23lecturepresentation-100812184052-phpapp02-180328062258 (1).pdf

1. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings PowerPoint® Lecture Presentations for Biology Eighth Edition Neil Campbell and Jane Reece Lectures by Chris Romero, updated by Erin Barley with contributions from Joan Sharp Chapter 23 The Evolution of Populations

2. Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Overview: The Smallest Unit of Evolution • One misconception is that organisms evolve, in the Darwinian sense, during their lifetimes • Natural selection acts on individuals, but only populations evolve • Genetic variations in populations contribute to evolution • Microevolution is a change in allele frequencies in a population over generations

3. Fig. 23-1

4. Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings • Two processes, mutation and sexual reproduction, produce the variation in gene pools that contributes to differences among individuals Concept 23.1: Mutation and sexual reproduction produce the genetic variation that makes evolution possible

5. Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Genetic Variation • Variation in individual genotype leads to variation in individual phenotype • Not all phenotypic variation is heritable • Natural selection can only act on variation with a genetic component

6. Fig. 23-2 (a) (b)

7. Fig. 23-2a (a)

8. Fig. 23-2b (b)

9. Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Variation Within a Population • Both discrete and quantitative characters contribute to variation within a population • Discrete characters can be classified on an either-or basis • Quantitative characters vary along a continuum within a population

10. Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings • Population geneticists measure polymorphisms in a population by determining the amount of heterozygosity at the gene and molecular levels • Average heterozygosity measures the average percent of loci that are heterozygous in a population • Nucleotide variability is measured by comparing the DNA sequences of pairs of individuals

11. Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Variation Between Populations • Most species exhibit geographic variation, differences between gene pools of separate populations or population subgroups

12. Fig. 23-3 13.17 19 XX 10.16 9.12 8.11 1 2.4 3.14 5.18 6 7.15 9.10 1 2.19 11.12 13.17 15.18 3.8 4.16 5.14 6.7 XX

13. Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings • Some examples of geographic variation occur as a cline, which is a graded change in a trait along a geographic axis

14. Fig. 23-4 1.0 0.8 0.6 0.4 0.2 0 46 44 42 40 38 36 34 32 30 Georgia Warm (21°C) Latitude (°N) Maine Cold (6°C) Ldh-B b allele frequency

15. Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Mutation • Mutations are changes in the nucleotide sequence of DNA • Mutations cause new genes and alleles to arise • Only mutations in cells that produce gametes can be passed to offspring Animation: Genetic Variation from Sexual Recombination Animation: Genetic Variation from Sexual Recombination

17. Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings • The effects of point mutations can vary: – Mutations in noncoding regions of DNA are often harmless – Mutations in a gene might not affect protein production because of redundancy in the genetic code

18. Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings • The effects of point mutations can vary: – Mutations that result in a change in protein production are often harmful – Mutations that result in a change in protein production can sometimes increase the fit between organism and environment

19. Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Mutations That Alter Gene Number or Sequence • Chromosomal mutations that delete, disrupt, or rearrange many loci are typically harmful • Duplication of large chromosome segments is usually harmful • Duplication of small pieces of DNA is sometimes less harmful and increases the genome size • Duplicated genes can take on new functions by further mutation

20. Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Mutation Rates • Mutation rates are low in animals and plants • The average is about one mutation in every 100,000 genes per generation • Mutations rates are often lower in prokaryotes and higher in viruses

21. Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Sexual Reproduction • Sexual reproduction can shuffle existing alleles into new combinations • In organisms that reproduce sexually, recombination of alleles is more important than mutation in producing the genetic differences that make adaptation possible

22. Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Concept 23.2: The Hardy-Weinberg equation can be used to test whether a population is evolving • The first step in testing whether evolution is occurring in a population is to clarify what we mean by a population

23. Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Gene Pools and Allele Frequencies • A population is a localized group of individuals capable of interbreeding and producing fertile offspring • A gene pool consists of all the alleles for all loci in a population • A locus is fixed if all individuals in a population are homozygous for the same allele

24. Fig. 23-5 Porcupine herd Porcupine herd range Beaufort Sea N O R T H W E S T T E R R I T O R I E S MAP AREA ALASKA CAN ADA Fortymile herd range Fortymile herd A L A S K A Y U K O N

25. Fig. 23-5a Porcupine herd range Beaufort Sea N O R T H W E S T T E R R I T O R I E S MAP AREA ALASKA CAN ADA Fortymile herd range A L A S K A Y U K O N

26. Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings • The frequency of an allele in a population can be calculated – For diploid organisms, the total number of alleles at a locus is the total number of individuals x 2 – The total number of dominant alleles at a locus is 2 alleles for each homozygous dominant individual plus 1 allele for each heterozygous individual; the same logic applies for recessive alleles

27. Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings • By convention, if there are 2 alleles at a locus, p and q are used to represent their frequencies • The frequency of all alleles in a population will add up to 1 – For example, p + q = 1

28. Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings The Hardy-Weinberg Principle • The Hardy-Weinberg principle describes a population that is not evolving • If a population does not meet the criteria of the Hardy-Weinberg principle, it can be concluded that the population is evolving

29. Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Hardy-Weinberg Equilibrium • The Hardy-Weinberg principle states that frequencies of alleles and genotypes in a population remain constant from generation to generation • In a given population where gametes contribute to the next generation randomly, allele frequencies will not change • Mendelian inheritance preserves genetic variation in a population

30. Fig. 23-6 Frequencies of alleles Alleles in the population Gametes produced Each egg: Each sperm: 80% chance 80% chance 20% chance 20% chance q = frequency of p = frequency of CR allele = 0.8 CW allele = 0.2

31. Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings • Hardy-Weinberg equilibrium describes the constant frequency of alleles in such a gene pool • If p and q represent the relative frequencies of the only two possible alleles in a population at a particular locus, then – p2 + 2pq + q2 = 1 – where p2 and q2 represent the frequencies of the homozygous genotypes and 2pq represents the frequency of the heterozygous genotype

32. Fig. 23-7-1 Sperm CR (80%) C W (20%) 80% CR (p = 0.8) CW (20%) 20% CW (q = 0.2) 16% (pq) CR CW 4% (q2 ) CW CW C R (80 %) 64% (p2 ) CR CR 16% (qp) CR CW Eggs

33. Fig. 23-7-2 Gametes of this generation: 64% CR CR , 32% CR CW , and 4% CW CW 64% CR + 16% CR = 80% CR = 0.8 = p 4% CW + 16% CW = 20% CW = 0.2 = q

34. Fig. 23-7-3 Gametes of this generation: 64% CR CR , 32% CR CW , and 4% CW CW 64% CR + 16% CR = 80% CR = 0.8 = p 4% CW + 16% CW = 20% CW = 0.2 = q 64% CR CR , 32% CR CW , and 4% CW CW plants Genotypes in the next generation:

35. Fig. 23-7-4 Gametes of this generation: 64% CR CR , 32% CR CW , and 4% CW CW 64% CR + 16% CR = 80% CR = 0.8 = p 4% CW + 16% CW = 20% CW = 0.2 = q 64% CR CR , 32% CR CW , and 4% CW CW plants Genotypes in the next generation: Sperm CR (80%) C W (20%) 80% CR (p = 0.8) CW (20%) 20% CW (q = 0.2) 16% (pq) CR CW 4% (q2 ) CW CW C R (80%) 64% ( p2 ) CR CR 16% (qp) CR CW Eggs

36. Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Conditions for Hardy-Weinberg Equilibrium • The Hardy-Weinberg theorem describes a hypothetical population • In real populations, allele and genotype frequencies do change over time

37. Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings • The five conditions for nonevolving populations are rarely met in nature: – No mutations – Random mating – No natural selection – Extremely large population size – No gene flow

39. Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Applying the Hardy-Weinberg Principle • We can assume the locus that causes phenylketonuria (PKU) is in Hardy-Weinberg equilibrium given that: – The PKU gene mutation rate is low – Mate selection is random with respect to whether or not an individual is a carrier for the PKU allele

40. Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings – Natural selection can only act on rare homozygous individuals who do not follow dietary restrictions – The population is large – Migration has no effect as many other populations have similar allele frequencies

41. Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings • The occurrence of PKU is 1 per 10,000 births – q2 = 0.0001 – q = 0.01 • The frequency of normal alleles is – p = 1 – q = 1 – 0.01 = 0.99 • The frequency of carriers is – 2pq = 2 x 0.99 x 0.01 = 0.0198 – or approximately 2% of the U.S. population

42. Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings • Three major factors alter allele frequencies and bring about most evolutionary change: – Natural selection – Genetic drift – Gene flow Concept 23.3: Natural selection, genetic drift, and gene flow can alter allele frequencies in a population

43. Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Natural Selection • Differential success in reproduction results in certain alleles being passed to the next generation in greater proportions

44. Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Genetic Drift • The smaller a sample, the greater the chance of deviation from a predicted result • Genetic drift describes how allele frequencies fluctuate unpredictably from one generation to the next • Genetic drift tends to reduce genetic variation through losses of alleles Animation: Causes of Evolutionary Change Animation: Causes of Evolutionary Change

45. Fig. 23-8-1 Generation 1 p (frequency of CR ) = 0.7 q (frequency of CW ) = 0.3 CW CW CR CR CR CW CR CR CR CR CR CR CR CR CR CW CR CW CR CW

46. Fig. 23-8-2 Generation 1 p (frequency of CR ) = 0.7 q (frequency of CW ) = 0.3 Generation 2 p = 0.5 q = 0.5 CW CW CR CR CR CW CR CR CR CR CR CR CR CR CR CW CR CW CR CW CR CW CR CW CR CW CR CW CW CW CW CW CW CW CR CR CR CR CR CR

47. Fig. 23-8-3 Generation 1 CW CW CR CR CR CW CR CR CR CR CR CR CR CR CR CW CR CW CR CW p (frequency of CR ) = 0.7 q (frequency of CW ) = 0.3 Generation 2 CR CW CR CW CR CW CR CW CW CW CW CW CW CW CR CR CR CR CR CR p = 0.5 q = 0.5 Generation 3 p = 1.0 q = 0.0 CR CR CR CR CR CR CR CR CR CR CR CR CR CR CR CR CR CR CR CR

48. Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings The Founder Effect • The founder effect occurs when a few individuals become isolated from a larger population • Allele frequencies in the small founder population can be different from those in the larger parent population

49. Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings The Bottleneck Effect • The bottleneck effect is a sudden reduction in population size due to a change in the environment • The resulting gene pool may no longer be reflective of the original population’s gene pool • If the population remains small, it may be further affected by genetic drift

50. Fig. 23-9 Original population Bottlenecking event Surviving population

52. Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Case Study: Impact of Genetic Drift on the Greater Prairie Chicken • Loss of prairie habitat caused a severe reduction in the population of greater prairie chickens in Illinois • The surviving birds had low levels of genetic variation, and only 50% of their eggs hatched

53. Fig. 23-10 Number of alleles per locus Range of greater prairie chicken Pre-bottleneck (Illinois, 1820) Post-bottleneck (Illinois, 1993) Minnesota, 1998 (no bottleneck) Nebraska, 1998 (no bottleneck) Kansas, 1998 (no bottleneck) Illinois 1930–1960s 1993 Location Population size Percentage of eggs hatched 1,000–25,000 <50 750,000 75,000– 200,000 4,000 5.2 3.7 93 <50 5.8 5.8 5.3 85 96 99 (a) (b)

54. Fig. 23-10a Range of greater prairie chicken Pre-bottleneck (Illinois, 1820) Post-bottleneck (Illinois, 1993) (a)

55. Fig. 23-10b Number of alleles per locus Minnesota, 1998 (no bottleneck) Nebraska, 1998 (no bottleneck) Kansas, 1998 (no bottleneck) Illinois 1930–1960s 1993 Location Population size Percentage of eggs hatched 1,000–25,000 <50 750,000 75,000– 200,000 4,000 5.2 3.7 93 <50 5.8 5.8 5.3 85 96 99 (b)

56. Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings • Researchers used DNA from museum specimens to compare genetic variation in the population before and after the bottleneck • The results showed a loss of alleles at several loci • Researchers introduced greater prairie chickens from population in other states and were successful in introducing new alleles and increasing the egg hatch rate to 90%

57. Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Effects of Genetic Drift: A Summary 1. Genetic drift is significant in small populations 2. Genetic drift causes allele frequencies to change at random 3. Genetic drift can lead to a loss of genetic variation within populations 4. Genetic drift can cause harmful alleles to become fixed

58. Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Gene Flow • Gene flow consists of the movement of alleles among populations • Alleles can be transferred through the movement of fertile individuals or gametes (for example, pollen) • Gene flow tends to reduce differences between populations over time • Gene flow is more likely than mutation to alter allele frequencies directly

59. Fig. 23-11

60. Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings • Gene flow can decrease the fitness of a population • In bent grass, alleles for copper tolerance are beneficial in populations near copper mines, but harmful to populations in other soils • Windblown pollen moves these alleles between populations • The movement of unfavorable alleles into a population results in a decrease in fit between organism and environment

61. Fig. 23-12 NON- MINE SOIL MINE SOIL NON- MINE SOIL Prevailing wind direction Index of copper toleranc e Distance from mine edge (meters) 70 60 50 40 30 20 10 0 20 0 20 0 20 40 60 80 100 120 140 160

62. Fig. 23-12a NON- MINE SOIL MINE SOIL NON- MINE SOIL Prevailing wind direction Index of copp er tolera nce Distance from mine edge (meters) 70 60 50 40 30 20 10 0 20 0 20 0 20 40 60 80 100 120 140 160

63. Fig. 23-12b

64. Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings • Gene flow can increase the fitness of a population • Insecticides have been used to target mosquitoes that carry West Nile virus and malaria • Alleles have evolved in some populations that confer insecticide resistance to these mosquitoes • The flow of insecticide resistance alleles into a population can cause an increase in fitness

65. Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings • Only natural selection consistently results in adaptive evolution Concept 23.4: Natural selection is the only mechanism that consistently causes adaptive evolution

66. Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings A Closer Look at Natural Selection • Natural selection brings about adaptive evolution by acting on an organism’s phenotype

67. Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Relative Fitness • The phrases “struggle for existence” and “survival of the fittest” are misleading as they imply direct competition among individuals • Reproductive success is generally more subtle and depends on many factors

68. Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings • Relative fitness is the contribution an individual makes to the gene pool of the next generation, relative to the contributions of other individuals • Selection favors certain genotypes by acting on the phenotypes of certain organisms

69. Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Directional, Disruptive, and Stabilizing Selection • Three modes of selection: – Directional selection favors individuals at one end of the phenotypic range – Disruptive selection favors individuals at both extremes of the phenotypic range – Stabilizing selection favors intermediate variants and acts against extreme phenotypes

70. Fig. 23-13 Original population (c) Stabilizing selection (b) Disruptive selection (a) Directional selection Phenotypes (fur color) Frequenc y of individu als Original population Evolved population

71. Fig. 23-13a Original population (a) Directional selection Phenotypes (fur color) Frequenc y of individu als Original population Evolved population

72. Fig. 23-13b Original population (b) Disruptive selection Phenotypes (fur color) Frequenc y of individu als Evolved population

73. Fig. 23-13c Original population (c) Stabilizing selection Phenotypes (fur color) Frequenc y of individu als Evolved population

74. Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings The Key Role of Natural Selection in Adaptive Evolution • Natural selection increases the frequencies of alleles that enhance survival and reproduction • Adaptive evolution occurs as the match between an organism and its environment increases

75. Fig. 23-14 (a) Color-changing ability in cuttlefish (b) Movable jaw bones in snakes Movable bones

76. Fig. 23-14a (a) Color-changing ability in cuttlefish

77. Fig. 23-14b (b) Movable jaw bones in snakes Movable bones

78. Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings • Because the environment can change, adaptive evolution is a continuous process • Genetic drift and gene flow do not consistently lead to adaptive evolution as they can increase or decrease the match between an organism and its environment

79. Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Sexual Selection • Sexual selection is natural selection for mating success • It can result in sexual dimorphism, marked differences between the sexes in secondary sexual characteristics

80. Fig. 23-15

81. Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings • Intrasexual selection is competition among individuals of one sex (often males) for mates of the opposite sex • Intersexual selection, often called mate choice, occurs when individuals of one sex (usually females) are choosy in selecting their mates • Male showiness due to mate choice can increase a male’s chances of attracting a female, while decreasing his chances of survival

82. Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings • How do female preferences evolve? • The good genes hypothesis suggests that if a trait is related to male health, both the male trait and female preference for that trait should be selected for

83. Fig. 23-16 SC male gray tree frog Female gray tree frog LC male gray tree frog EXPERIMENT SC sperm × Eggs × LC sperm Offspring of LC father Offspring of SC father Fitness of these half-sibling offspring compared RESULTS 1995 Fitness Measure 1996 Larval growth Larval survival Time to metamorphosis LC better NSD LC better (shorter) LC better (shorter) NSD LC better NSD = no significant difference; LC better = offspring of LC males superior to offspring of SC males.

84. Fig. 23-16a SC male gray tree frog Female gray tree frog LC male gray tree frog SC sperm × Eggs × LC sperm Offspring of LC father Offspring of SC father Fitness of these half-sibling offspring compared EXPERIMENT

85. Fig. 23-16b RESULTS 1995 Fitness Measure 1996 Larval growth Larval survival Time to metamorphosis LC better NSD LC better (shorter) LC better (shorter) NSD LC better NSD = no significant difference; LC better = offspring of LC males superior to offspring of SC males.

88. Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Balancing Selection • Balancing selection occurs when natural selection maintains stable frequencies of two or more phenotypic forms in a population

89. Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings • Heterozygote advantage occurs when heterozygotes have a higher fitness than do both homozygotes • Natural selection will tend to maintain two or more alleles at that locus • The sickle-cell allele causes mutations in hemoglobin but also confers malaria resistance Heterozygote Advantage

90. Fig. 23-17 0–2.5% Distribution of malaria caused by Plasmodium falciparum (a parasitic unicellular eukaryote) Frequencies of the sickle-cell allele 2.5–5.0% 7.5–10.0% 5.0–7.5% >12.5% 10.0–12.5%

91. Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings • In frequency-dependent selection, the fitness of a phenotype declines if it becomes too common in the population • Selection can favor whichever phenotype is less common in a population Frequency-Dependent Selection

92. Fig. 23-18 “Right-mouthed” 1981 “Left-mouthed” Frequency of “left-mouthed” individuals Sample year 1.0 0.5 0 ’82 ’83 ’84 ’85 ’86 ’87 ’88 ’89 ’90

93. Fig. 23-18a “Right-mouthed” “Left-mouthed”

94. Fig. 23-18b 1981 Frequency of “left-mouthed” individuals Sample year 1.0 0.5 0 ’8 2 ’8 3 ’8 4 ’8 5 ’8 6 ’8 7 ’8 8 ’8 9 ’9 0

95. Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Neutral Variation • Neutral variation is genetic variation that appears to confer no selective advantage or disadvantage • For example, – Variation in noncoding regions of DNA – Variation in proteins that have little effect on protein function or reproductive fitness

96. Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Why Natural Selection Cannot Fashion Perfect Organisms 1. Selection can act only on existing variations 2. Evolution is limited by historical constraints 3. Adaptations are often compromises 4. Chance, natural selection, and the environment interact

97. Fig. 23-19

98. Fig. 23-UN1 Stabilizing selection Original population Evolved population Directional selection Disruptive selection

99. Fig. 23-UN2 Sampling sites (1–8 represent pairs of sites) Salinity increases toward the open ocean N Long Island Sound Allele frequencies Atlantic Ocean Other lap alleles lap94 alleles Data from R.K. Koehn and T.J. Hilbish, The adaptive importance of genetic variation, American Scientist 75:134–141 (1987). E S W 1 2 3 4 5 9 10 6 7 8 11 1 11 10 2 3 4 5 6 7 8 9

100. Fig. 23-UN3

101. Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings You should now be able to: 1. Explain why the majority of point mutations are harmless 2. Explain how sexual recombination generates genetic variability 3. Define the terms population, species, gene pool, relative fitness, and neutral variation 4. List the five conditions of Hardy-Weinberg equilibrium

102. Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings 5. Apply the Hardy-Weinberg equation to a population genetics problem 6. Explain why natural selection is the only mechanism that consistently produces adaptive change 7. Explain the role of population size in genetic drift

103. Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings 8. Distinguish among the following sets of terms: directional, disruptive, and stabilizing selection; intrasexual and intersexual selection 9. List four reasons why natural selection cannot produce perfect organisms

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