1. 1 Genes Within Populations Chapter 20

2. 2 Natural selection: mechanism of evolutionary change Inheritance of acquired characteristics: Proposed by Jean-Baptiste Lamarck Individuals passed on physical and behavioral changes to their offspring Variation by experience…not genetic Darwin’s natural selection: variation a result of preexisting genetic differences

3. 3 Lamarck’s theory of how giraffes’ long necks evolved

4. 4 Darwin: Evolution is descent with modification Evolution: changes through time 1.Species accumulate difference 2.Descendants differ from their ancestors 3.New species arise from existing ones Genetic Variation and Evolution

5. 5 Natural selection: proposed by Darwin as the mechanism of evolution individuals have specific inherited characteristics they produce more surviving offspring the population includes more individuals with these specific characteristics the population evolves and is better adapted to its present environment Evolution requires study of Population Genetics Natural selection: mechanism of evolutionary change

6. 6 Darwin’s theory for how long necks evolved in giraffes

7. 7 Measuring levels of genetic variation –blood groups –enzymes Enzyme polymorphism – A locus with more variation than can be explained by mutation is termed polymorphic. –Natural populations tend to have more polymorphic loci than can be accounted for by mutation. DNA sequence polymorphism Gene Variation in Nature

8. 8 Godfrey H. Hardy: English mathematician Wilhelm Weinberg: German physician Concluded that: The original proportions of the genotypes in a population will remain constant from generation to generation as long as five assumptions are met Hardy-Weinberg Principle

9. 9 Five assumptions : 1.No mutation takes place 2.No genes are transferred to or from other sources 3.Random mating is occurring 4.The population size is very large 5.No selection occurs Hardy-Weinberg Principle

10. 10 Calculate genotype frequencies with a binomial expansion (p+q) 2 = p 2 + 2pq + q 2 p = individuals homozygous for first allele 2pq = individuals heterozygous for both alleles q = individuals homozygous for second allele because there are only two alleles: p plus q must always equal 1 Hardy-Weinberg Principle

11. 11 Hardy-Weinberg Principle Since there are 16 white (bb) their freq is q 2 =0.16. q=0.4 p=1.0 - 0.4=0.6 p 2 =0.362pq=0.24

12. 12 Using Hardy-Weinberg equation to predict frequencies in subsequent generations Hardy-Weinberg Principle

13. 13 A population not in Hardy-Weinberg equilibrium indicates that one or more of the five evolutionary agents are operating in a population Five agents of evolutionary change

14. 14 A population not in Hardy-Weinberg equilibrium indicates that one or more of the five evolutionary agents are operating in a population Five agents of evolutionary change

15. 15 A population not in Hardy-Weinberg equilibrium indicates that one or more of the five evolutionary agents are operating in a population Five agents of evolutionary change

16. 16 Agents of Evolutionary Change Mutation: A change in a cell’s DNA –Mutation rates are generally so low (1/100,000 divisions) they have little effect on Hardy- Weinberg proportions of common alleles. –Ultimate source of genetic variation Gene flow: A movement of alleles from one population to another –Powerful agent of change –Tends to homogenize allele frequencies

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18. 18 Agents of Evolutionary Change Nonrandom Mating: mating with specific genotypes –Shifts genotype frequencies –Assortative Mating: does not change frequency of individual alleles; increases the proportion of homozygous individuals –Disassortative Mating: phenotypically different individuals mate; produce excess of heterozygotes

19. 19 Genetic Drift Genetic drift: Random fluctuation in allele frequencies over time by chance important in small populations –founder effect - few individuals found new population (small allelic pool) –bottleneck effect - drastic reduction in population, and gene pool size

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21. 21 Genetic Drift: A bottleneck effect

22. 22 Bottleneck effect: case study

23. 23 Selection Artificial selection: a breeder selects for desired characteristics

24. 24 Selection Natural selection: environmental conditions determine which individuals in a population produce the most offspring 3 conditions for natural selection to occur –Variation must exist among individuals in a population –Variation among individuals must result in differences in the number of offspring surviving –Variation must be genetically inherited

25. 25 Selection

26. 26 Pocket mice from the Tularosa Basin Selection

27. 27 Selection to match climatic conditions Enzyme allele frequencies vary with latitude Lactate dehydrogenase in Fundulus heteroclitus (mummichog fish) varies with latitude Enzymes formed function differently at different temperatures North latitudes: Lactate dehydrogenase is a better catalyst at low temperatures

28. 28 Selection for pesticide resistance

29. 29 Selection for pesticide resistance

30. 30 Fitness and Its Measurement Fitness: A phenotype with greater fitness usually increases in frequency –Most fit is given a value of 1 Fitness is a combination of: –Survival: how long does an organism live –Mating success: how often it mates –Number of offspring per mating that survive

31. 31 Body size and egg-laying in water striders Fitness and its Measurement

32. 32 Body size and egg-laying in water striders Fitness and its Measurement

33. 33 Body size and egg-laying in water striders Favors medium body size Fitness and its Measurement

34. 34 Interactions Among Evolutionary Forces Mutation and genetic drift may counter selection The magnitude of drift is inversely related to population size In very small populations drift can be more important than natural selection. Usually natural selection is predominate force

35. 35 Gene flow may promote or constrain evolutionary change –Spread a beneficial mutation –Impede adaptation by continual flow of inferior alleles from other populations Extent to which gene flow can hinder the effects of natural selection depends on the relative strengths of gene flow –High in birds & wind-pollinated plants –Low in sedentary species Interactions Among Evolutionary Forces

36. 36 Degree of copper tolerance Interactions Among Evolutionary Forces

37. 37 Maintenance of Variation Frequency-dependent selection: depends on how frequently or infrequently a phenotype occurs in a population –Negative frequency-dependent selection: rare phenotypes are favored by selection Prey are used to looking for more common types –Positive frequency-dependent selection: common phenotypes are favored variation is eliminated from the population Strength of selection changes through time (Oscillating Selection)

38. 38 Negative frequency - dependent selection Most common water boatman are eaten because fish are more used to identifying these. Maintenance of Variation

39. 39 Positive frequency-dependent selection Off type is selected & eaten Maintenance of Variation

40. 40 Oscillating selection: selection favors one phenotype at one time, and a different phenotype at another time Galápagos Islands ground finches –Wet conditions favor big bills (abundant seeds) –Dry conditions favor small bills Maintenance of Variation

41. 41 Fitness of a phenotype does not depend on its frequency Environmental changes lead to oscillation in selection Maintenance of Variation

42. 42 Heterozygotes may exhibit greater fitness than homozygotes Heterozygote advantage: keep deleterious alleles in a population Example: Sickle cell anemia Homozygous recessive phenotype: exhibit severe anemia Maintenance of Variation

43. 43 Homozygous dominant phenotype: no anemia; susceptible to malaria Heterozygous phenotype: no anemia; less susceptible to malaria Maintenance of Variation

44. 44 Maintenance of Variation Frequency of sickle cell allele

45. 45 Disruptive selection acts to eliminate intermediate types Maintenance of Variation

46. 46 Disruptive selection for large and small beaks in black-bellied seedcracker finch of west Africa Maintenance of Variation

47. 47 Directional selection: acts to eliminate one extreme from an array of phenotypes Maintenance of Variation

48. 48 Directional selection for negative phototropism in Drosophila Maintenance of Variation

49. 49 Stabilizing selection: acts to eliminate both extremes Maintenance of Variation

50. 50 Stabilizing selection for birth weight in humans Maintenance of Variation

51. 51 Experimental Studies of Natural Selection In some cases, evolutionary change can occur rapidly Evolutionary studies can be devised to test evolutionary hypotheses Guppy studies (Poecilia reticulata) in the lab and field –Populations above the waterfalls: low predation –Populations below the waterfalls: high predation

52. 52 High predation environment - Males exhibit drab coloration and tend to be relatively small and reproduce at a younger age. Low predation environment - Males display bright coloration, a larger number of spots, and tend to be more successful at defending territories. Experimental Studies

53. 53 The evolution of protective coloration in guppies Experimental Studies

54. 54 The laboratory experiment –10 large pools –2000 guppies –4 pools with pike cichlids (predator) –4 pools with killifish (nonpredator) –2 pools as control (no other fish added) –10 generations Experimental Studies

55. 55 The field experiment –Removed guppies from below the waterfalls (high predation) –Placed guppies in pools above the falls –10 generations later, transplanted populations evolved the traits characteristic of low-predation guppies Experimental Studies

56. 56 Evolutionary change in spot number Experimental Studies

57. 57 The Limits of Selection Genes have multiple effects –Pleiotropy: sets limits on how much a phenotype can be altered Evolution requires genetic variation –Thoroughbred horse speed

58. 58 Selection for increased speed in racehorses is no longer effective Experimental Studies