1. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings PowerPoint ® Lecture Presentations for Biology Eighth Edition Neil Campbell and Jane Reece Lectures by Chris Romero, updated by Erin Barley with contributions from Joan Sharp Chapter 23 The Evolution of Populations

2. Copyright © 2008 Pearson Education Inc., publishing as Pearson 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 individuals, but only populations evolve Genetic variations in populations contribute to evolution Microevolution is a change in allele frequencies in a population over generations

3. Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Mutation and sexual reproduction, produce variation in gene pools Concept 23.1: Mutation and sexual reproduction produce the genetic variation that makes evolution possible

4. Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Variation Between Populations Most species exhibit geographic variation, differences between gene pools of separate populations or population subgroups

5. Fig. 23-3 13.1719XX10.169.12 8.11 1 2.4 3.145.18 67.15 9.10 12.19 11.1213.17 15.18 3.84.165.14 6.7 XX

6. Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Some examples of geographic variation occur as a cline, which is a graded change in a trait along a geographic axis

7. Fig. 23-4 1.0 0.8 0.6 0.4 0.2 0 46 444240 3836 34 3230 Georgia Warm (21°C) Latitude (°N) Maine Cold (6°C) Ldh-B b allele frequency

8. Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Mutation Mutations are changes in the nucleotide sequence of DNA Mutations cause new genes and alleles to arise Only mutations in cells that produce gametes can be passed to offspring Animation: Genetic Variation from Sexual Recombination Animation: Genetic Variation from Sexual Recombination

9. Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Point Mutations A point mutation is a change in one base in a gene

10. Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Effects of point mutations: – Mutations in noncoding regions of DNA are often harmless – Mutations in a gene might not affect protein production because of redundancy in the genetic code – Mutations that result in a change in protein production are often harmful – Mutations that result in a change in protein production can sometimes increase the fit between organism and environment

11. Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Mutations That Alter Gene Number or Sequence Chromosomal mutations that delete, disrupt, or rearrange many loci are typically harmful Duplication of large chromosome segments is usually harmful Duplication of small pieces of DNA is sometimes less harmful and increases the genome size Duplicated genes can take on new functions by further mutation

12. Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Mutation Rates Mutation rates are low in animals and plants The average is about one mutation in every 100,000 genes per generation Mutations rates are often lower in prokaryotes and higher in viruses

13. Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Sexual Reproduction Sexual reproduction can shuffle existing alleles into new combinations In organisms that reproduce sexually, recombination of alleles is more important than mutation in producing the genetic differences that make adaptation possible

14. Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Three major factors alter allele frequencies and bring about most evolutionary change: – Natural selection – Genetic drift – Gene flow Concept 23.3: Natural selection, genetic drift, and gene flow can alter allele frequencies in a population

15. Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Natural Selection Differential success in reproduction results in certain alleles being passed to the next generation in greater proportions

16. Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Genetic Drift The smaller a sample, the greater the chance of deviation from a predicted result Genetic drift describes how allele frequencies fluctuate unpredictably from one generation to the next Genetic drift tends to reduce genetic variation through losses of alleles Animation: Causes of Evolutionary Change Animation: Causes of Evolutionary Change

17. Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings The Founder Effect The founder effect occurs when a few individuals become isolated from a larger population Allele frequencies in the small founder population can be different from those in the larger parent population

18. Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings The Bottleneck Effect The bottleneck effect is a sudden reduction in population size due to a change in the environment The resulting gene pool may no longer be reflective of the original population’s gene pool If the population remains small, it may be further affected by genetic drift

19. Fig. 23-9 Original population Bottlenecking event Surviving population

20. Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Case Study: Impact of Genetic Drift on the Greater Prairie Chicken Loss of prairie habitat caused a severe reduction in the population of greater prairie chickens in Illinois The surviving birds had low levels of genetic variation, and only 50% of their eggs hatched

21. Fig. 23-10a Range of greater prairie chicken Pre-bottleneck (Illinois, 1820) Post-bottleneck (Illinois, 1993) (a)

22. Fig. 23-10b Number of alleles per locus Minnesota, 1998 (no bottleneck) Nebraska, 1998 (no bottleneck) Kansas, 1998 (no bottleneck) Illinois 1930–1960s 1993 Location Population size Percentage of eggs hatched 1,000–25,000 <50 750,000 75,000– 200,000 4,000 5.2 3.7 93 <50 5.8 5.3 85 96 99 (b)

23. Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Effects of Genetic Drift: A Summary 1.Genetic drift is significant in small populations 2.Genetic drift causes allele frequencies to change at random 3.Genetic drift can lead to a loss of genetic variation within populations 4.Genetic drift can cause harmful alleles to become fixed

24. Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Gene Flow Gene flow consists of the movement of alleles among populations Alleles can be transferred through the movement of fertile individuals or gametes (for example, pollen) Gene flow tends to reduce differences between populations over time Gene flow is more likely than mutation to alter allele frequencies directly

25. Fig. 23-11

26. Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Gene flow can decrease the fitness of a population In bent grass, alleles for copper tolerance are beneficial in populations near copper mines, but harmful to populations in other soils Windblown pollen moves these alleles between populations The movement of unfavorable alleles into a population results in a decrease in fit between organism and environment

27. Fig. 23-12b

28. Fig. 23-12a NON- MINE SOIL MINE SOIL NON- MINE SOIL Prevailing wind direction Index of copper tolerance Distance from mine edge (meters) 70 60 50 40 30 20 10 0 20 0 0 406080 100120 140 160

29. Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Gene flow can increase the fitness of a population Insecticides have been used to target mosquitoes that carry West Nile virus and malaria Alleles have evolved in some populations that confer insecticide resistance to these mosquitoes The flow of insecticide resistance alleles into a population can cause an increase in fitness

30. Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Only natural selection consistently results in adaptive evolution Concept 23.4: Natural selection is the only mechanism that consistently causes adaptive evolution

31. Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings A Closer Look at Natural Selection Natural selection brings about adaptive evolution by acting on an organism’s phenotype

32. Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Relative Fitness The phrases “struggle for existence” and “survival of the fittest” are misleading as they imply direct competition among individuals Reproductive success is generally more subtle and depends on many factors

33. Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Relative fitness is the contribution an individual makes to the gene pool of the next generation, relative to the contributions of other individuals Selection favors certain genotypes by acting on the phenotypes of certain organisms

34. Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Directional, Disruptive, and Stabilizing Selection Three modes of selection: – 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

35. Fig. 23-13 Original population (c) Stabilizing selection (b) Disruptive selection (a) Directional selection Phenotypes (fur color) Frequency of individuals Original population Evolved population

36. Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Sexual Selection Sexual selection is natural selection for mating success It can result in sexual dimorphism, marked differences between the sexes in secondary sexual characteristics

37. Fig. 23-15

38. Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Intrasexual selection is competition among individuals of one sex (often males) for mates of the opposite sex Intersexual selection, often called mate choice, occurs when individuals of one sex (usually females) are choosy in selecting their mates Male showiness due to mate choice can increase a male’s chances of attracting a female, while decreasing his chances of survival

39. Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings How do female preferences evolve? Good genes hypothesis – If a trait is related to male health, both the male trait and female preference for that trait should be selected for

40. Fig. 23-16a SC male gray tree frog Female gray tree frog LC male gray tree frog SC sperm  Eggs  LC sperm Offspring of LC father Offspring of SC father Fitness of these half-sibling offspring compared EXPERIMENT

41. Fig. 23-16b RESULTS 1995 Fitness Measure1996 Larval growth Larval survival Time to metamorphosis LC better NSD LC better (shorter) LC better (shorter) NSD LC better NSD = no significant difference; LC better = offspring of LC males superior to offspring of SC males.

42. Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings The Preservation of Genetic Variation Various mechanisms help to preserve genetic variation in a population

43. Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Heterozygote advantage occurs when heterozygotes have a higher fitness than do both homozygotes Natural selection will tend to maintain two or more alleles at that locus The sickle-cell allele causes mutations in hemoglobin but also confers malaria resistance Heterozygote Advantage

44. Fig. 23-17 0–2.5% Distribution of malaria caused by Plasmodium falciparum (a parasitic unicellular eukaryote) Frequencies of the sickle-cell allele 2.5–5.0% 7.5–10.0% 5.0–7.5% >12.5% 10.0–12.5%

45. Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Why Natural Selection Cannot Fashion Perfect Organisms 1.Selection can act only on existing variations 2.Evolution is limited by historical constraints 3.Adaptations are often compromises 4.Chance, natural selection, and the environment interact

46. Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings You should now be able to: 1.Explain why the majority of point mutations are harmless 2.Explain how sexual recombination generates genetic variability 3.Define the terms population, species, gene pool, relative fitness, and neutral variation 4.List the five conditions of Hardy-Weinberg equilibrium

47. Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings 5.Apply the Hardy-Weinberg equation to a population genetics problem 6.Explain why natural selection is the only mechanism that consistently produces adaptive change 7.Explain the role of population size in genetic drift

48. Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings 8.Distinguish among the following sets of terms: directional, disruptive, and stabilizing selection; intrasexual and intersexual selection 9.List four reasons why natural selection cannot produce perfect organisms