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Chapter 10

Chapter 10. Natural Selection An Evolving Enemy. Chapter 10 Section 1. Tuberculosis Return of an Ancient Killer. 10.1 Return of an Ancient Killer. Tuberculosis Tuberculosis (TB) has infected humans for 1000s of years Evidence of TB has been found in Egyptian mummies

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Chapter 10

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  1. Chapter 10 Natural Selection An Evolving Enemy

  2. Chapter 10 Section 1 Tuberculosis Return of an Ancient Killer

  3. 10.1 Return of an Ancient Killer Tuberculosis • Tuberculosis (TB) has infected humans for 1000s of years • Evidence of TB has been found in Egyptian mummies • Hippocrates described a TB-like condition • In 1906, TB accounted for 2 out of every 1000 deaths in the U.S.

  4. 10.1 Return of an Ancient Killer What is Tuberculosis? • TB is caused by bacterium, Mycobacterium tuberculosis • 2 billion people carry TB • New infections occur at rate of 1 per second • TB causes roughly 2 million deaths per year Figure 10.1

  5. 10.1 Return of an Ancient Killer - What is Tuberculosis? Symptoms of TB include: • Cough that produces blood • Fever • Fatigue • Period of wasting – patient becomes weaker and thinner • Led to calling the disease “consumption”

  6. 10.1 Return of an Ancient Killer - What is Tuberculosis? • Consumptive symptoms occur because of damage to lung tissues. Figure 10.2

  7. 10.1 Return of an Ancient Killer - What is Tuberculosis? • Testing is by x-ray of lungs and skin test • Transmission of TB occurs through the air, from infected individuals • A single sneeze can contain 40,000 droplets, all containing infectious bacteria Figure 10.4

  8. 10.1 Return of an Ancient Killer Treatment – and Treatment Failure • 19th and early 20th century – treatment consisted of “sanitariums” • Discovery of antibiotics revolutionized TB treatment • Since the 1980s, scientists have noticed an increase in TB that is resistant to antibiotics. • Because of resistance to antibiotics, the number of TB cases worldwide is increasing.

  9. END Chapter 10 Section 1 Tuberculosis Return of an Ancient Killer

  10. Chapter 10 Section 2 Natural Selection Causes Evolution

  11. 10.2 Natural Selection Causes Evolution Darwin discussed two ideas in Origin of Species • Common descent • Natural selection • Natural selection is considered to be the primary cause of evolution • Other factors include genetic drift and sexual selection (chapter 11)

  12. 10.2 Natural Selection Causes Evolution Natural selection is an inference based on four observations 1. Individuals within populations vary 2. Some of the variation within individuals can be passed on to their offspring 3. Populations of organisms produce more offspring than will survive 4. Survival and reproduction are not random

  13. 10.2 Natural Selection Causes Evolution 1. Individuals within populations vary. • This is true of human and non-human populations. • Variation can include traits other than appearance, such as blooming time in flowers. Figure 10.6

  14. 10.2 Natural Selection Causes Evolution 2. Some of the variation within individuals can be passed on to their offspring. • Darwin noticed that animal breeders could get exaggerated traits through selective breeding. Figure 10.7

  15. 10.2 Natural Selection Causes Evolution 3. Populations of organisms produce more offspring than will survive. • Even slow-breeding animals can produce large populations quickly.

  16. 10.2 Natural Selection Causes Evolution If a female elephant (colored pink) lives a full fertile lifetime, she will bear about six calvesin about 90 years. On average, half of her calves will be female. Shelf = Available resources Generation 0 =2 elephants Generation 1 =6 elephants Generation 2 =18 elephants Generation 3 =54 elephants 3. Populations of organisms produce more offspring than will survive. Figure 10.8

  17. 10.2 Natural Selection Causes Evolution 4. Survival and reproduction are not random • Fitness: Relative survival and reproduction of one variant • Adaptation: Traits that increase individual fitness in an environment • Individuals with adaptations for a particular environment are more likely to survive and reproduce

  18. 10.2 Natural Selection Causes Evolution 1976 Billdepth Number of individuals Averagebill depthof droughtsurvivors Average billdepth beforedrought 1978 Bill depth (mm) 4. Survival and reproduction are not random. Figure 10.9

  19. 10.2 Natural Selection Causes Evolution Adaptations do not only affect survival. • Any trait that increases the number of offspring produced is an adaptation • A flower’s reproduction is impacted by traits that affect the number of pollinators it receives. • Therefore, color or nectar production might be adaptations. Figure 10.10

  20. 10.2 Natural Selection Causes Evolution Darwin’s Inference: Natural Selection Causes Evolution • Result of natural selection is • Favorable inherited variations tend to increase in frequency • Unfavorable variations tend to be lost • End result is a change in the traits of individuals in a population over generations (i.e. evolution)

  21. 10.2 Natural Selection Causes Evolution Testing Natural Selection • Artificial Selection: Selection imposed by human choice • Breeds of pigeons studied by Darwin arose through artificial selection • Breeds of dog have been artificially selected by humans

  22. 10.2 Natural Selection Causes Evolution Testing Natural Selection in the Lab • In laboratory, environmental conditions can be manipulated and effects on population examined • Example: alcohol in fruit flies • Scientists examined alcohol metabolism in fruit flies • All animals have enzymes to metabolize alcohol • Variations in ability to metabolize alcohol exist in populations

  23. 10.2 Natural Selection Causes Evolution - Testing Natural Selection • Natural selection in the Lab Increase in percent offast-metabolizingflies After 57 generations, all fliesin the high-alcohol environmentare fast processors of alcohol.As a result, the average rateof alcohol metabolism is twicethe rate of the unmodifiedpopulation. Percent of population thatmetabolized alcohol rapidly No change inalcohol-metabolizing rate Generation 1 Generation 57 Generation1 Generation 57 Fly population innormal laboratoryenvironment Fly population in ahigh-alcohollaboratory environment Figure 10.12

  24. 10.2 Natural Selection Causes Evolution Natural Selection in Wild Populations 1976 Billdepth Number of individuals Averagebill depthof droughtsurvivors Average billdepth beforedrought 1978 Bill depth (mm) • Many examples exist: • Evolution of resistance of M. tuberculosis to antibiotics • Many other disease-causing pathogens have also evolved resistance • Galapagos finches provide another classic example Figure 10.9

  25. 10.2 Natural Selection Causes Evolution PLAY Animation—Natural Selection Figure 10.9

  26. End Chapter 10 Section 2 Natural Selection Causes Evolution

  27. Chapter 10 Section 3 Natural Selection Since Darwin

  28. 10.3 Natural Selection Since Darwin The Modern Synthesis • The union of genetics and evolution is called “the modern synthesis” • Knowledge of genetics facilitates understanding of the mechanisms of evolution • Alleles are the basis of variation of traits • Mutations can create new alleles and provide the basis for new traits • Natural selection provides a filter that selects for or against new traits

  29. 10.3 Natural Selection Since Darwin The Modern Synthesis • Mutation gives rise to new alleles • Generates raw material for natural selection • Natural Selection alters the frequency of alleles within a population over generations • Evolution of a population = an increase or decrease in the frequency of an allele of a particular gene

  30. 10.3 Natural Selection Since Darwin Mutation, Natural Selection & Evolution Figure 10.13

  31. 10.3 Natural Selection Since Darwin - Overview: The Subtleties of Natural Selection PLAY Animation—Drug Resistance and Natural Selection

  32. 10.3 Natural Selection Since Darwin A Closer Look: Subtleties of Natural Selection 1. Natural selection cannot cause new traits to arise • Not an issue of choice or “will” of organisms • Selection can ONLY act on variations that ALREADY exist • Mutation creates new alleles RANDOMLY • In fly example, alcohol-rich environment did not cause gene to arise, differential survival caused allele to become more common

  33. 10.3 Natural Selection Since Darwin A Closer Look: Subtleties of Natural Selection 2. Natural selection does not result in perfection • Natural selection does cause organisms to become a better fit to their environment • Organisms are not necessarily “better”, just better fit to a particular situation • Adaptation that is beneficial in one situation might be a liability in another • Adaptations are trade-offs between better fit in one situation versus another

  34. 10.3 Natural Selection Since Darwin A Closer Look: Subtleties of Natural Selection • Adaptations are constrained by underlying biology • Result is “jury-rigged design” • Example is panda’s “thumb” • Actually grown from wrist bones • Not as efficient as opposable thumb Figure 10.14

  35. 10.3 Natural Selection Since Darwin A Closer Look: Subtleties of Natural Selection 3. Natural selection does not cause progression towards a goal • Natural selection favors variants with the most appropriate adaptations for current environment • Organisms do not choose to change or adapt • Natural selection depends on the situation of the population

  36. 10.3 Natural Selection Since Darwin Patterns of Selection • Different environmental conditions can lead to different changes in populations • Directional selection– change in population traits by favoring one allele over another • Stabilizing selection– selection for the average traits • Diversifying selection– selection for extremes

  37. 10.3 Natural Selection Since Darwin Directional Selection (a) Directional selection Not favored bypollinator = low fitness Number of individuals in thepopulation of each type Number of individuals in thepopulation of each type Population evolvesin the direction of adarker pink color. Preferred bypollinator = high fitness Color range Color range Figure 10.16a

  38. 10.3 Natural Selection Since Darwin Stabilizing Selection (b) Stabilizing selection Pollinators more likelyto visit similar-colored flowers = high fitness Populationstabilizes;nearly all theindividuals aresame color. Pale flowersnot recognizedby pollinators= low fitness Number of individuals in thepopulation of each type Number of individuals in thepopulation of each type Dark flowers notrecognized bypollinators = low fitness Color range Color range Figure 10.16b

  39. 10.3 Natural Selection Since Darwin Diversifying Selection (c) Diversifying selection Neither pollinator choosesintermediate color = low fitness One type ofpollinatorspecializes inpale flowers= high fitness Population diversifiesinto pale variety anddark variety. Number of individuals in thepopulation of each type Number of individuals in thepopulation of each type Another pollinatorspecializes in darkflowers = high fitness Color range Color range Figure 10.16

  40. END Chapter 10 Section 3 Natural Selection Since Darwin

  41. Chapter 10 Section 4 Natural Selection and Human Health

  42. 10.4 Natural Selection and Human Health Tuberculosis Fits Darwin’s Observations • Mycobacterium tuberculosis has evolved resistance to antibiotics because it fulfills the same observations Darwin made • Bacteria in the population vary • Variation can be passed on to offspring • More bacteria are produced than survive • Bacterial survival is not random

  43. 10.4 Natural Selection and Human Health Selecting for Drug Resistance Single drug therapy 1 Start with different variantsof M. tuberculosis. 2 Single drug reduces fitnessof most variants. 3 Resistant variants proliferate. • Simple antibiotic treatment can result in directional selection in bacteria. Figure 10.17

  44. 10.4 Natural Selection and Human Health Stopping Drug Resistance • Continue treatment until infection is completely cured (maybe months) • Combination therapy (aka drug cocktail) is a powerful tool against drug resistance. Figure 10.18

  45. 10.4 Natural Selection and Human Health Can Natural Selection Save Us From Superbugs • If bacteria can evolve resistance to antibiotics, can humans evolve resistance to bacteria? • Humans do vary in their immune capacity • To evolve resistance to superbugs would require many humans to die over many generations • By using modern drugs are we allowing the “survival of the weakest”?

  46. END Chapter 10 Section 4 Natural Selection and Human Health

  47. END Chapter 10 Natural Selection An Evolving Enemy

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