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EVOLUTIONARY CHANGE IS RANDOM AP BIOLOGY BIG IDEA #1 – Part A – Section 3.

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Presentation on theme: "EVOLUTIONARY CHANGE IS RANDOM AP BIOLOGY BIG IDEA #1 – Part A – Section 3."— Presentation transcript:

1 EVOLUTIONARY CHANGE IS RANDOM AP BIOLOGY BIG IDEA #1 – Part A – Section 3

2 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

3 Natural Selection Differential success in reproduction results in certain alleles being passed to the next generation in greater proportions

4 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

5 Fig 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

6 Fig 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

7 Fig 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

8 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

9 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 populations gene pool If the population remains small, it may be further affected by genetic drift

10 Fig Original population Bottlenecking event Surviving population

11 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

12 Fig Number of alleles per locus Range of greater prairie chicken Pre-bottleneck (Illinois, 1820) Post-bottleneck (Illinois, 1993) 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, < (a) (b)

13 Fig a Range of greater prairie chicken Pre-bottleneck (Illinois, 1820) Post-bottleneck (Illinois, 1993) (a)

14 Fig b 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, < (b)

15 Researchers used DNA from museum specimens to compare genetic variation in the population before and after the bottleneck The results showed a loss of alleles at several loci Researchers introduced greater prairie chickens from population in other states and were successful in introducing new alleles and increasing the egg hatch rate to 90%

16 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

17 Gene Flow Gene flowGene 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

18 Fig

19 Examples of Gene flow that 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

20 Fig NON- MINE SOIL MINE SOIL NON- MINE SOIL Prevailing wind direction Index of copper tolerance Distance from mine edge (meters)

21 Fig a NON- MINE SOIL MINE SOIL NON- MINE SOIL Prevailing wind direction Index of copper tolerance Distance from mine edge (meters)

22 Fig b

23 Examples of Gene flow that 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

24 Only natural selection consistently results in adaptive evolution Concept 23.4: Natural selection is the only mechanism that consistently causes adaptive evolution

25 A Closer Look at Natural Selection Natural selection brings about adaptive evolution by acting on an organisms phenotype

26 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

27 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

28 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

29 Fig Original population (c) Stabilizing selection (b) Disruptive selection (a) Directional selection Phenotypes (fur color) Frequency of individuals Original population Evolved population

30 Fig a Original population (a) Directional selection Phenotypes (fur color) Frequency of individuals Original population Evolved population

31 Fig b Original population (b) Disruptive selection Phenotypes (fur color) Frequency of individuals Evolved population

32 Fig c Original population (c) Stabilizing selection Phenotypes (fur color) Frequency of individuals Evolved population

33 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

34 Fig (a) Color-changing ability in cuttlefish (b) Movable jaw bones in snakes Movable bones

35 Fig a (a) Color-changing ability in cuttlefish

36 Fig b (b) Movable jaw bones in snakes Movable bones

37 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

38 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

39 Fig

40 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 males chances of attracting a female, while decreasing his chances of survival

41 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

42 Fig SC male gray tree frog Female gray tree frog LC male gray tree frog EXPERIMENT SC sperm Eggs LC sperm Offspring of LC father Offspring of SC father Fitness of these half-sibling offspring compared RESULTS 1995Fitness 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.

43 Fig a 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

44 Fig b 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.

45 The Preservation of Genetic Variation Various mechanisms help to preserve genetic variation in a population

46 Diploidy Diploidy maintains genetic variation in the form of hidden recessive alleles

47 Balancing Selection Balancing selection occurs when natural selection maintains stable frequencies of two or more phenotypic forms in a population

48 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

49 Fig –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%

50 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 Frequency-Dependent Selection

51 Fig Right-mouthed 1981 Left-mouthed Frequency of left-mouthed individuals Sample year

52 Fig a Right-mouthed Left-mouthed

53 Fig b 1981 Frequency of left-mouthed individuals Sample year

54 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

55 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

56 Fig

57 Fig. 23-UN1 Stabilizing selection Original population Evolved population Directional selection Disruptive selection

58 Fig. 23-UN2 Sampling sites (1–8 represent pairs of sites) Salinity increases toward the open ocean N Long Island Sound Allele frequencies Atlantic Ocean Other lap alleles lap 94 alleles Data from R.K. Koehn and T.J. Hilbish, The adaptive importance of genetic variation, American Scientist 75:134–141 (1987). E S W

59 Fig. 23-UN3

60 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 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

62 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


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