Chapter 17: Evolution of Populations

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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 - PowerPoint PPT Presentation


  • Chapter 17:Evolution of PopulationsSection 17-2: Evolution as Genetic Change in Populations

  • How Natural Selection WorksEvolutionary fitness = success in passing on genes

    Evolutionary adaptation = any genetically controlled trait that increases an organisms ability to pass along its alleles

  • Natural Selection on Single-Gene TraitsChanges allele frequenciesEx: Body color in lizards

  • Natural Selection on Polygenic TraitsRange of phenotypesFitness varies throughout the curveNatural selection can affect phenotype range, changing shape of bell curve

  • Directional SelectionOccurs when individuals at one end of the curve have higher fitness than individuals in the middle or at the other end

  • Stabilizing SelectionOccurs when individuals near the center of the curve have higher fitness than individuals at either endKeeps center of curve, narrows overall graph

  • Disruptive SelectionOccurs when individuals at the upper and lower ends of the curve have higher fitness than individuals near the middleActs against the intermediate phenotype, can create two distinct phenotypes

  • Genetic DriftOccurs in small populationsAllele becomes more or less common by chanceRandom

  • Genetic BottlenecksThe bottleneck effect is a change in allele frequency following a dramatic reduction in population size

    Ex: Natural disaster

  • The Founder EffectOccurs 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

  • The Founder Effect

  • Evolution vs Genetic EquilibriumIf allele frequencies in a population remain the same it is in genetic equilibriumNo evolutionThe Hardy-Weinberg principle describes the conditions under which evolution will not occurStates that allele frequencies will remain constant unless something causes them the change

  • Hardy-Weinberg Principle5 conditions can disturb genetic equilibrium and cause evolution:Nonrandom mating sexual selectionSmall population sizeImmigration or emigrationMutationsNatural selection

  • Sexual Reproduction and Allele Frequency

    Sexual reproduction alone does not change relative allele frequency