Bio171-F10-Lec 21 - Biology 171 Monday November 1 2010...

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Biology 171 Monday, November 1, 2010 Lecture 21: Evolutionary Processes I Announcements Text Reading Lecture 21 : 4 th : Chapter 25 (440-443) 3 rd : Chapter 25 (520-523) Lecture 22: 4 th : Chapter 25 (452-455) 3 rd : Chapter 25 (508-511) The Logic of Darwin & Wallace Modern Synthesis Different Types of Selection Heterozygote Advantage Directional/Disruptive/Stabilizing This Week in Discussion: Population Genetics/ Evolution Simulations 1
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Historical Perspectives on Evolutionary Thought Charles Darwin & Alfred Russel Wallace Darwin at age 50 Wallace at age 24 2
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Like many upper class Victorian Englishmen, Darwin was a breeder of “fancy” pigeons and knew he could “select” for certain traits in breeding experiments. 3
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Darwin knew that Artificial Selection could produce extensive variation quickly in domesticated Plants & Animals. These vegetables have all been selected from one species of wild mustard. What process governed descent with modification in natural populations? Darwin and Wallace developed a new explanatory model that Darwin referred to as Natural Selection . 4
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Thomas Malthus - 1798 “Essay on the Principle of Population” Food supply Malthus had observed that in nature plants and animals produce far more offspring than can survive. This prompted Darwin to focus on differential survival as a key mechanism in evolution. The “struggle for existence” 5
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Overproduction of offspring . Just one branch of a maple tree bears dozens of winged seeds. If all the tree s offspring survived, we would quickly be overwhelmed by maple forests. 6
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Summary of the logic of natural selection. Observation 1: Individual variation . Individuals in a population of any species vary in many heritable traits. Observation 2: Overproduction and competition . A population of any species has the potential to produce far more offspring than will survive to produce offspring of their own. With more individuals than the environment can support, competition is inevitable. Inference 1: Unequal reproductive success . From the observable facts of heritable variation and overproduction of offspring, Darwin inferred that individuals are unequal in their likelihood of surviving and reproducing. Those individuals with heritable traits best suited to the local environment will generally produce a disproportionately large number of healthy, fertile offspring. Inference 2: Evolutionary adaptation . This unequal reproductive success can adapt a population to its environment. Over the generations, heritable traits that enhance survival and reproductive success tend to increase in frequency among a population s individuals.
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