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BIOE 109 Summer 2009 Lecture 6- Part I Microevolution – Random genetic drift

BIOE 109 Summer 2009 Lecture 6- Part I Microevolution – Random genetic drift. Random genetic drift Definition: random changes in the frequencies of neutral alleles from generation to generation caused by “ accidents of sampling ”. Random genetic drift

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BIOE 109 Summer 2009 Lecture 6- Part I Microevolution – Random genetic drift

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  1. BIOE 109 Summer 2009 Lecture 6- Part I Microevolution – Random genetic drift

  2. Random genetic drift Definition: random changes in the frequencies of neutral alleles from generation to generation caused by “accidents of sampling”

  3. Random genetic drift Definition: random changes in the frequencies of neutral alleles from generation to generation caused by “accidents of sampling” Q: What is a neutral allele?

  4. Random genetic drift Definition: random changes in the frequencies of neutral alleles from generation to generation caused by “accidents of sampling” Q: What is a neutral allele? A: A neutral allele has no effect on fitness.

  5. Random genetic drift Definition: random changes in the frequencies of neutral alleles from generation to generation caused by “accidents of sampling” Q: What is a neutral allele? A: A neutral allele has no effect on fitness. Q: How can alleles be neutral?

  6. How can alleles be neutral?

  7. How can alleles be neutral? 1. Mutations among very similar amino acids

  8. How can alleles be neutral? 1. Mutations among very similar amino acids (leu)  (val) CUC GUC

  9. How can alleles be neutral? 1. Mutations among very similar amino acids (leu)  (val) CUC GUC 2. Mutations involving silent (synonymous) changes

  10. How can alleles be neutral? 1. Mutations among very similar amino acids (leu)  (val) CUC GUC 2. Mutations involving silent (synonymous) changes (leu)  (leu) CUC CUU

  11. How can alleles be neutral? 1. Mutations among very similar amino acids (leu)  (val) CUC GUC 2. Mutations involving silent (synonymous) changes (leu)  (leu) CUC CUU 3. Mutations in non-coding (nonfunctional) DNA

  12. Codon bias shows that silent changes may not be neutral!

  13. Random genetic drift Definition: random changes in the frequencies of neutral alleles from generation to generation caused by “accidents of sampling” What are RANDOM changes in frequencies? What is sampling error?

  14. Some properties of random genetic drift

  15. Some properties of random genetic drift 1. Magnitude inversely proportional to effective population size (Ne).

  16. Some properties of random genetic drift 1. Magnitude inversely proportional to effective population size (Ne). 2. Ultimately results in loss of variation from natural populations.

  17. Some properties of random genetic drift 1. Magnitude inversely proportional to effective population size (Ne). 2. Ultimately results in loss of variation from natural populations. 3. The probability of fixation of a neutral allele is equal to its frequency in the population.

  18. Some properties of random genetic drift 1. Magnitude inversely proportional to effective population size (Ne). 2. Ultimately results in loss of variation from natural populations. 3. The probability of fixation of a neutral allele is equal to its frequency in the population. 4. Will cause isolated populations to diverge genetically.

  19. Some properties of random genetic drift 1. Magnitude inversely proportional to effective population size (Ne). 2. Ultimately results in loss of variation from natural populations. 3. The probability of fixation of a neutral allele is equal to its frequency in the population. 4. Will cause isolated populations to diverge genetically. 5. Is accentuated during population bottlenecks and founder events.

  20. What is effective population size?

  21. What is effective population size? • in any one generation, Ne is roughly equivalent to the number of breeding individuals in the population.

  22. What is effective population size? • in any one generation, Ne is roughly equivalent to the number of breeding individuals in the population. • this is equivalent to a contemporary effective size.

  23. What is effective population size? • in any one generation, Ne is roughly equivalent to the number of breeding individuals in the population. • this is equivalent to a contemporary effective size. • Ne is also strongly influenced by long-term history.

  24. What is effective population size? • in any one generation, Ne is roughly equivalent to the number of breeding individuals in the population. • this is equivalent to a contemporary effective size. • Neis also strongly influenced by long-term history. • this is equivalent to a species’ evolutionary effective size.

  25. Factors affecting effective population size

  26. Factors affecting effective population size 1. Fluctuations in population size

  27. Factors affecting effective population size 1. Fluctuations in population size - here, Ne equals the harmonic mean of the actual population numbers:

  28. Factors affecting effective population size 1. Fluctuations in population size - here, Ne equals the harmonic mean of the actual population numbers: 1/Ne = 1/t(1/N1 + 1/N2 + 1/N3 + … 1/Nt)

  29. Factors affecting effective population size 1. Fluctuations in population size - here, Ne equals the harmonic mean of the actual population numbers: 1/Ne = 1/t(1/N1 + 1/N2 + 1/N3 + … 1/Nt) Example: Over 3 generations, N = 2000, 30, 2000

  30. Factors affecting effective population size 1. Fluctuations in population size - here, Ne equals the harmonic mean of the actual population numbers: 1/Ne = 1/t(1/N1 + 1/N2 + 1/N3 + … 1/Nt) Example: Over 3 generations, N = 2000, 30, 2000 Arithmetic mean = 1343.3

  31. Factors affecting effective population size 1. Fluctuations in population size - here, Ne is equal to the harmonic mean of the actual population numbers: 1/Ne = 1/t(1/N1 + 1/N2 + 1/N3 + … 1/Nt) Example: Over 3 generations, N = 2000, 30, 2000 Arithmetic mean = 1343.3 Harmonic mean = 87.4

  32. Factors affecting effective population size 2. Unequal numbers of males and females

  33. Factors affecting effective population size 2. Unequal numbers of males and females • let Nm = No. of males, Nf = No. of females:

  34. Factors affecting effective population size 2. Unequal numbers of males and females • let Nm = No. of males, Nf = No. of females: Ne = 4NmNf Nm + Nf

  35. Factors affecting effective population size 2. Unequal numbers of males and females • let Nm = No. of males, Nf = No. of females: Ne = 4NmNf Nm + Nf Example: Breeding populations of northern elephant seals:

  36. Factors affecting effective population size 2. Unequal numbers of males and females • let Nm = No. of males, Nf = No. of females: Ne = 4NmNf Nm + Nf Example: Breeding populations of northern elephant seals: • 15 alpha males each controlling a harem of 20 females:

  37. Factors affecting effective population size 2. Unequal numbers of males and females • let Nm = No. of males, Nf = No. of females: Ne = 4NmNf Nm + Nf Example: Breeding populations of northern elephant seals: • 15 alpha males each controlling a harem of 20 females: census size (N) = 315

  38. Factors affecting effective population size 2. Unequal numbers of males and females • let Nm = No. of males, Nf = No. of females: Ne = 4NmNf Nm + Nf Example: Breeding populations of northern elephant seals: • 15 alpha males each controlling a harem of 20 females: census size (N) = 315 effective size (Ne) = 57.1

  39. Factors affecting effective population size 3. Large variance in reproductive success • reduces Ne because a small number of individuals have a disproportional effect on the reproductive success of the population.

  40. Factors affecting effective population size 4. Genetic Bottlenecks • genetic bottlenecks refer to severe reductions in effective population size. Ne Time

  41. Genetic Bottlenecks • genetic bottlenecks refer to severe reductions in effective population size.

  42. Genetic Bottlenecks • genetic bottlenecks refer to severe reductions in effective population size. Examples: the northern elephant seal (Mirounga angustirostis) and the cheetah (Acinonyx jubatus).

  43. Founder effects • occur when a new population is founded from a small number of individuals.

  44. Founder effects • occur when a new population is founded from a small number of individuals. Consequences:

  45. Founder effects • occur when a new population is founded from a small number of individuals. Consequences: 1. New population has a fraction of genetic variation present in the ancestral population.

  46. Founder effects • occur when a new population is founded from a small number of individuals. Consequences: 1. New population has a fraction of genetic variation present in the ancestral population. 2. Initial allele frequencies differ because of chance.

  47. Founder effects • occur when a new population is founded from a small number of individuals. Consequences: 1. New population has a fraction of genetic variation present in the ancestral population. 2. Initial allele frequencies differ because of chance. Example: the silvereye, Zosterops lateralis

  48. Founder effects in the silvereye, Zosterops lateralis

  49. Timing of island hopping…

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