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Genes Within Populations

Genes Within Populations

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Genes Within Populations

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  1. Genes Within Populations Chapter 20

  2. Darwin: Evolution is descent with modification Evolution: changes in gene frequencies through time Species accumulate difference Descendants differ from their ancestors New species arise from existing ones Genetic Variation and Evolution

  3. Natural selection: proposed by Darwin as the mechanism of evolution individuals possess traits with underlying genetic basis: genotype phenotype they produce more offspring than can survive therefore the population includes individuals that are variable for a given trait those individuals that are better adapted to the present environment survive & reproduce more offspring Natural selection: mechanism of evolutionary change

  4. Darwin’s theory for how long necks evolved in giraffes

  5. Inheritance of acquired characteristics:Proposed byJean-Baptiste Lamarck Individuals passed on physical and behavioral changes to their offspring Variation by experience…not genetic Darwin’s natural selection: variation a result of preexisting genetic differences Natural selection: mechanism of evolutionary change

  6. Lamarck’s theory of how giraffes’ long necks evolved

  7. Mutation - replication errors Natural selection - differential reproduction of genotypes Migration - movement of individuals among populations Genetic drift - random chance events in small populations Genetic Recombination - shuffling novel combinations of existing genes Mechanisms of Evolution

  8. Measuring levels of genetic variation blood groups enzymes Enzyme polymorphism A locus with more variation than can be explained by mutation is termed polymorphic. Natural populations tend to have more polymorphic loci than can be accounted for by mutation. DNA sequence polymorphism Gene Variation in Nature

  9. Godfrey H. Hardy: English mathematicianWilhelm Weinberg: German physicianConcluded that:The original proportions of the genotypes in a population will remain constant from generation to generation as long as five assumptions are met Hardy-Weinberg Principle

  10. Five assumptions : No mutation takes place No genes are transferred to or from other sources, no migration Random mating is occurring The population size is very large No natural selection occurs Hardy-Weinberg Principle

  11. Calculate genotype frequencies with a binomial expansion(p+q)2 = p2 + 2pq + q2 = 1 p2 = individuals homozygous for first allele 2pq = individuals heterozygous for both alleles q2 = individuals homozygous for second allele because there are only two alleles: p plus q must always equal 1 Hardy-Weinberg Principle

  12. Hardy-Weinberg Principle

  13. Hardy-Weinberg Principle Using Hardy-Weinberg equation to predict frequencies in subsequent generations

  14. A population not in Hardy-Weinberg equilibrium indicates that one or more of the five evolutionary agents are operating in a population Five agents of evolutionary change

  15. Mutation:A change in a cell’s DNA Mutation rates are generally so low they have little effect on Hardy-Weinberg proportions of common alleles. Ultimate source of genetic variation Migration (Gene flow):A movement of alleles from one population to another Powerful agent of change Tends to homogenize allele frequencies Agents of Evolutionary Change

  16. Genetic Recombination (Nonrandom Mating): mating with specific genotypes Shifts genotype frequencies Assortative Mating: does not change frequency of individual alleles; increases the proportion of homozygous individuals Disassortative Mating: phenotypically different individuals mate; produce excess of heterozygotes Agents of Evolutionary Change

  17. Genetic drift: Random fluctuation in allele frequencies over time by chance important in small populations founder effect - few individuals found new population (small allelic pool) bottleneck effect - drastic reduction in population, and gene pool size Genetic Drift

  18. Genetic Drift: A bottleneck effect

  19. Bottleneck effect: case study

  20. Artificial selection: a breeder selects for desired characteristics Selection

  21. Natural selection: environmental conditions determine which individuals in a population produce the most offspring 3 conditions for natural selection to occur Variation must exist among individuals in a population Variation must be genetically inherited Variation among individuals must result in differences in the number of offspring surviving Selection

  22. Selection

  23. Selection Pocket mice from the Tularosa Basin

  24. Enzyme allele frequencies vary with latitude Lactate dehydrogenase in Fundulus heteroclitus (mummichog fish) varies with latitude Enzymes formed function differently at different temperatures North latitudes: Lactate dehydrogenase is a better catalyst at low temperatures Selection to match climatic conditions

  25. Selection for pesticide resistance

  26. Fitness: A phenotype with greater fitness usually increases in frequency Most fit is given a value of 1 Fitness is a combination of: Survival: how long does an organism live Mating success: how often it mates Number of offspring per mating that survive Fitness and Its Measurement

  27. Fitness and its Measurement Body size and egg-laying in water striders

  28. Mutation and genetic drift may counter selection The magnitude of drift is inversely related to population size Interactions Among Evolutionary Forces

  29. Gene flow may promote or constrain evolutionary change Spread a beneficial mutation Impede adaptation by continual flow of inferior alleles from other populations Extent to which gene flow can hinder the effects of natural selection depends on the relative strengths of gene flow High in birds & wind-pollinated plants Low in sedentary species Interactions Among Evolutionary Forces

  30. Interactions Among Evolutionary Forces Degree of copper tolerance

  31. Frequency-dependent selection: depends on how frequently or infrequently a phenotype occurs in a population Negative frequency-dependent selection: rare phenotypes are favored by selection Positive frequency-dependent selection: common phenotypes are favored; variation is eliminated from the population Strength of selection changes through time Maintenance of Variation

  32. Maintenance of Variation Negative frequency - dependent selection

  33. Maintenance of Variation Positive frequency-dependent selection

  34. Oscillating selection: selection favors one phenotype at one time, and a different phenotype at another time Galápagos Islands ground finches Wet conditions favor small bills (abundant seeds) Dry conditions favor big bills Maintenance of Variation

  35. Fitness of a phenotype does not depend on its frequency Environmental changes lead to oscillation in selection Maintenance of Variation

  36. Heterozygotes may exhibit greater fitness than homozygotes Heterozygote advantage: keep deleterious alleles in a population Example: Sickle cell anemia Homozygous recessive phenotype: exhibit severe anemia Maintenance of Variation

  37. Maintenance of Variation • Homozygous dominant phenotype: no anemia; susceptible to malaria • Heterozygous phenotype: no anemia; less susceptible to malaria

  38. Maintenance of Variation Frequency of sickle cell allele

  39. Disruptive selection acts to eliminate intermediate types Types of Selection

  40. Types of Selection Disruptive selection for large and small beaks in black-bellied seedcracker finch of west Africa

  41. Directional selection: acts to eliminate one extreme from an array of phenotypes Types of Selection

  42. Types of Selection Directional selection for negative phototropism in Drosophila

  43. Stabilizing selection: acts to eliminate both extremes Types of Selection

  44. Types of Selection Stabilizing selection for birth weight in humans

  45. In some cases, evolutionary change can occur rapidly Evolutionary studies can be devised to test evolutionary hypotheses Guppy studies (Poecilia reticulata) in the lab and field Populations above the waterfalls: low predation Populations below the waterfalls: high predation Experimental Studies of Natural Selection

  46. High predation environment - Males exhibit drab coloration and tend to be relatively small and reproduce at a younger age. Low predation environment - Males display bright coloration, a larger number of spots, and tend to be more successful at defending territories. Experimental Studies

  47. The evolution of protective coloration or sexually favored coloration in guppies Experimental Studies

  48. The laboratory experiment 10 large pools 2000 guppies 4 pools with pike cichlids (predator) 4 pools with killifish (nonpredator) 2 pools as control (no other fish added) 10 generations Experimental Studies