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 common misconception about evolution is that individual organisms evolve, in the Darwinian sense, during their lifetimes Natural selection acts on individuals, but populations evolve

3. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Concept 23.1: Population genetics provides a foundation for studying evolution Microevolution – Is change in the genetic makeup of a population from generation to generation Figure 23.2

4. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings The Modern Synthesis Population genetics – Is the study of how populations change genetically over time – Reconciled Darwin’s and Mendel’s ideas The modern synthesis – Integrates Mendelian genetics with the Darwinian theory of evolution by natural selection – Focuses on populations as units of evolution

5. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Gene Pools and Allele Frequencies A population – Is a localized group of individuals that are capable of interbreeding and producing fertile offspring MAP AREA ALASKA CANADA Beaufort Sea Porcupine herd range Fairbanks Whitehorse Fortymile herd range NORTHWEST TERRITORIES ALASKA YUKON Figure 23.3

6. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings The gene pool – Is the total aggregate of genes in a population at any one time – Consists of all gene loci in all individuals of the population

7. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings The Hardy-Weinberg Theorem The Hardy-Weinberg theorem – Describes a population that is not evolving – States that the frequencies of alleles and genotypes in a population’s gene pool remain constant from generation to generation provided that only Mendelian segregation and recombination of alleles are at work In a given population where gametes contribute to the next generation randomly, allele frequencies will not change

8. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Hardy-Weinberg Equilibrium Hardy-Weinberg equilibrium – Describes a population in which random mating occurs – Describes a population where allele frequencies do not change

9. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings A population in Hardy-Weinberg equilibrium Figure 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%) p2p2 64% C R 16% C R C W 16% C R C W 4% C W qp C R (80%) Eggs C W (20%) pq If the gametes come together at random, the genotype frequencies of this generation are in Hardy-Weinberg equilibrium: q2q2 64% C R C R, 32% C R C W, and 4% C W C W Gametes of the next generation: 64% C R from C R C R homozygotes 16% C R from C R C W homozygotes += 80% C R = 0.8 = p 16% C W from C R C W heterozygotes += 20% C W = 0.2 = q With random mating, these gametes will result in the same mix of plants in the next generation: 64% C R C R, 32% C R C W and 4% C W C W plants p2p2 4% C W from C W C W homozygotes

10. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings If p and q represent the relative frequencies of the only two possible alleles in a population at a particular locus, then – p 2 + 2pq + q 2 = 1 – And p 2 and q 2 represent the frequencies of the homozygous genotypes and 2pq represents the frequency of the heterozygous genotype

11. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Conditions for Hardy-Weinberg Equilibrium The Hardy-Weinberg theorem – Describes a hypothetical population In real populations – Allele and genotype frequencies do change over time

12. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings The five conditions for non-evolving populations are rarely met in nature – Extremely large population size – No gene flow – No mutations – Random mating – No natural selection

13. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Population Genetics and Human Health We can use the Hardy-Weinberg equation – To estimate the percentage of the human population carrying the allele for an inherited disease – EX. PKU is a recessive genetic disorder affecting 1/10,000 Americans. – Calculate the frequency of p and q. – Calculate the frequency of homozygous recessive, heterozygous and homozygous dominant in the population.

14. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Concept 23.2: Mutation and sexual recombination produce the variation that makes evolution possible Two processes, mutation and sexual recombination – Produce the variation in gene pools that contributes to differences among individuals

15. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Mutation Mutations – Are changes in the nucleotide sequence of DNA – Cause new genes and alleles to arise Figure 23.6

16. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Point Mutations A point mutation – Is a change in one base in a gene – Can have a significant impact on phenotype – Is usually harmless, but may have an adaptive impact Chromosomal mutations that affect many loci – Are almost certain to be harmful – May be neutral and even beneficial

17. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Mutation Rates Mutation rates – Tend to be low in animals and plants – Average about one mutation in every 100,000 genes per generation – Are more rapid in microorganisms

18. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Sexual Recombination In sexually reproducing populations, sexual recombination – Is far more important than mutation in producing the genetic differences that make adaptation possible (crossing over, independent assortment and random fertilization)

19. 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 Three major factors alter allele frequencies and bring about most evolutionary change – Natural selection – Genetic drift – Gene flow

20. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Genetic Drift Statistically, the smaller a sample – The greater the chance of deviation from a predicted result Genetic drift – Describes how allele frequencies can fluctuate unpredictably from one generation to the next – Tends to reduce genetic variation Figure 23.7 CRCRCRCR CRCWCRCW CRCRCRCR CWCWCWCW CRCRCRCR CRCWCRCW CRCWCRCW CRCWCRCW CRCRCRCR CRCRCRCR Only 5 of 10 plants leave offspring CWCWCWCW CRCRCRCR CRCWCRCW CRCRCRCR CWCWCWCW CRCWCRCW CWCWCWCW CRCRCRCR CRCWCRCW CRCWCRCW 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

21. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings The Bottleneck Effect In the bottleneck effect – A sudden change in the environment may drastically reduce the size of a population – The gene pool may no longer be reflective of the original population’s gene pool Original population Bottlenecking event Surviving population Figure 23.8 A (a) Shaking just a few marbles through the narrow neck of a bottle is analogous to a drastic reduction in the size of a population after some environmental disaster. By chance, blue marbles are over-represented in the new population and gold marbles are absent.

22. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Understanding the bottleneck effect – Can increase understanding of how human activity affects other species Figure 23.8 B (b) Similarly, bottlenecking a population of organisms tends to reduce genetic variation, as in these northern elephant seals in California that were once hunted nearly to extinction.

23. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings The Founder Effect The founder effect – Occurs when a few individuals become isolated from a larger population – Can affect allele frequencies in a population

24. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Gene Flow Gene flow – Causes a population to gain or lose alleles – Results from the movement of fertile individuals or gametes – Tends to reduce differences between populations over time – Over the last years gene flow has increase in human populations.

25. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Concept 23.4: Natural selection is the primary mechanism of adaptive evolution Natural selection – Accumulates and maintains favorable genotypes in a population

26. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Genetic Variation Genetic variation – Occurs in individuals in populations of all species – Is not always heritable Figure 23.9 A, B (a)Map butterflies that emerge in spring: orange and brown (b)Map butterflies that emerge in late summer: black and white

27. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Variation Within a Population Both discrete and quantitative characters – Contribute to variation within a population Discrete characters – Can be classified on an either-or basis Quantitative characters – Vary along a continuum within a population

28. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Polymorphism Phenotypic polymorphism – Describes a population in which two or more distinct morphs for a character are each represented in high enough frequencies to be readily noticeable Genetic polymorphisms – Are the heritable components of characters that occur along a continuum in a population

29. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Measuring Genetic Variation Population geneticists – Measure the number of polymorphisms in a population by determining the amount of heterozygosity at the gene level and the molecular level Average heterozygosity – Measures the average percent of loci that are heterozygous in a population

30. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Variation Between Populations Most species exhibit geographic variation – Differences between gene pools of separate populations or population subgroups 1 2.4 3.14 5.18 6 7.15 XX19 13.1710.169.12 8.11 12.193.84.165.146.7 XX 15.18 13.17 11.12 9.10 Figure 23.10

31. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Some examples of geographic variation occur as a cline, which is a graded change in a trait along a geographic axis Figure 23.11 EXPERIMENT Researchers observed that the average size of yarrow plants (Achillea) growing on the slopes of the Sierra Nevada mountains gradually decreases with increasing elevation. To eliminate the effect of environmental differences at different elevations, researchers collected seeds from various altitudes and planted them in a common garden. They then measured the heights of the resulting plants. RESULTS The average plant sizes in the common garden were inversely correlated with the altitudes at which the seeds were collected, although the height differences were less than in the plants’ natural environments. CONCLUSION The lesser but still measurable clinal variation in yarrow plants grown at a common elevation demonstrates the role of genetic as well as environmental differences. Mean height (cm) Atitude (m) Heights of yarrow plants grown in common garden Seed collection sites Sierra Nevada Range Great Basin Plateau

32. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings A Closer Look at Natural Selection From the range of variations available in a population – Natural selection increases the frequencies of certain genotypes, fitting organisms to their environment over generations

33. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Evolutionary Fitness The phrases “struggle for existence” and “survival of the fittest” – Are commonly used to describe natural selection – Can be misleading Reproductive success – Is generally more subtle and depends on many factors

34. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Fitness – Is the contribution an individual makes to the gene pool of the next generation, relative to the contributions of other individuals Relative fitness – Is the contribution of a genotype to the next generation as compared to the contributions of alternative genotypes for the same locus

35. 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 Three modes of selection are – Directional – Disruptive – Stabilizing

36. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Directional selection – Favors individuals at one end of the phenotypic range Disruptive selection – Favors individuals at both extremes of the phenotypic range Stabilizing selection – Favors intermediate variants and acts against extreme phenotypes

37. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings The three modes of selection Fig 23.12 A–C (a) Directional selection shifts the overall makeup of the population by favoring variants at one extreme of the distribution. In this case, darker mice are favored because they live among dark rocks and a darker fur color conceals them from predators. (b) Disruptive selection favors variants at both ends of the distribution. These mice have colonized a patchy habitat made up of light and dark rocks, with the result that mice of an intermediate color are at a disadvantage. (c) Stabilizing selection removes extreme variants from the population and preserves intermediate types. If the environment consists of rocks of an intermediate color, both light and dark mice will be selected against. Phenotypes (fur color) Original population Original population Evolved population Frequency of individuals

38. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings The Preservation of Genetic Variation Various mechanisms help to preserve genetic variation in a population Diploidy – Maintains genetic variation in the form of hidden recessive alleles Balancing selection – Occurs when natural selection maintains stable frequencies of two or more phenotypic forms in a population – Leads to a state called balanced polymorphism

39. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Heterozygote Advantage Some individuals who are heterozygous at a particular locus – Have greater fitness than homozygotes Natural selection – Will tend to maintain two or more alleles at that locus

40. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings The sickle-cell allele – Causes mutations in hemoglobin but also confers malaria resistance – Exemplifies the heterozygote advantage Figure 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)

41. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Neutral Variation Neutral variation – Is genetic variation that appears to confer no selective advantage

42. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Sexual Selection Sexual selection – Is natural selection for mating success – Can result in sexual dimorphism, marked differences between the sexes in secondary sexual characteristics

43. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Intrasexual selection – Is a direct competition among individuals of one sex for mates of the opposite sex

44. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Intersexual selection – Occurs when individuals of one sex (usually females) are choosy in selecting their mates from individuals of the other sex – May depend on the showiness of the male’s appearance Figure 23.15

45. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings The Evolutionary Enigma of Sexual Reproduction Sexual reproduction – Produces fewer reproductive offspring than asexual reproduction, a so-called reproductive handicap Figure 23.16 Asexual reproduction Female Sexual reproduction Female Male Generation 1 Generation 2 Generation 3 Generation 4

46. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings If sexual reproduction is a handicap, why has it persisted? – It produces genetic variation that may aid in disease resistance

47. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Why Natural Selection Cannot Fashion Perfect Organisms Evolution is limited by historical constraints Adaptations are often compromises

48. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Chance and natural selection interact Selection can only edit existing variations