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Elementary Science Education

Observations and Reasoning

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      Bernard Nebel

      To not believe in evolution is a valid preference from a variety of points of view. However, to refuse to look at evidence and exercise rational reasoning is the antithesis of what science is about. Therefore, keep the emphasis of this lesson on examining the evidence and checking the rationality of reasoning.

      Observations and Reasoning

      The theory of evolution is based on things we can all easily observe and reasoning we can all follow.

      1. The concept that living world consists of distinct, (created) definable species does not correspond to reality (see text). The following video describes one case of the sort of confusion that exists in defining a species:
      2. Offspring of every plant/animal exhibit variation. Type into your browser: variation among littermates images                 Plant and animal breeders utilize such variations to selectively breed, over many generations, new varieties/breeds of the plants/animals. The following video describes selective breeding:     Type into your browser: breeds of _____ images (put any domestic animal/plant in the blank.
      3. Over reproduction. Witness that every organism is capable of producing far more seeds/eggs/offspring than are required for simple replacement. (It is instructive to have students calculate and graph population growth assuming all the offspring grow up and reproduce in turn.) It readily becomes self-evident that a large portion of offspring in nature do not survive to reproduce.
      4. Review adaptations required for survival (see text).
      5. Reasoning: Given variation among offspring and inevitable competition for survival, it follows that organisms with variations that enhance “winning” in any of the above respects will have a greater likelihood of being a survivor and reproducing. Thus, nature unavoidably exercises a selective breeding process leading organisms to become increasing well adapted to their particular niche. Emphasize the “to their particular niche.”
      6. Speciation. If/when a population spreads it faces different conditions in different parts of its territory. Selective pressures will differ on separate portions of the population. For example, consider an ancestral fox population spread from north to south.  Have students discuss and reason how the differing climate factors north to south create differing selective pressures. Namely, northern climate will favor variations that better adapt animals to cold and snow. Over many generations, this will lead to a another breed then species. (Type into your browser: comparison of arctic and grey fox images) Note white color, heavier fir, shorter legs, ears, and nose are adaptations to cold and snow.
      7. Homologous structures. When we examine the skeletons of various animals we find they are constructed, not as unique creations, but with the same bones bones albeit that the bones are modified to enable a different function. See:
      8. Comparative embryology. When the early stages of embryology (the development of the fertilized egg to an embryo) are studied, a remarkable similarity is found among a host different animals. Type into your browser: comparative embryology images                                                         This indicates that these animals shared a common origin.
      9. Any plant community provides a niche and opportunity for herbivores. We think of wild herbivores as being deer, elk, and such, but if such animals are not present but turtles are present, what might occur over a few millions of years of natural selection. Type into your browser: tortoises of the galapagos islands images
      10. Perfection not so perfect. Looking at a flounder (Type into your browser: flounder images) one sees a vertically oriented fish that took to swimming on its side to hug the bottom. Then, its anatomy has started but not yet completed its adaptation for the new way of life.
      11. Have students discuss how variation and potential speciation being underway can explain difficulties we experience in assigning  given plant/animal as belonging to one or another species.
      12. Most of all, the theory of evolution brings the geological history of the earth and fossil record together into a logical, self consistant context (see Lesson D-19).
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