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AP Bio Ch 23 part 2. Only natural selection consistently results in adaptive evolution Concept 23.4: Natural selection is the only mechanism that consistently.

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Presentation on theme: "AP Bio Ch 23 part 2. Only natural selection consistently results in adaptive evolution Concept 23.4: Natural selection is the only mechanism that consistently."— Presentation transcript:

1 AP Bio Ch 23 part 2

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

3 A Closer Look at Natural Selection Natural selection brings about adaptive evolution by acting on an organism’s phenotype

4 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

5 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

6 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

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

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

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

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

11 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

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

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

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

15 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

16 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

17 Fig

18 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

19 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

20 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.

21 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

22 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.

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

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

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

26 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

27 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%

28 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

29 Fig “Right-mouthed” 1981 “Left-mouthed” Frequency of “left-mouthed” individuals Sample year ’82 ’83 ’84 ’85 ’86 ’87 ’88 ’89’90

30 Fig a “Right-mouthed” “Left-mouthed”

31 Fig b 1981 Frequency of “left-mouthed” individuals Sample year ’82 ’83 ’84’85 ’86’87’88’89’90

32 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

33 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

34 Fig

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

36 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

37 Fig. 23-UN3

38 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

39 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

40 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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