Biology - Chp 16 - Evolution Of Populations - Powerpoint


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Biology - Chp 16 - Evolution Of Populations - Powerpoint

  1. 1. Chapter 16Evolution of Populations
  2. 2. 16-1 Genes and Variation• As Darwin developed his theory of evolution, he worked under a serious handicap• He didn’t know how heredity worked• This lack of knowledge left two big gaps in Darwin’s thinking
  3. 3. 1. He had no idea how heritable traits pass from one generation to the next2. He had no idea how variation appeared, even though variation in heritable traits was central to Darwin’s theory• During the 1930’s Evolutionary biologists connected Mendel’s work to Darwin’s• By then biologists understood that genes control heritable traits
  4. 4. How Common Is Genetic Variation?• Many genes have at least 2 forms or alleles• Animals such as horses, dogs, mice, and humans often have several alleles for traits such as body size or coat color
  5. 5. Variation and Gene Pools• Genetic variation is studied in populations
  6. 6. Population• A group of individuals of the same species that interbreed• Because members of a population interbreed, they share a common group of genes called a gene pool
  7. 7. Gene pool• All the genes including all the different alleles, that are present in a population
  8. 8. Relative Frequency• 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• In genetic terms, evolution is any change in the relative frequency of alleles in a population
  9. 9. Sources of Genetic Variation• The 2 main sources of genetic variation are mutations and the genetic shuffling that results from sexual reproduction
  10. 10. Mutations• Any change in a sequence of DNA• Mutations can occur because of• Mistakes in DNA replication• Radiation or chemicals in the environment• Some mutations don’t affect the phenotype but some do
  11. 11. Gene shuffling during sexual reproduction• Mutations are not the only source of variation• Most heritable differences are due to gene shuffling that occurs during the production of gametes• The 23 pairs of chromosomes can produce 8.4 milliondifferent combinations of genes• Crossing over further increases the number of different genotypes that can also appear in offspring
  12. 12. Single – Gene and Polygenic Traits• The number of phenotypes produced for a given trait depends on how many genes control the trait
  13. 13. Single – gene trait• Controlled by a single gene that has two alleles• Variation in these genes leads to only 2 distinct phenotypes
  14. 14. Polygenic traits• Traits controlled by two or more genes• Each gene of a polygenic trait has two or more alleles• As a result one polygenic trait can have many possible genotypes and phenotypesEx.) height
  15. 15. Polygenic traits
  16. 16. 16-2 Evolution as Genetic Change• A genetic view of evolution offers a new way to look at key evolutionary concepts• If each time an organism reproduces, it passes copies of its genes to its offspring…• We can therefore view evolutionary fitness as an organism’s success in passing genes to the next generation• 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
  17. 17. Evolution as Genetic Change• Remember that evolution is any change over time in the relative frequency of alleles in a population.• This reminds us that it is populations, not individual organisms that can evolve overtime
  18. 18. Natural Selection on Single – Gene Traits• Natural selection on single gene traits can lead to changes in allele frequencies and thus to evolution
  19. 19. Natural Selection on Polygenic Traits• Natural selection can affect the distributions of phenotypes in any of three ways
  20. 20. 1. Directional Selection• When individuals at one end of the curve have higher fitness than individuals in the middle or at the other end
  21. 21. 2. Stabilizing Selection• When individuals near the center of the curve have higher fitness than individuals at either end of the curve
  22. 22. 3. Disruptive Selection• When individuals at the upper and lower ends of the curve have higher fitness than individuals near the middle• Can create 2 distinct phenotypes
  23. 23. Genetic Drift• Natural Selection is not the only source of evolutionary change• In small populations, an allele can become more or less common by chance
  24. 24. Genetic Drift• A random change in allele frequency
  25. 25. Genetic Drift• These individuals may carry alleles in different relative frequencies than did the larger population from which they came• If so, the population that they found will be genetically different from the parent population• This cause is not natural selection, but chance
  26. 26. Founder effect• A situation in which allele frequency changes as a result of the migration of a small subgroup of a population
  27. 27. Evolution vs. Genetic Equilibrium• To clarify how evolutionary change operates, scientists often find it helpful to determine what happens when no change takes place
  28. 28. Hardy – Weinberg principle• States that allele frequency in a population will remain constant unless one or more factors cause these frequencies to change
  29. 29. Genetic equilibrium• The situation in which allele frequencies remain constant
  30. 30. 5 conditions are required to maintain genetic equilibrium1. There must be random mating2. The population must be very large3. There can be no movement into or out of the population4. No mutations5. No natural selections
  31. 31. 16-3 The Process of Speciation• Factors such as natural selection and chance events can change the relative frequencies of alleles in a population• But how do these changes lead to speciation?
  32. 32. Speciation• The formation of new species
  33. 33. Isolating Mechanisms• 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• As new species evolve, populations become reproductively isolated from each other
  34. 34. Reproductive isolation• When the members of 2 populations cannot interbreed and produce fertile offspring
  35. 35. Behavioral Isolation• When two populations are capable of interbreeding but have differences in courtship rituals or other reproductive strategies
  36. 36. Eastern & Western Meadowlark
  37. 37. Geographical Isolation• When two populations are separated by geographic barriers such as rivers, mountains, or bodies of water
  38. 38. Albert & Kaibab Squirrels
  39. 39. Temporal Isolation• When 2 or more species reproduce at different times Rana aurora - breeds January - Rana boylii - breeds late March - March May
  40. 40. Testing Natural Selection in NatureQ: Can evolution be observed in nature?A: YES
  41. 41. The Grants
  42. 42. Testing Natural Selection in Nature • 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 • The Grants, realized that Darwin’s hypothesis relied on two testable assumptions
  43. 43. 1. There must be enough heritable variation in these traits to provide raw materials for natural selection2. Differences in beak size and shape must produce differences in fitness that cause natural selection to occur
  44. 44. Variation• The Grants identified and measured every variable characteristic of the birds on the island• Their data indicated that there is a great variation of heritable traits among the Galapagos finches
  45. 45. Natural Selection• During the…• Rainy season – enough food for everyone, no competition• Dry season – some foods become scarce• At that time, differences in beak sizes can mean the difference between life and death• Birds become feeding specialists
  46. 46. Natural Selection• The Grants discovered that individual birds with different size beaks had different chances of survival during a drought
  47. 47. Speciation in Darwin’s Finches• 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
  48. 48. Founders Arrive• Many years ago, a few finches from South American mainland• Species A, flew or were blown to one of the Galapagos Islands
  49. 49. Geographic Isolation• Later on, some birds from species A crossed to another island in the Galapagos group• 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
  50. 50. Changes in the Gene Pool • Overtime, populations on each island became adapted to their local environments
  51. 51. Reproductive Isolation• Now imagine that a few birds from the second island cross back to the first island• Q: Will the population A birds, breed with the population B birds?• A: Probably not
  52. 52. Ecological Competition• As these two new species live together in the same environment, they compete with each other for available seeds• The more different birds are, the higher fitness they have, due to less compitition
  53. 53. Continued Evolution• 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• Over many generations, it produced the 13 different finch species found there today
  54. 54. Studying Evolution Since Darwin• It is useful to review and critique the strength and weakness of evolutionary theory• Darwin made bold assumptions about heritable variation, the age of the Earth, and the relationships among organisms• New data from genetics, physics, and biochemistry could have proved him wrong on many counts, and they did not• Scientific evidence supports the theory that living species descended with modification from common ancestors that lived in the past
  55. 55. Limitations of Research• The Grants data shows how competition and climate change affects natural selection• However, they did not observe the formation of a new species
  56. 56. Unanswered Questions• Many new discoveries have led to new hypotheses that refine and expand Darwin’s original ideas• No scientist suggests that all evolutionary processes are fully understood. Many unanswered questions remain
  57. 57. Why Understanding Evolution is Important?• Evolution continues todayEx.)• Drug resistance in bacteria and viruses• Pesticide resistance in insects• Evolutionary theory helps us understand and respond to these changes in ways that improve human life