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Chapter 17: Evolution of Populations

Chapter 17: Evolution of Populations. Section 17-2: Evolution as Genetic Change in Populations. How Natural Selection Works. Evolutionary fitness = success in passing on genes

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Chapter 17: Evolution of Populations

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  1. Chapter 17:Evolution of Populations Section 17-2: Evolution as Genetic Change in Populations

  2. How Natural Selection Works • Evolutionary fitness = success in passing on genes • Evolutionary adaptation = any genetically controlled trait that increases an organism’s ability to pass along its alleles

  3. Natural Selection on Single-Gene Traits • Changes allele frequencies • Ex: Body color in lizards

  4. Natural Selection on Polygenic Traits • Range of phenotypes • Fitness varies throughout the curve • Natural selection can affect phenotype range, changing shape of bell curve

  5. Directional Selection • Occurs when individuals at one end of the curve have higher fitness than individuals in the middle or at the other end

  6. Stabilizing Selection • Occurs when individuals near the center of the curve have higher fitness than individuals at either end • Keeps center of curve, narrows overall graph

  7. Disruptive Selection • Occurs when individuals at the upper and lower ends of the curve have higher fitness than individuals near the middle • Acts against the intermediate phenotype, can create two distinct phenotypes

  8. Genetic Drift • Occurs in small populations • Allele becomes more or less common by chance • Random

  9. Genetic Bottlenecks • The bottleneck effect is a change in allele frequency following a dramatic reduction in population size • Ex: Natural disaster

  10. The Founder Effect • Occurs when allele frequencies change as a result of the migration of a small subgroup of a population • Can create new populations very different from original group

  11. The Founder Effect

  12. Evolution vs Genetic Equilibrium • If allele frequencies in a population remain the same it is in genetic equilibrium • No evolution • The Hardy-Weinberg principle describes the conditions under which evolution will not occur • States that allele frequencies will remain constant unless something causes them the change

  13. Hardy-Weinberg Principle • 5 conditions can disturb genetic equilibrium and cause evolution: • Nonrandom mating – sexual selection • Small population size • Immigration or emigration • Mutations • Natural selection

  14. Sexual Reproduction and Allele Frequency • Sexual reproduction alone does not change relative allele frequency

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