Biology - Chp 16 - Evolution Of Populations - Powerpoint

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Biology - Chp 16 - Evolution Of Populations - Powerpoint

  1. 1. Chapter 16 Evolution of Populations
  2. 2. 16-1 Genes and Variation <ul><li>As Darwin developed his theory of evolution, he worked under a serious handicap </li></ul><ul><li>He didn’t know how heredity worked </li></ul><ul><li>This lack of knowledge left two big gaps in Darwin’s thinking </li></ul>
  3. 3. <ul><li>He had no idea how heritable traits pass from one generation to the next </li></ul><ul><li>He had no idea how variation appeared, even though variation in heritable traits was central to Darwin’s theory </li></ul><ul><li>During the 1930’s Evolutionary biologists connected Mendel’s work to Darwin’s </li></ul><ul><li>By then biologists understood that genes control heritable traits </li></ul>
  4. 4. How Common Is Genetic Variation? <ul><li>Many genes have at least 2 forms or alleles </li></ul><ul><li>Animals such as horses, dogs, mice, and humans often have several alleles for traits such as body size or coat color </li></ul>
  5. 5. Variation and Gene Pools <ul><li>Genetic variation is studied in populations </li></ul>
  6. 6. Population <ul><li>A group of individuals of the same species that interbreed </li></ul><ul><li>Because members of a population interbreed, they share a common group of genes called a gene pool </li></ul>
  7. 7. Gene pool <ul><li>All the genes including all the different alleles, that are present in a population </li></ul>
  8. 9. Relative Frequency <ul><li>The number of times that the allele occurs in a gene pool, compared with the number of times other alleles for the same gene occur </li></ul><ul><li>In genetic terms, evolution is any change in the relative frequency of alleles in a population </li></ul>
  9. 10. Sources of Genetic Variation <ul><li>The 2 main sources of genetic variation are mutations and the genetic shuffling that results from sexual reproduction </li></ul>
  10. 11. Mutations <ul><li>Any change in a sequence of DNA </li></ul><ul><li>Mutations can occur because of </li></ul><ul><li>Mistakes in DNA replication </li></ul><ul><li>Radiation or chemicals in the environment </li></ul><ul><li>Some mutations don’t affect the phenotype but some do </li></ul>
  11. 12. Gene shuffling during sexual reproduction <ul><li>Mutations are not the only source of variation </li></ul><ul><li>Most heritable differences are due to gene shuffling that occurs during the production of gametes </li></ul><ul><li>The 23 pairs of chromosomes can produce 8.4 million different combinations of genes </li></ul><ul><li>Crossing over further increases the number of different genotypes that can also appear in offspring </li></ul>
  12. 13. Single – Gene and Polygenic Traits <ul><li>The number of phenotypes produced for a given trait depends on how many genes control the trait </li></ul>
  13. 15. Single – gene trait <ul><li>Controlled by a single gene that has two alleles </li></ul><ul><li>Variation in these genes leads to only 2 distinct phenotypes </li></ul>
  14. 16. Polygenic traits <ul><li>Traits controlled by two or more genes </li></ul><ul><li>Each gene of a polygenic trait has two or more alleles </li></ul><ul><li>As a result one polygenic trait can have many possible genotypes and phenotypes </li></ul><ul><li>Ex.) height </li></ul>
  15. 17. Polygenic traits
  16. 18. 16-2 Evolution as Genetic Change <ul><li>A genetic view of evolution offers a new way to look at key evolutionary concepts </li></ul><ul><li>If each time an organism reproduces, it passes copies of its genes to its offspring… </li></ul><ul><li>We can therefore view evolutionary fitness as an organism’s success in passing genes to the next generation </li></ul><ul><li>We can also view an evolutionary adaptation as any genetically controlled physiological, anatomical, or behavioral trait that increases an individuals ability to pass along its genes </li></ul>
  17. 19. Evolution as Genetic Change <ul><li>Remember that evolution is any change over time in the relative frequency of alleles in a population. </li></ul><ul><li>This reminds us that it is populations, not individual organisms that can evolve overtime </li></ul>
  18. 20. Natural Selection on Single – Gene Traits <ul><li>Natural selection on single gene traits can lead to changes in allele frequencies and thus to evolution </li></ul>
  19. 22. Natural Selection on Polygenic Traits <ul><li>Natural selection can affect the distributions of phenotypes in any of three ways </li></ul>
  20. 23. 1. Directional Selection <ul><li>When individuals at one end of the curve have higher fitness than individuals in the middle or at the other end </li></ul>
  21. 25. 2. Stabilizing Selection <ul><li>When individuals near the center of the curve have higher fitness than individuals at either end of the curve </li></ul>
  22. 27. 3. Disruptive Selection <ul><li>When individuals at the upper and lower ends of the curve have higher fitness than individuals near the middle </li></ul><ul><li>Can create 2 distinct phenotypes </li></ul>
  23. 29. Genetic Drift <ul><li>Natural Selection is not the only source of evolutionary change </li></ul><ul><li>In small populations, an allele can become more or less common by chance </li></ul>
  24. 30. Genetic Drift <ul><li>A random change in allele frequency </li></ul>
  25. 31. Genetic Drift <ul><li>These individuals may carry alleles in different relative frequencies than did the larger population from which they came </li></ul><ul><li>If so, the population that they found will be genetically different from the parent population </li></ul><ul><li>This cause is not natural selection, but chance </li></ul>
  26. 32. Founder effect <ul><li>A situation in which allele frequency changes as a result of the migration of a small subgroup of a population </li></ul>
  27. 33. Evolution vs. Genetic Equilibrium <ul><li>To clarify how evolutionary change operates, scientists often find it helpful to determine what happens when no change takes place </li></ul>
  28. 34. Hardy – Weinberg principle <ul><li>States that allele frequency in a population will remain constant unless one or more factors cause these frequencies to change </li></ul>
  29. 35. Genetic equilibrium <ul><li>The situation in which allele frequencies remain constant </li></ul>
  30. 36. 5 conditions are required to maintain genetic equilibrium <ul><li>There must be random mating </li></ul><ul><li>The population must be very large </li></ul><ul><li>There can be no movement into or out of the population </li></ul><ul><li>No mutations </li></ul><ul><li>No natural selections </li></ul>
  31. 37. 16-3 The Process of Speciation <ul><li>Factors such as natural selection and chance events can change the relative frequencies of alleles in a population </li></ul><ul><li>But how do these changes lead to speciation? </li></ul>
  32. 38. Speciation <ul><li>The formation of new species </li></ul>
  33. 39. Isolating Mechanisms <ul><li>Since members of the same species share a common gene pool, in order for a species to evolve into 2 new species, the gene pools must be separated into 2 </li></ul><ul><li>As new species evolve, populations become reproductively isolated from each other </li></ul>
  34. 40. Reproductive isolation <ul><li>When the members of 2 populations cannot interbreed and produce fertile offspring </li></ul>
  35. 41. Behavioral Isolation <ul><li>When two populations are capable of interbreeding but have differences in courtship rituals or other reproductive strategies </li></ul>
  36. 42. Eastern & Western Meadowlark
  37. 43. Geographical Isolation <ul><li>When two populations are separated by geographic barriers such as rivers, mountains, or bodies of water </li></ul>
  38. 44. Albert & Kaibab Squirrels
  39. 45. Temporal Isolation <ul><li>When 2 or more species reproduce at different times </li></ul>Rana aurora - breeds January - March Rana boylii - breeds late March - May
  40. 46. Testing Natural Selection in Nature <ul><li>Q: Can evolution be observed in nature? </li></ul><ul><li>A: YES </li></ul>
  41. 47. The Grants
  42. 48. Testing Natural Selection in Nature <ul><li>Darwin hypothesized that finches had descended from a common ancestor and overtime, natural selection shaped the beaks of different bird populations as they adapted to eat different foods </li></ul><ul><li>The Grants, realized that Darwin’s hypothesis relied on two testable assumptions </li></ul>
  43. 49. <ul><li>There must be enough heritable variation in these traits to provide raw materials for natural selection </li></ul><ul><li>Differences in beak size and shape must produce differences in fitness that cause natural selection to occur </li></ul>
  44. 50. Variation <ul><li>The Grants identified and measured every variable characteristic of the birds on the island </li></ul><ul><li>Their data indicated that there is a great variation of heritable traits among the Galapagos finches </li></ul>
  45. 51. Natural Selection <ul><li>During the… </li></ul><ul><li>Rainy season – enough food for everyone, no competition </li></ul><ul><li>Dry season – some foods become scarce </li></ul><ul><li>At that time, differences in beak sizes can mean the difference between life and death </li></ul><ul><li>Birds become feeding specialists </li></ul>
  46. 52. Natural Selection <ul><li>The Grants discovered that individual birds with different size beaks had different chances of survival during a drought </li></ul>
  47. 53. Speciation in Darwin’s Finches <ul><li>Speciation in the Galapagos finches occurred by founding of a new population, geographical isolation, changes in the new population’s gene pool, reproductive isolation and ecological competition </li></ul>
  48. 54. Founders Arrive <ul><li>Many years ago, a few finches from South American mainland </li></ul><ul><li>Species A, flew or were blown to one of the Galapagos Islands </li></ul>
  49. 55. Geographic Isolation <ul><li>Later on, some birds from species A crossed to another island in the Galapagos group </li></ul><ul><li>The finches then became unable to fly from island to island and become isolated from each other and no longer share a common gene pool </li></ul>
  50. 56. Changes in the Gene Pool <ul><li>Overtime, populations on each island became adapted to their local environments </li></ul>
  51. 57. Reproductive Isolation <ul><li>Now imagine that a few birds from the second island cross back to the first island </li></ul><ul><li>Q: Will the population A birds, breed with the population B birds? </li></ul><ul><li>A: Probably not </li></ul>
  52. 58. Ecological Competition <ul><li>As these two new species live together in the same environment, they compete with each other for available seeds </li></ul><ul><li>The more different birds are, the higher fitness they have, due to less compitition </li></ul>
  53. 59. Continued Evolution <ul><li>This process of isolation on different islands, genetic change, and reproductive isolation probably repeated itself time and time again across the entire Galapagos island chain </li></ul><ul><li>Over many generations, it produced the 13 different finch species found there today </li></ul>
  54. 60. Studying Evolution Since Darwin <ul><li>It is useful to review and critique the strength and weakness of evolutionary theory </li></ul><ul><li>Darwin made bold assumptions about heritable variation, the age of the Earth, and the relationships among organisms </li></ul><ul><li>New data from genetics, physics, and biochemistry could have proved him wrong on many counts, and they did not </li></ul><ul><li>Scientific evidence supports the theory that living species descended with modification from common ancestors that lived in the past </li></ul>
  55. 61. Limitations of Research <ul><li>The Grants data shows how competition and climate change affects natural selection </li></ul><ul><li>However, they did not observe the formation of a new species </li></ul>
  56. 62. Unanswered Questions <ul><li>Many new discoveries have led to new hypotheses that refine and expand Darwin’s original ideas </li></ul><ul><li>No scientist suggests that all evolutionary processes are fully understood. Many unanswered questions remain </li></ul>
  57. 63. Why Understanding Evolution is Important? <ul><li>Evolution continues today </li></ul><ul><li>Ex.) </li></ul><ul><li>Drug resistance in bacteria and viruses </li></ul><ul><li>Pesticide resistance in insects </li></ul><ul><li>Evolutionary theory helps us understand and respond to these changes in ways that improve human life </li></ul>

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