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Genes Within Populations

Genes Within Populations. Chapter 20. Genetic Variation and Evolution. Darwin : Evolution is descent with modification Evolution : changes through time Species accumulate difference Descendants differ from their ancestors New species arise from existing ones.

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Genes Within Populations

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  1. Genes Within Populations Chapter 20

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

  3. Natural selection: proposed by Darwin as the mechanism of evolution individuals have specific inherited characteristics they produce more surviving offspring the population includes more individuals with these specific characteristics the population evolves and is better adapted to its present environment Natural selection: mechanism of evolutionary change

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

  5. Natural selection: mechanism of evolutionary change 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

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

  7. Gene Variation in Nature • 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

  8. 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

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

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

  11. Hardy-Weinberg Principle

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

  13. 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

  14. Agents of Evolutionary Change • 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 • Gene flow:A movement of alleles from one population to another • Powerful agent of change • Tends to homogenize allele frequencies

  15. Agents of Evolutionary Change • 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

  16. Genetic Drift • 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

  17. Genetic Drift: A bottleneck effect

  18. Bottleneck effect: case study

  19. Selection • Artificial selection: a breeder selects for desired characteristics

  20. Selection • 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 among individuals must result in differences in the number of offspring surviving • Variation must be genetically inherited

  21. Selection

  22. Selection Pocket mice from the Tularosa Basin

  23. Selection to match climatic conditions • 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

  24. Selection for pesticide resistance

  25. Fitness and Its Measurement • 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

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

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

  28. Interactions Among Evolutionary Forces • 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

  29. Interactions Among Evolutionary Forces Degree of copper tolerance

  30. Maintenance of Variation • 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

  31. Maintenance of Variation Negative frequency - dependent selection

  32. Maintenance of Variation Positive frequency-dependent selection

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

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

  35. Maintenance of Variation • 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

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

  37. Maintenance of Variation Frequency of sickle cell allele

  38. Maintenance of Variation Disruptive selection acts to eliminate intermediate types

  39. Maintenance of Variation Disruptive selection for large and small beaks in black-bellied seedcracker finch of west Africa

  40. Maintenance of Variation Directional selection: acts to eliminate one extreme from an array of phenotypes

  41. Maintenance of Variation Directional selection for negative phototropism in Drosophila

  42. Maintenance of Variation Stabilizing selection: acts to eliminate both extremes

  43. Maintenance of Variation Stabilizing selection for birth weight in humans

  44. Experimental Studies of Natural Selection • 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

  45. Experimental Studies • 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.

  46. Experimental Studies The evolution of protective coloration in guppies

  47. Experimental Studies 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

  48. Experimental Studies The field experiment • Removed guppies from below the waterfalls (high predation) • Placed guppies in pools above the falls • 10 generations later, transplanted populations evolved the traits characteristic of low-predation guppies

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