Origin Study Guide 5

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Organismic and Evolutionary Biology
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Organismic and Evolutionary Biology OEB 53
Andrew Berry

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Julie Vu TF: Pan Pan Jiang THE O RIGIN OF THE SPECIES :CHAPTER 5(L AWS OF VARIATION ) The effects of “use” and “disuse”seem topuzzleDarwin.What arethe observationsthathe istrying to explainandwhat arethe competing hypotheses? Darwin is unsure whether change is a result of disuse leading to atrophy or rather, change results from natural selection reducing the structure. He notes that in cases of artificial selection, it is easy to conclude that use in domestic animals enhances certain parts, while disuse diminishes them, but it is more difficult to observe natural organisms. Darwin, in examining different examples, comes to differing conclusions as to what causes change. For example, when talking about blind animals in caves, he attributes loss of eyes to disuse, because he thinks that the useless eyes could not negatively impact the organisms and thus, not be acted upon by selection. Other modifications, such as the reduction of wing size in some birds, are said to have been acted upon by selection, such as progenitors of the ostrich using its leg more and wings less, resulting in the modern day form. What aretheimplicatio
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