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The Evolution of a Theory OR Don’t Blame it All on Darwin
In our last episode... Buffon (18th century) developed the theory of the “internal mould.” Lamarck (18th-19th centuries) coined the term “evolution,” and proposed a progressive sort of evolution in which organisms strive to improve. Cuvier (18th-19th centuries) stated that organisms were perfectly adapted and could not survive change. Darwin (19th century) proposed the theory of Natural Selection as a mechanism of evolution.
After the voyage... Darwin returned to England on October 2, 1836. Cataloged some of his specimens, sent others out to other naturalists to catalog and describe. One of these naturalists, by the name of Gould, was the first to notice patterns of distribution in the Galapagos finches. Darwin begins a series of notebooks to collect facts from other authors and to record his thoughts about speciation.
Darwin’s questions Why the replacement of species geographically; that is, why are the species on islands not exactly the same as those on the nearby mainland? Why the replacement of species in time? Where do species come from in the first place? Change in species appears to be the answer — but how do species change?
The fourth notebook 1838: Reads Thomas Malthus’ Essay on the Principle of Population. Malthus notes that in nature, plants and animals produce more offspring than can survive. Humans, too, can overproduce if left unchecked. Poverty and famine were natural outcomes. This, Malthus believed, was a divine plan to keep humans from becoming lazy and irresponsible. Darwin extended the idea beyond what Malthus said, framing the concept in purely natural terms and outcomes. Large numbers offspring sets up a competitive environment. Process is open-ended, not progressive, not directional. New ideas about enormous time needed. Moves away from ideas of “harmonious” nature, looks to selection. Change acts on species, not individuals. Not all individuals affected equally.
In 1844, a shocking new book is published concerning the origins of the organic world......and it is not Darwin’s!
Robert Chambers 1810-1873 Anonymously published Vestiges of the Natural History of Creation in 1844, a naturalistic account of the world. Used data from other naturalists, yet drew different conclusions, which drew anger and criticism. Used Uniformitarianism to promote the idea of the organic world arising from the inorganic world and changing over time. Highly speculative, and drew much criticism.
Darwin takes notes... Chambers was criticized for not being a naturalist himself. Darwin knew he’d have to establish himself as a working naturalist and amass huge quantities of his own data to support his theory of natural selection. Set to work on barnacle taxonomy, which was a mess at the time. Spent eight years on barnacles, made many new discoveries. 1856: First draft of Origin.
Alfred Russel Wallace 1823- 1913 Self-educated, read Humboldt, Malthus, other naturalists and philosophers. Also read Voyage of the Beagle. Traveled to S.E. Asia to catalog new species. Pondered the questions of what species are, how they arise. Developed a concept of selection, wrote an essay and sent it to Darwin for comments.
Darwin’s response... Darwin feels obliged to publish Wallace’s essay. On the advice of friends, reads both the essay and his own proposal at a meeting of the Linnean Society in 1859. Attracts mostly yawns. Finally, in 1859, Darwin publishes On the Origin of Species as an abstract (condensed version) of his theory of Natural Selection.
Natural selection: the basics More organisms are born than can survive. Some individuals have traits that give them a survival advantage. These individuals are more likely to survive and have offspring, so pass their traits on to their offspring. The world is in a perpetual state of change, so the environment perpetually changes. This creates changing selection pressures over time.
Initial criticisms of Origin Age of the Earth: Lord Kelvin believed that the earth was too young (based on measurements of the sun’s heat, but Kelvin did not know about radiation). Heredity: How can traits be selected for if heredity involves blending? Variation: not all variants in parents are inherited in individuals. Further, variation can only stray so far from the mean.
Thomas Henry Huxley 1825 - 1895 Huxley understood Darwin’s arguments, promoted Natural Selection as a plausible mechanism for change in species. Became known as “Darwin’s Bulldog.” Where Darwin was shy, Huxley was bold. Huxley’s efforts popularized the Origin and opened it up to wide debate.
Between 1859 and 1863, there came slow acceptance of the theory in the scientific community. By 1875, evolution (as change in species over time) had acceptance in the broader population, at least among the educated, possibly because scientists frequently gave public lectures to educational societies. However, natural selection as the mechanism was widely debated. Theories of germplasm, acquired characteristics, and other mechanisms discussed. The aftermath...
Dutch scientist Hugo de Vries was one of three scientists who rediscovered Mendel’s work in 1900. de Vries published Mutation Theory, which was widely read. Argued that natural selection acts on mutations. Change is by sudden leaps, not by blending or gradual change. Early 20th century: forging links between principles of heredity and principles of natural selection. Rediscovery of Mendel
Modern Synthesis Theory Ernst Mayr Theodosius Dobzhansky George Simpson
Ernst Mayr Interested in isolation mechanisms as a means of speciation: isolation disrupts random mating, often begins with small populations where change happens rapidly. Developed the concept of adaptive radiation as the result of speciation. Wrote Systematics and the Origin of the Species in 1942.
Theodosius Dobzhansky Worked to integrate evolution with genetics into a unified theory. Rejected Lamarckianism that was still prevalent. Rejected “orthogenesis” (inner-directed, progressive evolution) Said that mutations were not enough to explain speciation. Selection must act on mutations. Wrote Genetics and the Origin of Species in 1937.
George Simpson Employed by the American Museum of Natural History. Rejected orthogenesis (progressive) and teleological (goal-directed) evolution. Replaced the “ladder of life” model with a many-branched “tree.” Tied population genetics to evolution.
Premises of the theory All inheritable characteristics are carried on genes. Acquired traits are not passed on because they are not in the genes. Other mechanisms besides selection are at work, including mutation, isolation, genetic drift. Speciation is the result of the accumulation of genetic change over time. Microevolution leads to macroevolution.
Punctuated Equilibria Stephen Jay Gould Niles Eldredge
Stephen Jay Gould Paleobiologist at the American Museum of Natural History. Known for his popular literature on science: The Panda’s Thumb, Ever Since Darwin, etc. Interested in why fossil species appear to remain unchanged for long periods of time, then change suddenly.
Niles Eldredge Curator in the Department of Invertebrates at the American Museum of Natural History and adjunct professor at CUNY. Like Gould, Eldredge noticed that most fossil sequences do not demonstrate constant, gradual change. Also dissatisfied with Richard Dawkins’ views that evolution is all about genes making more genes. Takes an ecological view of evolution as a process involving populations interacting with their environment.
The problems with gradualism... How do we describe species across time, if species are in constant, slow change? Gradualism explains replacement of species, but not diversity and adaptive radiation. Gradualism is inferred from the fossil record. Few actual examples exist.
Punctuated Equilibria “The norm for a species or, by extension, a community, is stability. Speciation is a rare and difficult event that punctuates a system in homeostatic equilibrium.” Large populations of species tend to undergo little change over long periods of time — stabilizing selection is a factor. Rapid change occurs when groups are isolated and out of equilibrium with their environment. Allopatric speciation (speciation following isolation) is proposed as a driving factor. Small, isolated populations experience more rapid changes to the gene pool because of genetic drift.
“Punk Eek,” con’t Stasis does not mean no change; rather, changes in large populations tend to be stabilizing responses to changes in the environment. Loss of species leaves open niches that other species can exploit. Historical major extinctions were often followed by rapid adaptive radiation. The theory has great explanatory power, as it explains about 90% of known fossil sequences.
The Selfish Gene Richard Dawkins: view of the gene as encoded, binary information, view of evolution from the genetic level rather than the organismal or population level. Accused of being “hyper-Darwinian.” “An organism is a gene’s way of making another gene.” Places organisms in a subordinate position to their genes. Organisms are “survival machines” for genes. An organism is a transitory thing; genes (or at least the information in them) endure.
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