1. BIOE 109 Evolution Summer 2009 Lecture 3- Part I Natural selection – theory and definitions

2. Some important principles of natural selection

3. Some important principles of natural selection 1. Natural selection (usually) acts at the level of individuals, not populations.

4. Some important principles of natural selection 1. Natural selection (usually) acts at the level of individuals, not populations. 2. Populations, not individuals, evolve.

5. Some important principles of natural selection 1.Natural selection (usually) acts at the level of individuals, not populations. 2. Populations, not individuals, evolve. 3. Natural selection is retrospective and cannot predict the future.

6. Some important principles of natural selection 1.Natural selection (usually) acts at the level of individuals, not populations. 2. Populations, not individuals, evolve. 3. Natural selection is retrospective and cannot predict the future. 4. Natural selection is not necessarily progressive.

7. Some important principles of natural selection 1.Natural selection (usually) acts at the level of individuals, not populations. 2. Populations, not individuals, evolve. 3. Natural selection is retrospective and cannot predict the future. 4. Natural selection is not necessarily progressive. 5. Product of selection is a “compromise”.

8. What is fitness?

9. What is fitness?

10. Natural selection and the concept of fitness Darwinian fitness: the number of gene copies (i.e., offspring) a phenotype places into the next generation.

11. Natural selection and the concept of fitness Darwinian fitness: the number of gene copies (i.e., offspring) a phenotype places into the next generation. Relative fitness: a phenotype’s Darwinian fitness relative to other phenotypes. http://www.blackwellpublishing.com/ridley/video_gallery/LP_What_is_fitness.asp

12. What is fitness? 1. Fitness is a description not an explanation

13. What is fitness? 1. Fitness is a description not an explanation 2. Fitness is an average property

14. What is fitness? 1. Fitness is a description not an explanation. 2. Fitness is an average property. 3. Total fitness is comprised of several individual components:

15. What is fitness? 1. Fitness is a description not an explanation. 2. Fitness is an average property. 3. Total fitness is comprised of several individual components: Total fitness = viability + fecundity + longevity + mating success

16. Natural selection at a single locus 1. Purifying selection

17. Natural selection at a single locus 1. Purifying selection a form of selection acting against deleterious (harmful) alleles.

18. Natural selection at a single locus 1. Purifying selection a form of selection acting against deleterious (harmful) alleles. the majority of deleterious alleles are recessive.

19. Natural selection at a single locus 1. Purifying selection a form of selection acting against deleterious (harmful) alleles. the majority of deleterious alleles are recessive. purifying selection drives deleterious recessives to low frequencies where they are maintained at mutation-selection balance:

20. Natural selection at a single locus 1. Purifying selection a form of selection acting against deleterious (harmful) alleles. the majority of deleterious alleles are recessive. purifying selection drives deleterious recessives to low frequencies where they are maintained at mutation-selection balance: rate of introduction = rate of removal by mutation by selection e.g., Tay-Sachs disease, cystic fibrosis, etc.

21. Natural selection at a single locus 2. Directional selection

22. Natural selection at a single locus 2. Directional selection a form of selection acting on advantageous mutations.

23. Natural selection at a single locus 2. Directional selection a form of selection acting on advantageous mutations. the selectively favored allele “sweeps” through the population to become fixed (i.e., reach a frequency of 1.0).

24. Natural selection at a single locus 2. Directional selection a form of selection acting on advantageous mutations. the selectively favored allele “sweeps” through the population to become fixed (i.e., reach a frequency of 1.0). Example:Genotype:AA Aa aa

25. Natural selection at a single locus 2. Directional selection a form of selection acting on advantageous mutations. the selectively favored allele “sweeps” through the population to become fixed (i.e., reach a frequency of 1.0). Example:Genotype:AA Aa aa Fitness:w AA w Aa w aa

26. Natural selection at a single locus 2. Directional selection a form of selection acting on advantageous mutations. the selectively favored allele “sweeps” through the population to become fixed (i.e., reach a frequency of 1.0). Example:Genotype:AA Aa aa Fitness:w AA w Aa w aa 1.01.0051.010

27. Natural selection at a single locus 2. Directional selection a form of selection acting on advantageous mutations. the selectively favored allele “sweeps” through the population to become fixed (i.e., reach a frequency of 1.0). Example:Genotype:AA Aa aa Fitness:w AA w Aa w aa 1.01.0051.010 here, the small a allele would reach fixation in about 3,000 generations.

29. Natural selection at a single locus 3. Balancing selection

30. Natural selection at a single locus 3. Balancing selection - various forms of selection that lead to the active maintenance of genetic variation in natural populations.

31. Natural selection at a single locus 3. Balancing selection - various forms of selection that lead to the active maintenance of genetic variation in natural populations. - alleles are said to be “balanced” because a stable equilibrium state is reached.

32. Natural selection at a single locus 3. Balancing selection - various forms of selection that lead to the active maintenance of genetic variation in natural populations. - alleles are said to be “balanced” because a stable equilibrium state is reached. - if allele frequencies are perturbed from this equilibrium, selection will return them back to that state.

33. Forms of balancing selection 1. Overdominance

34. Forms of balancing selection 1. Overdominance - occurs when the heterozygote is more fit than either alternate homozygote.

35. Forms of balancing selection 1. Overdominance - occurs when the heterozygote is more fit than either alternate homozygote. Genotype:AA Aa aa

36. Forms of balancing selection 1. Overdominance - occurs when the heterozygote is more fit than either alternate homozygote. Genotype:AA Aa aa Fitness:w AA w Aa w aa

37. Forms of balancing selection 1. Overdominance - occurs when the heterozygote is more fit than either alternate homozygote. Genotype:AA Aa aa Fitness:w AA w Aa w aa 0.88 1 0.14

38. Forms of balancing selection 1. Overdominance - occurs when the heterozygote is more fit than either alternate homozygote. Genotype:AA Aa aa Fitness:w AA w Aa w aa 0.88 1 0.14 Example: Sickle cell hemoglobin in west-central Africa

39. Alleles: Hb A = normal allele Hb S = sickle cell allele

40. Example: Sickle cell hemoglobin in west-central Africa Alleles: Hb A = normal allele Hb S = sickle cell allele Genotypes: Hb A Hb A :susceptible to malaria Hb A Hb S :resistant to malaria, mild anemia Hb S Hb S :susceptible to severe anemia

41. Example: Sickle cell hemoglobin in west-central Africa Alleles: Hb A = normal allele Hb S = sickle cell allele Genotypes: Hb A Hb A :susceptible to malaria Hb A Hb S :resistant to malaria, mild anemia Hb S Hb S :susceptible to severe anemia results in stable polymorphic equilibrium where Hb A = 0.89 and Hb S = 0.11

42. Forms of balancing selection 2. Frequency-dependent selection

43. Forms of balancing selection 2. Frequency-dependent selection the relative fitness of genotypes are not constant but vary with their frequencies in the population.

44. Forms of balancing selection 2. Frequency-dependent selection the relative fitnesses of genotypes are not constant but vary with their frequencies in the population. Genotype: AAAa aa Fitness: w AA w Aa w aa 1-p 2 1-2pq 1-q 2

45. Forms of balancing selection 2. Frequency-dependent selection the relative fitnesses of genotypes are not constant but vary with their frequencies in the population. Genotype: AAAa aa Fitness: w AA w Aa w aa 1-p 2 1-2pq 1-q 2 Example: Self-incompatibility (S) loci in flowering plants

46. S loci in flowering plants

47. ● leads to obligate out-crossing

48. S loci in flowering plants ● leads to obligate out-crossing ● at equilibrium, all S alleles occur at equal frequencies

49. Forms of balancing selection 3. Spatially or temporally varying selection - some genotypes are more fit than others in some habitats, or under some environmental conditions, than others.

50. Environment A Genotype: AA Aa aa Fitness: w AA w Aa w aa 1 0.95 0.91   gene flow Environment B Genotype: AA Aa aa Fitness: w AA w Aa w aa 0.84 0.93 1

51. 2-year female morph cycle: Uta stansburiana Orange females small eggs large clutches Yellow females large eggs small clutches

52. 2-year female cycle: Uta stansburiana 89909192939495969798 Year 990001 0.10 0.40 0.50 0.55 0.45 0.35 0.30 0.25 0.20 0.15 0.00 0.05 Orange female frequency Number of Breeding Females (Modified from Sinervo et al., 2000) Population status Orange commonYellow common Fitness of rare O strategy 1.001.61 Fitness of rare Y strategy 1.601.00

53. Convergent evolution Parallel evolution

54. Convergent evolution Parallel evolution

55. Convergent evolution Parallel evolution

56. Convergent evolution: the evolution of similar traits independently in distantly related taxa from different ancestral features or from different developmental pathways Example: marsupial and placental mammals (common ancestor ~ 170 mya)

57. Convergent evolution between placental and marsupial mammals

58. Convergent evolution in crocodiles and hippos Both have eyes on top of their heads

59. Parallel evolution: the evolution of similar traits independently in closely related taxa involving the same genes or developmental pathways

60. Example: hemoglobins in high altitude geese

61. Bar-headed goose, Anser indicus Lives > 4,000 m in Himalayas Andean goose, Chloephaga melanoptera Lives > 3,500 m in Andes

62. 3D structure of hemoglobin

63.  119  55 x x

64. Bar-headed gooseprolineleucine  alanine x x

65.  119  55 Bar-headed gooseprolineleucine  alanine Andean gooseprolineleucine  serine x x

66. Recap Concept of fitness: Darwinian and relative fitness Types of selection: Purifying selection Directional selection Balancing selection over-dominancefrequency-dependentspatially/ temporally varying Convergent and parallel evolution