lecture 2-1 speciation - Chapters 23(continued...

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Chapters 23 (continued) & 24 (pp.462-483) Learning objectives • Distinguish among directional, disruptive, and stabilizing selection. Give an example of each mode of selection [Fig.23-12,a-c]. • Explain how diploidy can protect a rare recessive allele from elimination by natural selection [p.466]. Describe how heterozygote advantage and frequency-dependent selection promote balanced polymorphism [p.466]. Define neutral variations. Explain why natural selection does not act on these alleles [p.468]. • Distinguish between intrasexual selection and intersexual selection [p.468]. • List four reasons why natural selection cannot produce perfect organisms [p.469]. Chapter 24: • Distinguish between anagenesis and cladogenesis [p.472]. Distinguish between prezygotic and postzygotic isolating mechanisms [Fig.24.4]. Describe the different definitons of “species” and how each is used [p.476]. • Distinguish between allopatric and sympatric speciation [pp.477-480]. • Terms to know include… adaptive radiation, relative fitness, neutral variations, anagenesis, cladogenesis, autopolyploid, allopolyploid..
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Evolutionary Fitness The phrases “struggle for existence” and “survival of the fittest” Are commonly used to describe natural selection Can be misleading Reproductive success Is generally more subtle and depends on many factors
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Fitness Is the contribution an individual makes to the gene pool of the next generation, relative to the contributions of other individuals Relative fitness Is the contribution of a genotype to the next generation as compared to the contributions of alternative genotypes for the same locus
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Directional, Disruptive, and Stabilizing Selection Selection Favors certain genotypes by acting on the phenotypes of certain organisms Three modes of selection are Directional Disruptive Stabilizing
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The three modes of selection Fig 23.12 A–C (a) Directional selection shifts the overall makeup of the population by favoring variants at one extreme of the distribution. In this case, darker mice are favored because they live among dark rocks and a darker fur color conceals them from predators. (b) Disruptive selection favors variants at both ends of the distribution. These mice have colonized a patchy habitat made up of light and dark rocks, with the result that mice of an intermediate color are at a disadvantage. (c) Stabilizing selection removes extreme variants from the population and preserves intermediate types. If the environment consists of rocks of an intermediate color, both light and dark mice will be selected against. Phenotypes (fur color) Original population Original population Evolved population Frequency of individuals Question: Average human birth weight an example of which mode?
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The Preservation of Genetic Variation Genetic variation in a population is preserved by… - Diploidy
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