Animals World

Some interesting facts about Animals:

  • The Struggle to Conceptualize Evolution The theory that new biological species could arise from changes in existing species was not readily accepted at first. Linnaeus and other classical biologists emphasized the immutability of species under the Platonic-Aristotelian concept of essentialism. Those who believed in the concept of evolution realized that no such idea could gain acceptance until a suitable mechanism of evolution could be found. Many possible mechanisms were therefore proposed. Etienne Geoffroy Saint-Hilaire proposed that the environment directly induced physiological changes, which he thought would be inherited, a theorynowknown as Geoffroyism. Lamarck proposed that there was an overall linear ascent of the scale of being but that organisms could also adapt to local environments by voluntary exercise, which would strengthen the organs used; unused organs would deteriorate. He thought that the characteristics acquired by use and disuse would be passed on to later generations, but the inheritance of acquired characteristics was later disproved. Central to both these explanations was the concept of adaptation, or the possession by organisms of characteristics that suit them to their environments or to their ways of life. In eighteenth century England, the Reverend William Paley and his numerous scientific supporters believed that such adaptations could be explained only by the action of an omnipotent, benevolent God. In criticizing Lamarck, the supporters of Paley pointed out that birds migrated toward warmer climates before winter set in and that the heart of the human fetus had features that anticipated the changes of function that take place at birth. No amount of use and disuse could explain these cases of anticipation, they claimed; only an omniscient God who could foretell future events could have designed things with their future utility in mind. The nineteenth century witnessed a number of books asserting that living species had evolved from earlier ones. Before 1859, these works were often more geological than biological in content. Most successful among them was the anonymously published Vestiges of the Natural History of Creation (1844), written by Robert Chambers. Books of this genre sold well but contained many flaws. They proposed no mechanism to account for evolutionary change. They supported the outmoded concept of a scale of being, often as a single sequence of evolutionary "progress." In geology, they supported the outmoded theory of catastrophism, an idea that the history of the earth had been characterized by great cataclysmic upheavals. From 1830 on, however, that theory was being replaced by the modern theory of uniformitarianism, championed by Charles Lyell. Charles Darwin read these books and knew their faults, especially their lack of a mechanism that was compatible with Lyell's geology. In hisownwork, Darwin carefully tried to avoid the shortcomings of these books.

  • Mutations Used to Determine an Anabolic Pathway Between 1937 and 1941, George W. Beadle and Edward L. Tatum isolated fungal mutants that were unable to synthesize various vitamins and amino acids. They isolated these mutants to try to understand howgenes control specific reactions in various metabolic (synthetic) pathways. The mutants would not grow on a simple medium unless supplied with the nutrient they were unable to synthesize. Beadle and Tatum induced large numbers of mutations in the fungus they were studying and isolated hundreds of mutants. Seven mutants were isolated, each carrying a defective form of a gene involved in the synthesis of the amino acid arginine. Using these mutants, Beadle and Tatum were able to order the chemical reactions by feeding the fungal mutants different metabolic intermediates. If the fungal mutants grew when a particular metabolic intermediate was provided, then the gene mutation affected a step leading to the synthesis of the intermediate. If the mutant did not grow, however, then the gene mutation affected a step that converted the intermediate into arginine. They concluded from their experiments that each gene controlled a different chemical reaction. They confirmed that a specific chemical reaction fails to take place in diploid organisms if both representatives of a given gene were defective. Beadle and Tatum's research strengthened the idea that genes specify enzymes. Their idea became known as the "one gene-one enzyme" hypothesis.

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