1. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings PowerPoint Lectures for Biology, Seventh Edition Neil Campbell and Jane Reece Lectures by Chris Romero Chapter 23 The Evolution of Populations

2. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Overview: The Smallest Unit of Evolution One misconception is that organisms evolve, in the Darwinian sense, during their lifetimes Natural selection acts on indiv’s, but only pop’s evolve Genetic variations in pop’s contribute to evolution

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4. Concept 23.1: Population genetics provides a foundation for studying evolution Microevolution- change in the genetic makeup of a pop from generation to generation

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6. The Modern Synthesis Pop genetics- study of how pop’s change genetically over time Population genetics integrates Mendelian genetics with the Darwinian theory of evolution by natural selection This modern synthesis focuses on pop’s as units of evolution

7. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Gene Pools and Allele Frequencies Pop– localized group of indiv’s capable of interbreeding & producing fertile offspring Gene pool- total aggregate of genes in a pop at any one time gene pool has all gene loci in all indiv’s of the pop

8. LE 23-3 MAP AREA CANADA ALASKA Beaufort Sea Porcupine herd range NORTHWEST TERRITORIES Fairbanks Fortymile herd range Whitehorse ALASKA YUKON

9. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings The Hardy-Weinberg Theorem The Hardy-Weinberg theorem describes a pop that is not evolving freq’s of alleles & genotypes in a pop’s gene pool remain constant from generation to gen, provided that only Mendelian segregation & recombination of alleles are at work Mendelian inheritance p reserves genetic variation in a pop

10. LE 23-4 Generation 3 25% C R C R Generation 4 50% C R C W 25% C W C W 50% C W gametes 50% C R come together at random 25% C R C R 50% C R C W 25% C W C W Alleles segregate, and subsequent generations also have three types of flowers in the same proportions gametes Generation 2 Generation 1 CRCRCRCR CWCWCWCW genotype Plants mate All C R C W (all pink flowers) 50% C R 50% C W gametes come together at random X

11. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Preservation of Allele Frequencies In a given pop where gametes contribute to the next generation randomly, allele freq’s will not change

12. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Hardy-Weinberg Equilibrium HW equilibrium describes a pop in which random mating occurs It describes a pop where allele freq’s do not change

13. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings If p & q represent the relative freq’s of the only 2 possible alleles in a pop at a particular locus, then – p 2 + 2pq + q 2 = 1 – p 2 & q 2 represent the freq’s of the homozygous genotypes & 2pq represents the freq of the heterozygous genotype

14. LE 23-5 Gametes for each generation are drawn at random from the gene pool of the previous generation: 80% C R (p = 0.8) 20% C W (q = 0.2) Sperm C R (80%) C W (20%) pqp2p2 16% C R C W 64% C R Eggs C W (20%) C R (80%) 16% C R C W qp 4% C W q2q2

15. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Conditions for Hardy-Weinberg Equilibrium HW theorem describes a hypothetical pop In real populations, allele & genotype freq’s change over time

16. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 5 conditions for non-evolving pop’s are rarely met in nature: – Extremely lg pop size – No gene flow – No mutations – Random mating – No natural selection

17. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Population Genetics and Human Health We can use the HW equation to estimate the % of the human pop carrying the allele for an inherited disease

18. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Concept 23.2: Mutation and sexual recombination produce the variation that makes evolution possible mutation & sexual recombination, produce the variation in gene pools that contributes to diff’s among indiv’s

19. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Mutation Mutations are changes in the nt seq of DNA Mutations cause new genes & alleles to arise Animation: Genetic Variation from Sexual Recombination Animation: Genetic Variation from Sexual Recombination

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21. Point Mutations A point mutation is a change in 1 base in a gene It is usually harmless but may have significant impact on phenotype

22. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Mutations That Alter Gene Number or Sequence Chromosomal mutations that delete, disrupt, or rearrange many loci are typically harmful Gene duplication is nearly always harmful

23. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Mutation Rates Mutation rates are low in animals & plants avg is ~ 1 mutation in every 100,000 genes per generation Mutations are more rapid in microorganisms

24. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Sexual Recombination Sexual recomb is far more impt than mutation in producing the genetic diff’s that make adaptation possible

25. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Concept 23.3: Natural selection, genetic drift, and gene flow can alter a population’s genetic composition 3 major factors alter allele frequencies & bring about most evolutionary change: – Natural selection – Genetic drift – Gene flow

26. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Natural Selection Differential success in reproduction results in certain alleles being passed to the next generation in greater proportions

27. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Genetic Drift The smaller a sample, the > the chance of deviation from a predicted result Genetic drift– describes how allele frequencies fluctuate unpredictably from 1 generation to the next Genetic drift can reduce genetic variation by losses of alleles Animation: Causes of Evolutionary Change Animation: Causes of Evolutionary Change

28. LE 23-7 CRCRCRCR CRCRCRCR CWCWCWCW CRCRCRCR CRCWCRCW CRCRCRCR CRCWCRCW CWCWCWCW CWCWCWCW CRCWCRCW CRCWCRCW CRCRCRCR CRCWCRCW CRCWCRCW CRCRCRCR CRCRCRCR CRCWCRCW CWCWCWCW CRCWCRCW CRCRCRCR Only 5 of 10 plants leave offspring Only 2 of 10 plants leave offspring CRCRCRCR CRCRCRCR CRCRCRCR CRCRCRCR CRCRCRCR CRCRCRCR CRCRCRCR CRCRCRCR CRCRCRCR CRCRCRCR Generation 2 p = 0.5 q = 0.5 Generation 3 p = 1.0 q = 0.0 Generation 1 p (frequency of C R ) = 0.7 q (frequency of C W ) = 0.3

29. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings The Bottleneck Effect bottleneck effect- sudden change in the environment that may drastically reduce the size of a pop resulting gene pool may no longer be reflective of the original pop’s gene pool

30. LE 23-8 Original population Bottlenecking event Surviving population

31. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Understanding the bottleneck effect can ↑ understanding of how human activity affects other species

32. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings The Founder Effect founder effect– occurs when a few indiv’s become isolated from a larger pop It can affect allele freq’s in a pop

33. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Gene Flow Gene flow– consists of genetic additions or subtractions from a pop, resulting from mvmt of fertile indiv’s or gametes Gene flow causes a pop to gain or lose alleles It tends to reduce diff’s b/w pop’s over time

34. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Concept 23.4: Natural selection is the primary mechanism of adaptive evolution Natural selection accumulates & maintains favorable genotypes in a pop

35. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Genetic Variation Genetic variation occurs in indiv’s in pop’s of all species It is not always heritable Like the butterflies on the following page have the same genotype

36. LE 23-9 Map butterflies that emerge in spring: orange and brown Map butterflies that emerge in late summer: black and white

37. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Variation Within a Population Both discrete & quantitative characters contribute to variation w/in a pop Discrete characters can be classified on an either- or basis Quantitative characters vary along a continuum w/in a pop

38. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Polymorphism Phenotypic polymorphism- pop in which 2 or more distinct morphs for a char are represented in high enough freq’s to be readily noticeable (red or black) Genetic polymorphisms- heritable components of char’s that occur along a continuum in a pop (height)

39. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Measuring Genetic Variation Pop geneticists measure polymorphisms in a pop by determining the amt of heterozygosity at the gene & molecular levels Avg heterozygosity measures the avg % of loci that are heterozygous in a pop For ex: Drosophila is heterzygous at ~14% of its loci (so the avg hetero is 14%) So ~1900 of its genes out of 13,700 are hetero

40. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Variation Between Populations Most spp exhibit geographic variation diff’s b/w gene pools of separate pop’s or pop subgroups

41. LE 23-10 12.43.145.1867.15 8.119.1210.1613.1719XX 1 9.1011.1213.1715.18XX 2.193.84.165.146.7

42. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Some examples of geographic variation occur as a cline cline- a graded change in a trait along a geographic axis

43. LE 23-11 Heights of yarrow plants grown in common garden Sierra Nevada Range Great Basin Plateau Seed collection sites 100 50 0 3,000 2,000 1,000 0 Mean height (cm) Altitude (m)

44. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings A Closer Look at Natural Selection From the range of variations available in a pop, natural selection increases freq’s of certain genotypes, fitting organisms to their environment over generations

45. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Evolutionary Fitness The phrases “struggle for existence” & “survival of the fittest” are commonly used to describe natural selection but can be misleading Reproductive success is generally more subtle & depends on many factors

46. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Fitness- contribution an indiv makes to the gene pool of the next generation, relative to the contributions of other indiv’s Relative fitness- contribution of a genotype to the next generation, compared w/ contributions of alternative genotypes for the same locus EX: plants w/pink & white flowers make more offspring than red flowers, reproductive success is set at 1; rel fitness of the white & pink is 1 b/c they make the same amt of offspring & is only 0.8 for red b/c they make only 80% as much

47. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Directional, Disruptive, and Stabilizing Selection Selection favors certain genotypes by acting on the phenotypes of certain organisms 3 modes of selection: – Directional – Disruptive – Stabilizing

48. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Directional selection favors indiv’s at 1 end of the phenotypic range Disruptive selection favors indiv’s at both extremes of the phenotypic range Stabilizing selection favors intermediate variants & acts against extreme phenotypes

49. LE 23-12 Original population Evolved population Phenotypes (fur color) Original population Directional selectionDisruptive selectionStabilizing selection Frequency of individuals

50. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings The Preservation of Genetic Variation Various mechanisms help to preserve genetic variation in a pop

51. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Diploidy Diploidy maintains genetic variation in the form of hidden recessive alleles

52. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Balancing Selection Balancing selection- natural selection maintains stable freq’s of 2 or more phenotypic forms in a pop; leads to a state called balanced polymorphism 2 kinds: This includes heterozygote advantage & freq dependent selection

53. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Heterozygote Advantage 1.) Heterozygote Advantage– indiv’s who are heterozygous at a locus have greater fitness than homozygotes Natural selection will tend to maintain 2 or more alleles at that locus

54. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings The sickle-cell allele causes mutations in hemoglobin but also confers malaria resistance The sickle-cell allele exemplifies the heterozygote advantage

55. LE 23-13 Frequencies of the sickle-cell allele 0–2.5% 2.5–5.0% 5.0–7.5% 7.5–10.0% 10.0–12.5% >12.5% Distribution of malaria caused by Plasmodium falciparum (a protozoan)

56. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Frequency-Dependent Selection 2.) frequency-dependent selection– fitness of any morph declines if it becomes too common in the pop

57. LE 23-14 Parental population sample Experimental group sample On pecking a moth image the blue jay receives a food reward. If the bird does not detect a moth on either screen, it pecks the green circle to continue a new set of images (a new feeding opportunity). Plain backgroundPatterned background 0.6 0.5 0.4 0.3 0.2 020406080100 Generation number Frequency- independent control Phenotypic variation

58. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Neutral Variation Neutral variation– genetic variation that appears to confer no selective advantage Such as mutations in introns etc….

59. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Sexual Selection Sexual selection– natural selection for mating success can result in sexual dimorphism, marked diff’s b/w the sexes in 2 ndary sexual characteristics

60. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Intrasexual selection- competition among indiv’s of 1 sex for mates of the opposite sex

61. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Intersexual selection- indiv’s of 1 sex (usually females) are choosy in selecting their mates from indiv’s of the other sex Selection may depend on the showiness of the male’s appearance

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63. The Evolutionary Enigma of Sexual Reproduction Sexual reproduction produces fewer reproductive offspring than asexual reproduction, a so-called “reproductive handicap”

64. LE 23-16 Asexual reproduction Female Generation 1 Generation 2 Generation 3 Generation 4 Sexual reproduction Female Male

65. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Sexual reproduction produces genetic variation that may aid in disease resistance

66. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Why Natural Selection Cannot Fashion Perfect Organisms Why Natural Selection Can’t make Perfect Organisms: Evolution is limited by historical constraints Adaptations are often compromises Chance & natural selection interact Selection can only edit existing variations