including both fire and floods) molded the planet over time; the biblical Flood was the last of these to occur. Moreover, the writings of Georges Buffon, Erasmus Darwin, Jean-Baptiste Lamarck, and others gave rise to the idea that species change over time, particularly in response to environmental conditions; by mid-19th century, a theory of organic evolution had fully developed (7).
Avers then spends a page outlining Darwin’s voyage on the Beagle, and what it showed Darwin about world; she also outlines the influence that Malthus’s Essay on the Principle of Population had upon him, and his subsequent sitting on the idea for 20 years until Wallace and he read their papers together in London in 1858.
Three seminal books shaped the eventual Modern Synthesis: Dobzhansky’s Genetics and the Origin of Species (1937), Mayr’s Systematics and the Origin of Species (1942), and Simpson’s Tempo and Mode in Evolution (1944); this Modern Synthesis included the following basic posits: 1. Random processes of evolution give rise to mutations; 2. Evolution is characterized by changes in gene frequencies; 3. Adaptive genetic variations produce stepwise (gradual) changes in phenotype, which gradually accumulate; 4. Geographically isolated populations lead to speciation of isolated groups; and 5. Continued selective pressures result in new taxa above the specie level (16).
CD’s contribution to biology was to provide the mechanism to account for the evolutionary progressions from simpler to more complex and sophisticated systems; subsequently, a great body of evidence has been amasses to support CD’s general thrust of argumentation (211).
“Two different questions were presented and discussed by Darwin [in the Origin]: Has evolution occurred, and if so, by what mechanism have organism changed and diversified over time” (213)?
Avers defines natural selection as “the differential reproduction of genetically diverse organisms” (213); she contends that this definition would fit CD’s emphasis on selection at the level of the individual (213). She notes that whereas there is no definitive consensus on the level of selection today, most biologists accept the organismal level (213).
Avers contends that the improbability of direction toward more complexity, given the nature of mutations, is rendered highly probably by the differential reproduction of genetically diverse organisms; she also claims that CD himself thought so, even though CD wrote that nature selects the more fit over the less fit, taking CD to have been using a literary device for comparison, and not speaking literally (214). She notes that fitness is an outcome of inheritance, which is expressed as differential reproduction, and those that are better fit will propagate and predominate (214). But Bradford disagrees with Avers here, note, and thinks that CD was being forthright when he wrote of nature as the active tool of selection.
CD, when referring to the “struggle for existence,” was referring to the competition between individuals for limited resources, upon which their continued existence depends.
Avers argues that three strategies of selective mechanisms – diversifying selection, frequency-dependent selection, and balancing selection – increase the diversity of genotypes (and subsequently phenotypes) in heterogeneous environments and populations (224). Diversifying selection is disruptive, and generally favors homozygotic individuals. Frequency-dependent selection selects for a particular phenotype when it is rare, but against it when it is common, keeping it in a somewhat steady-state within the population. Balancing selection, or heterozygote advantage selection, is a strategy by which polymorphisms are maintained within populations.
Within their punctuated Equilibria hypothesis, Eldredge and Gould emphasize the relationship between the tempo and mode of evolution in speciation, the ideas of which were first distilled in Simpson’s Tempo and Mode in Evolution (1944); rather than evolving by gradualistic means, speciation occurs by means of the splitting of the ancestral population into two or more in short bursts.
Books and articles to get by Niles Eldredge:
- Niles Eldredge, “Evolutionary Tempos and Modes: A Paleontological Perspective.” In What Darwin Began, ed. L.R., Godfrey, p. 113. Boston: Allyn & Bacon, 1985.
- Niles Eldredge, Unfinished Synthesis: Biological Hierarchies and Modern Evolutionary Thought. New York: OUP, 1985.
- Niles Eldredge and Stephen J. Gould, “Punctuated Equilibria: An alternative to Gradualism.” In Models in Paleobiology, ed. T.J.M. Schopf, p. 82. San Francisco: Freeman Cooper.
In order to distinguish resemblances in constructing possible genetic relationships, biologists classify them in the following manner: as either homologous or analogous divergences, and as either being divergent or convergent in their lineage. Homologous are genetically related structures and habits, whereas analogous refers merely to similar structures and habits. Divergent evolution gives rise to homologous structures and habits, whereas convergent evolution gives rise to analogous structures and habits.