1. Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Overview: The Smallest Unit of Evolution 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 If there is a change in the allele frequencies in a population from one generation to another then evolution has occurred

2. Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Two processes, mutation and sexual reproduction, produce the variation in gene pools that contributes to differences among individuals Concept 23.1: Mutation and sexual reproduction produce the genetic variation that makes evolution possible

3. Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Genetic Variation Variation in individual genotype leads to variation in individual phenotype Not all phenotypic variation is heritable Natural selection can only act on variation with a genetic component

4. Copyright © 2008 Pearson Education Inc., publishing as Pearson 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

5. 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. The origin of variation is by mutation Only mutations in cells that produce gametes can be passed to offspring Animation: Genetic Variation from Sexual Recombination

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

7. Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings The effects of point mutations can vary: – 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

8. Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings The effects of point mutations can vary: – 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

9. 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

10. 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 eukaryotes and higher in bacteria and viruses

11. 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

12. Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Concept 23.2: The Hardy-Weinberg equation can be used to test whether a population is evolving The first step in testing whether evolution is occurring in a population is to clarify what we mean by a population

13. Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Gene Pools and Allele Frequencies A population is a localized group of individuals capable of interbreeding and producing fertile offspring A gene pool consists of all the alleles for all loci in a population A1A1, A2A1, A2A2-genotype

14. Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings The frequency of an allele in a population can be calculated – For diploid organisms, the total number of alleles at a locus is the total number of individuals x 2 A1A1, A2A1, A2A2-genotype Out of 6 alleles 3 are A1 and 3 are A2 so Frequency of A1 is 3/6 or.5 and A2 is 3/6 or.5

15. Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings By convention, if there are 2 alleles at a locus (A1, A2), p and q are used to represent their frequencies (p=.5, q=.5) The frequency of all alleles in a population will add up to 1 – For example, p + q = 1

16. Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings The Hardy-Weinberg Principle Two scientists came up with a mathematical formula that would help in determining if a population is evolving and the mechanisms that cause the evolution The Hardy-Weinberg principle describes a population that is not evolving If a population does not meet the criteria of the Hardy-Weinberg principle, it can be concluded that the population is evolving

17. Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Hardy-Weinberg Equilibrium The Hardy-Weinberg principle states that frequencies of alleles and genotypes in a population remain constant from generation to generation In a given population where gametes contribute to the next generation randomly, allele frequencies will not change Mendelian inheritance preserves genetic variation in a population

18. Fig. 23-6 Frequencies of alleles Alleles in the population Gametes produced Each egg:Each sperm: 80% chance 80% chance 20% chance 20% chance q = frequency of p = frequency of C R allele = 0.8 C W allele = 0.2

19. Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Hardy-Weinberg equilibrium describes the constant frequency of alleles in such a gene pool 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 – where p 2 and q 2 represent the frequencies of the homozygous genotypes and 2pq represents the frequency of the heterozygous genotype

20. Fig. 23-7-4 Gametes of this generation: 64% C R C R, 32% C R C W, and 4% C W C W 64% C R + 16% C R = 80% C R = 0.8 = p 4% C W + 16% C W = 20% C W = 0.2 = q 64% C R C R, 32% C R C W, and 4% C W C W plants Genotypes in the next generation: Sperm C R (80%) C W (20%) 80% C R ( p = 0.8) C W (20%) 20% C W ( q = 0.2) 16% ( pq ) C R C W 4% ( q 2 ) C W C W C R (80%) 64% ( p 2 ) C R C R 16% ( qp ) C R C W Eggs

21. Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Conditions for Hardy-Weinberg Equilibrium The Hardy-Weinberg theorem is a Null Hypothesis and describes a hypothetical population In real populations, allele and genotype frequencies do change over time

22. Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings The five conditions for nonevolving populations are rarely met in nature: – No mutations – Random mating – No natural selection – Extremely large population size – No gene flow

23. Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Natural populations can evolve at some loci, while being in Hardy-Weinberg equilibrium at other loci

24. Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Applying the Hardy-Weinberg Principle We can assume the locus that causes phenylketonuria (PKU) is in Hardy-Weinberg equilibrium given that: – The PKU gene mutation rate is low – Mate selection is random with respect to whether or not an individual is a carrier for the PKU allele

25. Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings – Natural selection can only act on rare homozygous individuals who do not follow dietary restrictions – The population is large – Migration has no effect as many other populations have similar allele frequencies

26. Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings The occurrence of PKU is 1 per 10,000 births – q 2 = 0.0001 – q = 0.01 The frequency of normal alleles is – p = 1 – q = 1 – 0.01 = 0.99 The frequency of carriers is – 2pq = 2 x 0.99 x 0.01 = 0.0198 – or approximately 2% of the U.S. population

27. 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

28. 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

29. Fig. 23-8-1 Generation 1 p (frequency of C R ) = 0.7 q (frequency of C W ) = 0.3 C W C R C R C W C R C R C W

30. Fig. 23-8-2 Generation 1 p (frequency of C R ) = 0.7 q (frequency of C W ) = 0.3 Generation 2 p = 0.5 q = 0.5 C W C R C R C W C R C R C W C W C R

31. Fig. 23-8-3 Generation 1 C W C R C R C W C R C R C W p (frequency of C R ) = 0.7 q (frequency of C W ) = 0.3 Generation 2 C R C W C W C R p = 0.5 q = 0.5 Generation 3 p = 1.0 q = 0.0 C R

32. 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

33. 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

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

35. 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 Understanding the bottleneck effect can increase understanding of how human activity affects other species

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

37. 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

38. 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 Gene flow can increase or decrease the fitness of a population

39. 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

40. Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Only natural selection consistently results in adaptive evolution Natural selection brings about adaptive evolution by acting on an organism’s phenotype Concept 23.4: Natural selection is the only mechanism that consistently causes adaptive evolution

41. 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

42. 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

43. 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

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

45. Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings The Key Role of Natural Selection in Adaptive Evolution Natural selection increases the frequencies of alleles that enhance survival and reproduction Adaptive evolution occurs as the match between an organism and its environment increases

46. Fig. 23-14 (a) Color-changing ability in cuttlefish (b) Movable jaw bones in snakes Movable bones

47. Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Because the environment can change, adaptive evolution is a continuous process Genetic drift and gene flow do not consistently lead to adaptive evolution as they can increase or decrease the match between an organism and its environment

48. 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

49. Fig. 23-15

50. 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

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

52. Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings The Preservation of Genetic Variation What prevents natural selection from reducing genetic variation by working against all unfavorable genotypes? Various mechanisms help to preserve genetic variation in a population

53. Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Diploidy & Balancing Selection 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. This selection includes Heterozygote Advantage and Frequency-Dependent Selection

54. Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Heterozygote advantage occurs when heterozygotes have a higher fitness than do both homozygotes The sickle-cell allele causes mutations in hemoglobin but also confers malaria resistance

55. 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%

56. Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings In frequency-dependent selection, the fitness of a phenotype declines if it becomes too common in the population Selection can favor whichever phenotype is less common in a population

57. Fig. 23-18 “Right-mouthed” 1981 “Left-mouthed” Frequency of “left-mouthed” individuals Sample year 1.0 0.5 0 ’82 ’83 ’84 ’85 ’86 ’87 ’88 ’89’90

58. Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Neutral Variation Neutral variation is genetic variation that appears to confer no selective advantage or disadvantage For example, – Variation in noncoding regions of DNA – Variation in proteins that have little effect on protein function or reproductive fitness

59. 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

60. 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

61. Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings 1.Apply the Hardy-Weinberg equation to a population genetics problem 2.Explain why natural selection is the only mechanism that consistently produces adaptive change 3.Explain the role of population size in genetic drift

62. Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings 1.Distinguish among the following sets of terms: directional, disruptive, and stabilizing selection; intrasexual and intersexual selection 2.List four reasons why natural selection cannot produce perfect organisms