Chapter 23


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Chapter 23

  1. 1. II. The Evolution of Populations (Chapter 23)
  2. 2. What are the functional units of matter? <ul><li>Atoms </li></ul>
  3. 3. What are the functional units of living things? <ul><li>cells </li></ul>
  4. 4. What are the functional units of schools? <ul><li>Classes </li></ul>
  5. 5. What are the functional units of evolution? <ul><li>Populations </li></ul>
  6. 6. Important Misconception! <ul><li>Evolution does not occur in individual organisms—even though natural selection acts on individuals </li></ul>
  7. 7. A. Population genetics <ul><li>1. Microevolution is evolutionary change on the smallest scale </li></ul>
  8. 8. <ul><li>2. Microevolution refers to the change in the genetic make-up of a population from generation to generation </li></ul>
  9. 9. B. The modern evolutionary synthesis integrated Darwinian selection and Mendelian inheritance <ul><li>1. Biologists rapidly accepted Darwin’s theory of evolution, but rejected natural selection as its mechanism because there was no explanation for chance variation and their transmission to offspring </li></ul>
  10. 10. <ul><li>2. Mendel’s work was rediscovered in early 1900’s </li></ul><ul><li>Darwin’s ideas required quantitative characteristics that vary along a continuum </li></ul>
  11. 12. <ul><li>Mendel’s ideas were that the factors were “discrete” and “either or” such as purple or white flowers </li></ul>
  12. 14. <ul><li>3. Many geneticists did not believe the continuum of traits Darwin focused on could be explained by Mendel’s theory which seemed to be “either—or” by nature. </li></ul>
  13. 15. 5. We know now that Quantitative traits such as: <ul><li>a. Height </li></ul><ul><li>b. Fur length </li></ul><ul><li>c. Weight </li></ul><ul><li>Are controlled by several genes and are </li></ul><ul><li>called: </li></ul><ul><li>Polygenic traits </li></ul>
  14. 16. <ul><li>5. Discoveries of the 1930’s led to the development of Population genetics which reconciled Mendelism with Darwinism. </li></ul>
  15. 17. <ul><li>6. Population genetics emphasized quantitative inheritance and genetic variation within a population . </li></ul>
  16. 18. <ul><li>7. 1940 a comprehensive theory known as the modern synthesis was developed that emphasized: </li></ul>
  17. 19. <ul><li>the importance of population s as the units of evolution, </li></ul><ul><li>the essential role of natural selection </li></ul><ul><li>the gradualness of evolution </li></ul>
  18. 20. C. The genetic structure of a population is defined by its allele and genotype frequencies <ul><li>1. A localized group of the same species is called a population . </li></ul>
  19. 21. <ul><li>2. A species is a group of organisms whose individuals have the ability to interbreed in nature. </li></ul><ul><li>3. Populations may be totally isolated or contiguous. </li></ul><ul><li>4. All the genes present in a population at any given time are called the gene pool. </li></ul>
  20. 24. Example 1 In a population of 200 mice, 98 are homozygous dominant for brown coat color (BB), 84 are heterozygous (Bb), and 18 are homozygous recessive (bb).
  21. 25. <ul><li>98 BB individuals = 196 B alleles </li></ul><ul><li>84 Bb individuals = 84 B alleles </li></ul><ul><li>and 84 b alleles </li></ul><ul><li>18 bb individuals = 36 bb alleles </li></ul><ul><li>200 individuals represents a gene pool of 400 alleles </li></ul><ul><li>280/400 alleles are B—70% </li></ul><ul><li>120/400 alleles are b—30% </li></ul>
  22. 26. <ul><li>0.7 x 0.7 = 0.49 (BB) </li></ul><ul><li>2(0.7 X 0.3) = 0.42 (Bb) </li></ul><ul><li>0.3 X 0.3 = 0.09 (bb) </li></ul>
  23. 27. D. The Hardy—Weinberg theorem describes a non-evolving Population <ul><li>1. In the absence of selection pressure and other agents of change, the genetic structure of a population will remain constant from one generation to the next in spite of the shuffling of alleles by meiosis and random mating . </li></ul>
  24. 28. <ul><li>2. When allele frequencies of both alleles and genotypes remain stable in a population that population is said to be in Hardy—Weinberg equilibrium . </li></ul><ul><li>3. In a simple case of only two alleles the letters p and q represent the proportion of the two alleles within the population. </li></ul>
  25. 29. <ul><li>4. The combined frequencies of the alleles equals one. (p + q = 1) </li></ul>
  26. 30. <ul><li>The frequency of homozygous dominant individuals (pp) equals p 2 </li></ul><ul><li>The frequency of homozygous recessive individuals (qq) equals q 2 </li></ul><ul><li>The frequency of heterozygous individuals equals pq individuals plus qp individuals, or 2pq. </li></ul>
  27. 31. <ul><li>Everyone must be homozygous dominant, recessive, or heterozygous, therefore the sum of their frequencies equals 1. </li></ul><ul><li>p 2 + 2pq + q 2 = 1 </li></ul>
  28. 32. 5. Summary: <ul><li>( p + q) x ( p + q) = p 2 +2pq + q 2 </li></ul><ul><li>Allele Allele </li></ul><ul><li>frequency frequency </li></ul><ul><li>For males for females </li></ul>
  29. 33. <ul><li>6. Allele frequencies can be determined from genotype frequencies </li></ul><ul><li>7. Always start analysis of allele frequency with the recessive phenotype because the frequency of q is the square root of recessive individuals </li></ul>
  30. 34. Example 3 <ul><li>If the number of individuals with sickle cell anemia is 4%, then q 2 =0.04 </li></ul><ul><li>the frequency of q is 0.2 (the square root of 0.04 is 0.2) </li></ul><ul><li>The frequency of p is 0.8 </li></ul><ul><li>You cannot determine the frequency of p from the homozygous dominant individuals because: </li></ul><ul><li>p 2 and 2pq have the same phenotype. </li></ul>
  31. 35. Example 4 <ul><li>Review example 1, which illustrates how Mendelian inheritance preserves genetic variation from one generation to the next. In example 1, the gene frequency for p = 0.5 and q = 0.5 . This frequency stays the same for 4 generations and is in equilibrium . Now, assume that p = 0.8 and q = 0.2 and annotate the diagram below: </li></ul>
  32. 37. Example 5 <ul><li>The allele frequency in a population are </li></ul><ul><li>A = 0.6 and a = 0.4. </li></ul><ul><li>Predict the genotype frequencies for the </li></ul><ul><li>next generation </li></ul>
  33. 38. <ul><li>AA ? </li></ul><ul><li>Aa ? </li></ul><ul><li>aa ? </li></ul>
  34. 39. <ul><li>AA 0.36 </li></ul><ul><li>Aa ? </li></ul><ul><li>aa 0 .16 </li></ul>
  35. 40. 0.36 + 0.16 = 0.52 <ul><li>1.0- 0.52 = 0.48 </li></ul><ul><li>Aa = 0.48 </li></ul>
  36. 41. Or, Aa = 2pq <ul><li>Aa = 2 x 0.6 X 0.4 = 0.48 </li></ul>
  37. 42. Example 6 <ul><li>Suppose that one gene locus determines stripe pattern in skunks. SS skunks have two broad stripes; Ss have two narrow stripes; and ss skunks have white speckles down their backs. A sampling of a population of skunks found 65 broad-striped skunks , 14 narrow-striped skunks, and 1 speckled. Determine the gene frequencies. </li></ul>
  38. 43. Total number of skunks in population = 80 <ul><li>65 SS skunks represent 130 A alleles </li></ul><ul><li>14 Ss skunks represent 14 S alleles and 14 s alleles </li></ul><ul><li>1 s skunk represent 2 s alleles </li></ul><ul><li>S alleles 130 + 14 = 144 </li></ul><ul><li>s alleles 14 + 2 = 16 </li></ul>
  39. 44. <ul><li>S alleles = 144 </li></ul><ul><li>s alleles = 16 </li></ul><ul><li>Total Population of skunks = 80 </li></ul><ul><li>80 skunks, each with 2 alleles means a gene pool of 160 alleles </li></ul><ul><li>144/160 = 0.9 frequency of S </li></ul><ul><li>16/160 = 0.1 frequency of s </li></ul>
  40. 45. You can also solve using Hardy- Weinberg equation: <ul><li>1 skunk out of 80 is homozygous recessive </li></ul><ul><li>1/80 = 0.0125 = q 2 </li></ul><ul><li>Therefore, q = 0.1118 or 0.1 and p = 0.9 </li></ul>
  41. 46. E. Conditions for Hardy—Weinberg Equilibrium <ul><li>Extremely large population size </li></ul><ul><li>The smaller the population, the greater the role played by chance fluctuations in allele frequencies </li></ul>
  42. 47. <ul><li>2. No gene flow </li></ul><ul><li>Gene flow is the transfer of alleles between populations and can alter allele frequencies </li></ul>
  43. 48. <ul><li>3. No mutations </li></ul><ul><li>By introducing or removing genes from chromosomes or by changing one allele into another, mutations modify the gene pool. </li></ul>
  44. 49. <ul><li>4. Random mating </li></ul><ul><li>If individuals preferentially choose mates with certain genotypes, including close relatives, (inbreeding), random mixing of gametes does not occur </li></ul>
  45. 50. <ul><li>5. No natural selection </li></ul><ul><li>Differential survival and reproductive </li></ul><ul><li>success of individuals carrying different </li></ul><ul><li>genotypes will alter allele frequencies. </li></ul>
  46. 51. Concept Check <ul><li>What did Mendel's findings about genetics add to Darwin’s theory of evolution by natural selection? </li></ul><ul><li>Mendel showed that the inheritance is particulate, and subsequently it was shown that this type of inheritance can preserve the variation on which natural selection acts </li></ul>
  47. 52. Concept Check <ul><li>Which parts of the Hardy-Weinberg equation correspond to the frequency of individuals that have at least one PKU allele? </li></ul><ul><li>2pq + q 2 </li></ul>
  48. 53. F. Primary Causes of microevolution
  49. 54. 1. Mutations <ul><li>New genes and alleles originate by mutations, such as a point mutation which is the cause of Sickle Cell Anemia . Some point mutations may be harmless, such as those found in the wobble position and those found in non-coding DNA. </li></ul>
  50. 55. <ul><li>“On rare occasions however, a mutant allele may actually make its bearer better suited to the environment, enhancing reproductive success.” (Campbell 459) </li></ul>
  51. 56. 2. Gene flow <ul><li>Genetic additions or subtractions resulting form the movement of individuals or gametes </li></ul>
  52. 57. 3.Genetic drift <ul><li>The smaller the sample size the greater the chance of deviation from the predicted result. 2 causes of genetic drift are: </li></ul><ul><ul><li>Bottleneck effect (p.461) </li></ul></ul><ul><ul><li>Founder effect (p. 462) </li></ul></ul>
  53. 59. Bottleneck effect—sudden changes in the environment
  54. 60. Elephant seals in California Have no genetic variation at 24 gene loci—only one allele
  55. 61. 4. Sexual Recombination <ul><li>Produces much more recombination than mutation from generation to generation. </li></ul>
  56. 62. 5. Natural selection <ul><li>Individuals in a population exhibit variation in their heritable traits, and those with variations that are better suited to the environmen t tend to produce more offspring than those that are less well suited </li></ul>
  57. 63. G. Genetic Variation occurs within and between populations <ul><li>1. Polymorphism occurs when two or more forms are evident in a population (freckles and absence of freckles are different “morphs” and examples of polymorphism ) </li></ul>
  58. 64. <ul><li>2. Genetic variation can be measured by gel electrophoresis </li></ul>
  59. 65. <ul><li>a. can identify the variation in protein products of specific loci. </li></ul><ul><li>b. Proteins produced by different alleles may vary in amino acid composition and still have the same overall charge and be indistinguishable by electrophoresis. </li></ul><ul><li>c. Meiosis and random mating generate genetic variation </li></ul>
  60. 66. <ul><li>most mutations are not viable </li></ul><ul><li>Mutation can produce genetic variation in bacteria and other microorganisms which have short generation times </li></ul><ul><li>Variation in plants and animals is usually due to sexual recombination because mutations are deleterious and usually result in defective protein products. </li></ul>
  61. 67. <ul><li>d. Sexual selection may lead to pronounced secondary differences between the sexes called sexual dimorphism . (example: size, plumage, lion manes, deer antlers or other adornments in males) </li></ul>
  62. 69. <ul><li>e. Natural selection cannot fashion perfect organisms </li></ul><ul><li>Selection can only edit variations that already exist </li></ul><ul><li>New genes are not formed by mutation on demand . </li></ul>