OHHS AP Biology Chapter 23 (Class Presentation)

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  • 9% =.09 = ss = q2(f)s = q = Square root of .09 = .3p = 1 - .3 = .72pq = 2 (.7 x .3) = .42 = 42% of the population are heterozyotes (carriers)
  • (f)A = (2 * (1469) + (138))/(2 * (1469 + 138 + 5)) = .954 or 95.4%(f)a = 1 - .954 = .046 or 4.6%(f)AA = (.954) 2 = .910 or 91%(f)Aa = 2 (.954)(.046) = .087 or 8.7%(f)aa = (.046) 2 = .002 or .2%
  • OHHS AP Biology Chapter 23 (Class Presentation)

    1. 1. Chapter 23<br />The Evolution of Populations<br />1<br />
    2. 2. Microevolution<br />Natural selection acts on individuals, but only populations evolve.<br />Microevolution is a change in allele frequencies in a population over generations<br />2<br />
    3. 3. 3<br />
    4. 4. Concept 23.1<br />Mutation and sexual reproduction produce the genetic variation that makes evolution possible<br />4<br />
    5. 5. Two processes, mutation and sexual reproduction, produce the variation in gene pools that contributes to differences among individuals<br />5<br />
    6. 6. Genetic Variation<br />Variation in individual genotype leads to variation in individual phenotype<br />Not all phenotypic variation is heritable<br />Natural selection can only act on variation with a genetic component<br />6<br />
    7. 7. 7<br />
    8. 8. Two Types of Variation in Populations<br />Discrete characters can be classified on an either-or basis<br />Quantitative characters vary along a continuum within a population<br />8<br />
    9. 9. Heterozygosity<br />Population geneticists measure polymorphisms in a population by determining the amount of heterozygosity at the gene and molecular levels<br />Average heterozygositymeasures the average percent of loci that are heterozygous in a population<br />9<br />
    10. 10. Variation Between Populations<br />Most species exhibit geographic variation,differences between gene pools of separate populations or population subgroups<br />10<br />
    11. 11. 11<br />
    12. 12. Some geographic variation occurs as a cline, grade change in a trait along a geographic axis.<br />12<br />
    13. 13. Mutations<br />Mutations are changes in the nucleotide sequence of DNA<br />Mutations cause new genes and alleles to arise<br />Only mutations in cells that produce gametes can be passed to offspring<br />Point mutations – often silent.<br />13<br />
    14. 14. Sexual Reproduction<br />Sexual reproduction can shuffle existing alleles into new combinations<br />In organisms that reproduce sexually, recombination of alleles is more important than mutation in producing the genetic differences that make adaptation possible<br />14<br />
    15. 15. You should now be able to:<br />Explain why the majority of point mutations are harmless<br />Explain how sexual recombination generates genetic variability<br />15<br />
    16. 16. Concept 23.2<br />The Hardy-Weinberg equation can be used to test whether a population is evolving<br />16<br />
    17. 17. Key Terms<br /><ul><li>A population is a localized group of individuals capable of interbreeding and producing fertile offspring</li></ul>A gene pool consists of all the alleles for all loci in a population<br />A locus is fixed if all individuals in a population are homozygous for the same allele<br />17<br />
    18. 18. By convention, if there are 2 alleles at a locus, p and q are used to represent their frequencies<br />The frequency of all alleles in a population will add up to 1<br />For example, p + q = 1<br />18<br />
    19. 19. The Hardy-Weinberg Principle<br />The Hardy-Weinberg principle describes a population that is not evolving<br />If a population does not meet the criteria of the Hardy-Weinberg principle, it can be concluded that the population is evolving<br />19<br />
    20. 20. Hardy-Weinberg Equilibrium<br />The Hardy-Weinberg principle states that frequencies of alleles and genotypes in a population remain constant from generation to generation<br />In a given population where gametes contribute to the next generation randomly, allele frequencies will not change<br />20<br />
    21. 21. 21<br />
    22. 22. Hardy-Weinberg equilibrium describes the constant frequency of alleles in such a gene pool<br />If p and q represent the relative frequencies of the only two possible alleles in a population at a particular locus, then<br />p2 + 2pq + q2 = 1<br />where p2 and q2 represent the frequencies of the homozygous genotypes and 2pq represents the frequency of the heterozygous genotype<br />22<br />
    23. 23. 23<br />
    24. 24. Conditions for Hardy-Weinberg Equilibrium<br />The Hardy-Weinberg theorem describes a hypothetical population<br />In real populations, allele and genotype frequencies do change over time<br />24<br />
    25. 25. The five conditions for nonevolving populations are rarely met in nature:<br />No mutations <br />Random mating <br />No natural selection <br />Extremely large population size<br />No gene flow<br />25<br />
    26. 26. Example 1<br />If 9% of an African population is born with a severe form of sickle-cell anemia (ss), what percentage of the population will be more resistant to malaria because they are heterozygous(Ss) for the sickle-cell gene?<br />26<br />
    27. 27. Example 2<br />This is a classic data set on wing coloration in the scarlet tiger moth (Panaxiadominula). Coloration in this species had been previously shown to behave as a single-locus, two-allele system with incomplete dominance. Data for 1612 individuals are given below:<br />White-spotted (AA) =1469 Intermediate (Aa) = 138 Little spotting (aa) =5<br />Calculate the following frequencies: (f)A = (f)a = (f)AA = (f)Aa= (f)aa= <br />27<br />
    28. 28. You should now be able to:<br />Define the terms population, species, gene pool, relative fitness, and neutral variation<br />List the five conditions of Hardy-Weinberg equilibrium<br />Apply the Hardy-Weinberg equation to a population genetics problem<br />28<br />
    29. 29. Concept 23.3<br />Natural selection, genetic drift, and gene flow can alter allele frequencies in a population<br />29<br />
    30. 30. Three major factors alter allele frequencies and bring about most evolutionary change:<br />Natural selection<br />Genetic drift<br />Gene flow<br />30<br />
    31. 31. Natural Selection<br />31<br />Differential success<br />
    32. 32. Genetic Drift<br />Genetic drift describes how allele frequencies fluctuate unpredictably from one generation to the next<br />32<br />
    33. 33. 33<br />
    34. 34. The Founder Effect<br />The founder effect occurs when a few individuals become isolated from a larger population<br />34<br />
    35. 35. The Bottleneck Effect<br />The bottleneck effect is a sudden reduction in population size due to a change in the environment<br />35<br />
    36. 36. 36<br />
    37. 37. Effects of Genetic Drift: A Summary<br />Genetic drift is significant in small populations<br />Genetic drift causes allele frequencies to change at random<br />Genetic drift can lead to a loss of genetic variation within populations<br />Genetic drift can cause harmful alleles to become fixed<br />37<br />
    38. 38. Gene Flow<br />Gene flow consists of the movement of alleles among populations<br />Gene flow tends to reduce differences between populations over time<br />Gene flow is more likely than mutation to alter allele frequencies directly<br />Gene flow can decrease the fitness of a population<br />38<br />
    39. 39. You should now be able to:<br />Explain why natural selection is the only mechanism that consistently produces adaptive change<br />Explain the role of population size in genetic drift<br />39<br />
    40. 40. Concept 23.4<br />Natural selection is the only mechanism that consistently causes adaptive evolution<br />40<br />
    41. 41. Only natural selection consistently results in adaptive evolution<br />41<br />
    42. 42. Relative Fitness<br />Reproductive success is generally subtle and depends on many factors<br />Relative fitness is the contribution an individual makes to the gene pool of the next generation, relative to the contributions of other individuals<br />Selection favors certain genotypes by acting on the phenotypes of certain organisms<br />42<br />
    43. 43. Three modes of selection:<br />Directional selection<br />Disruptive selection<br />Stabilizing selection<br />43<br />
    44. 44. 44<br />
    45. 45. 45<br />
    46. 46. 46<br />
    47. 47. Sexual Selection<br />Sexual selection is natural selection for mating success<br />It can result in sexual dimorphism, marked differences between the sexes in secondary sexual characteristics<br />47<br />
    48. 48. 48<br />
    49. 49. Intrasexual selection is competition among individuals of one sex (often males) for mates of the opposite sex<br />Intersexual selection, often called mate choice,occurs when individuals of one sex (usually females) are choosy in selecting their mates <br />49<br />
    50. 50. Preserving Genetic Variation<br />Diploidy: hidden recessive alleles.<br />Balancing Selection: natural selection maintains stable frequencies of two or more phenotypic forms in a population.<br />Heterozygote advantage occurs when heterozygotes have a higher fitness than do both homozygotes<br />Sickle-cell allele.<br />Frequency-dependent selection: the fitness of a phenotype declines if it becomes too common in the population<br />50<br />
    51. 51. Why Natural Selection Cannot Create the Perfect Organism:<br />Selection can act only on existing variations<br />Evolution is limited by historical constraints<br />Adaptations are often compromises<br />Chance, natural selection, and the environment interact<br />51<br />
    52. 52. You should now be able to:<br />Distinguish among the following sets of terms: directional, disruptive, and stabilizing selection; intrasexual and intersexual selection<br />List four reasons why natural selection cannot produce perfect organisms<br />52<br />

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