1. Chapter 23 The Evolution of Populations

2. Population Genetics u The study of genetic variation in populations. u Represents the reconciliation of Mendelism and Darwinism.

3. Population Genetics u Use population genetics as the means to track and study evolution. u Looks at the genetic basis of variation and natural selection.

4. Sources of Variation u Sexual Reproduction u Random Assortment of Chromosomes u Random Fertilization u Crossing Over u Mutation

5. Mutations u Inherited changes in a gene. u Rates low in most cases due to DNA repair etc.

6. Population u A localized group of individuals of the same species.

7. Species u A group of similar organisms. u A group of populations that could interbreed.

8. Gene Pool u The total collective genes in a population. u If evolution is occurring, then changes must occur in the gene pool of the population over time.

9. Microevolution u Changes in the relative frequencies of alleles in the gene pool.

10. Hardy-Weinberg Theorem u A mathematical way to measure evolution. u Population is evolving if the allele frequency is changing (favoring one allele)

11. Basic Equation u p + q = 1 u p = % dominant allele u q = % recessive allele

12. Expanded Equation u p + q = 1 u (p + q) 2 = (1) 2 u p 2 + 2pq + q 2 = 1

13. Genotypes u p 2 = Homozygous Dominants 2pq = Heterozygous q 2 = Homozygous Recessives

14. Importance of Hardy-Weinberg u Yardstick to measure rates of evolution. u Predicts that gene frequencies should NOT change over time as long as the HW assumptions hold (no evolution should occur). u Way to calculate gene frequencies through time.

15. Example u What is the frequency of the PKU allele? u PKU is expressed only if the individual is homozygous recessive (aa).

16. PKU Frequency u PKU is found at the rate of 1/10,000 births. u PKU = aa = q 2 q 2 =.0001 q =.01

17. Dominant Allele u p + q = 1 p = 1- q p = 1-.01 p =.99

18. Expanded Equation u p 2 + 2pq + q 2 = 1 (.99) 2 + 2(.99x.01) + (.01) 2 = 1.9801 +.0198 +.0001 = 1

19. Final Results u Normals (AA) = 98.01% u Carriers (Aa) = 1.98% u PKU (aa) =.01%

20. Hardy-Weinberg Assumptions 1. Large Population 2. Isolation 3. No Net Mutations 4. Random Mating 5. No Natural Selection

21. If H-W assumptions hold true: u The gene frequencies will not change over time. u Evolution will not occur. u But, how likely will natural populations hold to the H-W assumptions?

22. Microevolution u Caused by violations of the 5 H-W assumptions.

23. Causes of Microevolution 1. Genetic Drift 2. Gene Flow 3. Mutations 4. Nonrandom Mating 5. Natural Selection

24. Genetic Drift u Changes in the gene pool of a small population by chance. u Types: u 1. Bottleneck Effect u 2. Founder's Effect

25. By Chance

26. Bottleneck Effect u Loss of most of the population by disasters. u Surviving population may have a different gene pool than the original population.

28. Result u Some alleles lost. u Other alleles are over- represented. u Genetic variation usually lost.

29. Importance u Reduction of population size may reduce gene pool for evolution to work with. u Ex: Cheetahs

30. Founder's Effect u Genetic drift in a new colony that separates from a parent population. u Ex: Old-Order Amish

31. Result u Genetic variation reduced. u Some alleles increase in frequency while others are lost (as compared to the parent population).

32. Importance u Very common in islands and other groups that don't interbreed.

33. Gene Flow u Movement of genes in/out of a population. u Ex: u Immigration u Emigration

34. Result u Changes in gene frequencies within a population. u Immigration often brings new alleles into populations increasing genetic diversity.

35. Result - Mutations u May change gene frequencies (small population). u Source of new alleles for selection. u Often lost by genetic drift.

36. Nonrandom Mating u Failure to choose mates at random from the population.

37. Causes u Inbreeding within the same “neighborhood”. u Assortative mating (like with like).

38. Result u Increases the number of homozygous loci. u Does not in itself alter the overall gene frequencies in the population.

39. Natural Selection u Differential success in survival and reproduction. u Result - Shifts in gene frequencies.

40. Fitness - Darwinian u The relative contribution an individual makes to the gene pool of the next generation.

41. u End of part 1

42. Rate of Selection u Differs between dominant and recessive alleles. u Selection pressure by the environment.

43. Modes of Natural Selection 1. Stabilizing 2. Directional 3. Diversifying 4. Sexual

44. Homework u Read – Chapter 23, 24, 26 u Lab – Population Genetics u Discussion Board – Fri. 2/25 u Chapter 23 – Fri. 2/25 u Chapter 26 – Mon. 2/28

46. Stabilizing u Selection toward the average and against the extremes. u Ex: birth weight in humans

47. Directional Selection u Selection toward one extreme. u Ex: running speeds in race animals. u Ex. Galapagos Finch beak size and food source.

49. Diversifying u Selection toward both extremes and against the norm. u Ex: bill size in birds

51. Comment u Diversifying Selection - can split a species into several new species if it continues for a long enough period of time and the populations don’t interbreed.

53. Sexual Mate selection u May not be adaptive to the environment, but increases reproduction success of the individual. u This is a VERY important selection type for species.

54. Result u Sexual dimorphism. u Secondary sexual features for attracting mates.

55. Comments u Females may drive sexual selection and dimorphism since they often "choose" the mate.

56. Preserving Genetic Variation 1. Diploidy - preserves recessives as heterozygotes. 2. Balanced Polymorphisms - preservation of diversity by natural selection.

57. Example u Heterozygote Advantage - When the heterozygote or hybrid survives better than the homozygotes. Also called Hybrid vigor.

58. Result u Can't bred "true“ and the diversity of the population is maintained. u Ex – Sickle Cell Anemia

60. Comment u Population geneticists believe that ALL genes that persist in a population must have had a selective advantage at one time. u Ex – Sickle Cell and Malaria, Tay-Sachs and Tuberculosis

61. Question u Does evolution result in perfect organisms?

62. Answer - No 1. Historical Constraints 2. Compromises 3. Non-adaptive Evolution (chance) 4. Available variations – most come from using a current gene in a new way.

63. Summary u Know the difference between a species and a population. u Know that the unit of evolution is the population and not the individual.

64. Summary u Know the H-W equations and how to use them in calculations. u Know the H-W assumptions and what happens if each is violated.

65. Summary u Know the various modes of natural selection. u Identify various means to introduce genetic variation into populations.