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Lecture+2.Natural+Selection - Topic 2 Natural Selection I...

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Topic 2. Natural Selection I. Definitions and terminology . The answers to a lot of questions like the last one (#4) of Topic 1 -- questions about traits (=characteristics) of individuals -- is thought to lie in the mechanism of natural selection , or selection at the individual level = individual selection . This mechanism is one that can produce adaptations. Def. adaptation : trait that fits or suits the individual organism to the environment. Example: camouflaged morphology. We all know about this from elementary courses, but want to get you to think about it again. II. Three conditions for Natural Selection according to Darwin (Lewontin summary). Handout 1 . 1. Different phenotypes – variability in traits. Some moths of given species light, some dark, for example. 2. Different phenotypic variants have different survival and reproduction so total lifetime reproduction varies. Individual fitness varies, def: number of offspring produced over its lifetime that themselves reach reproductive age. Relative fitness is the important quantity in natural selection, that is, how your individual fitness compares to other members of your population. 3. The traits that affect fitness are passed on (heritable). There are hundreds if not thousands of examples of natural selection. We will discuss 7 of them:
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III . Examples of Natural Selection . 1. Beak size in Darwin’s finches of Galápagos Islands . Daphne Island studied by Peter Grant (book in 2008). In dry years birds eat more large seeds ( Slide ) but hard to crack, and bigger beaks better. Greater beak depth implies harder, larger seeds can be crushed. During drought, most birds die (in 1977, 85%). Those surviving have mostly big beaks (a few with very small ones as well good at eating the remaining small seeds). Beak depth is heritable, so parents pass on trait to offspring. Slide .
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Lecture+2.Natural+Selection - Topic 2 Natural Selection I...

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