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Chapter 18: Microevolution

Chapter 18: Microevolution. Populations Evolve. Biological evolution does not change individuals. It changes a population. Traits in a population vary among individuals Evolution is change in frequency of traits. The Gene Pool. All of the genes in the population

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Chapter 18: Microevolution

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  1. Chapter 18: Microevolution

  2. Populations Evolve • Biological evolution does not change individuals. It changes a population. • Traits in a population vary among individuals • Evolution is change in frequency of traits

  3. The Gene Pool • All of the genes in the population • Genetic resource that is shared (in theory) by all members of population

  4. Gene Mutations • Infrequent but inevitable • Each gene has own mutation rate • Lethal mutations • Neutral mutations • Advantageous mutations

  5. Variation in Phenotype • Each kind of gene in gene pool may have two or more alleles • Individuals inherit different allele combinations • This leads to variation in phenotype • Offspring inherit genes, not phenotypes

  6. Variation in Phenotype Fig. 18-2f, p.285

  7. What Determines Alleles in New Individual? • Mutation • Crossing over at meiosis I • Independent assortment • Fertilization • Change in chromosome number or structure

  8. Genetic Equilibrium • Allele frequencies at a locus are not changing • Population is not evolving

  9. Microevolutionary Processes • Drive a population away from genetic equilibrium • Small-scale changes in allele frequencies brought about by: • Natural selection • Gene flow • Genetic drift

  10. Five Conditions • No mutation • Random mating • Gene doesn’t affect survival or reproduction • Large population • No immigration/emigration

  11. Hardy-Weinberg Rule At genetic equilibrium, proportions of genotypes at a locus with two alleles are given by the equation: p2AA + 2pq Aa + q2aa = 1 Frequency of allele A = p Frequency of allele a = q

  12. a A q p AA(p2) Aa(pq) A p aa(q2) Aa(pq) a q Punnett Square p.287

  13. a a a 0.49 AA 0.42 Aa 0.09 aa A A A 0.49 + 0.21 0.21 + 0.09 0.7A 0.3a Frequencies in Gametes F1 genotypes: Gametes:

  14. STARTING POPULATION No Change through Generations 490 AA butterflies Dark-blue wings 420 Aa butterflies Medium-blue wings 90 aa butterflies White wings THE NEXT GENERATION 490 AA butterflies 420 Aa butterflies 90 aa butterflies NO CHANGE THE NEXT GENERATION 490 AA butterflies 420 Aa butterflies 90 aa butterflies NO CHANGE

  15. No Change through Generations Fig. 18-3, p.286

  16. STARTING POPULATION THE NEXT GENERATION THE NEXT GENERATION 490 AA butterflies dark-blue wings 490 AA butterflies dark-blue wings 490 AA butterflies dark-blue wings 420 Aa butterflies medium-blue wings 420 Aa butterflies medium-blue wings 420 Aa butterflies Medium-blue wings 90 aa butterflies white wings Fig.18-3 p.286 90 aa butterflies white wings 90 aa butterflies white wings

  17. Starting population Next generation Next generation 490 AA butterflies dark-blue wings 490 AA butterflies dark-blue wings 490 AA butterflies dark-blue wings 420 Aa butterflies medium-blue wings 420 Aa butterflies medium-blue wings 420 Aa butterflies medium-blue wings 90 aa butterflies white wings 90 aa butterflies white wings 90 aa butterflies white wings Stepped Art Fig. 18-3, p.286

  18. Natural Selection • A difference in the survival and reproductive success of different phenotypes • Acts directly on phenotypes and indirectly on genotypes

  19. Reproductive Capacity & Competition • All populations have the capacity to increase in numbers • No population can increase indefinitely • Eventually the individuals of a population will end up competing for resources

  20. Results of Natural Selection Three possible outcomes: • Directional shift in the range of values for a given trait in some direction • Stabilization of an existing range of values • Disruption of an existing range of values

  21. Results of Natural Selection Fig. 18-4a, p.287

  22. Number of individuals Range of values at time 1 Number of individuals Range of values at time 2 Number of individuals Range of values at time 3 Directional selection Stepped Art Fig. 18-4a, p.287

  23. Number of individuals Range of values at time 2 Number of individuals Range of values at time 3 Stabilizing Selection Number of individuals Range of values at time 1 Stepped Art Fig. 18-4b, p.287

  24. Number of individuals Range of values at time 2 Number of individuals Range of values at time 3 Disruptive Selection Number of individuals Range of values at time 1 Stepped Art Fig. 18-4c, p.287

  25. Directional Selection Number of individuals in the population • Allele frequencies shift in one direction Range of values for the trait at time 1 Number of individuals in the population Range of values for the trait at time 2 Number of individuals in the population Range of values for the trait at time 3

  26. Stabilizing Selection Number of individuals in the population • Intermediate forms are favored and extremes are eliminated Range of values for the trait at time 1 Range of values for the trait at time 2 Range of values for the trait at time 3

  27. Disruptive Selection Number of individuals in the population • Forms at both ends of the range of variation are favored • Intermediate forms are selected against Range of values for the trait at time 1 Number of individuals in the population Range of values for the trait at time 2 Number of individuals in the population Range of values for the trait at time 3

  28. Fig. 18-9, p.291

  29. Sexual Selection • Selection favors certain secondary sexual characteristics • Through nonrandom mating, alleles for preferred traits increase • Leads to increased sexual dimorphism

  30. Sexual Selection Fig. 18-12, p.292

  31. Sickle-Cell Trait: Heterozygote Advantage • Allele HbS causes sickle-cell anemia when heterozygous • Heterozygotes are more resistant to malaria than homozygotes Malaria case Sickle-cell trait less than 1 in 1,600 1 in 400-1,600 1 in 180-400 1 in 100-180 1 in 64-100 more than 1 in 64

  32. Sickle-Cell Trait: Heterozygote Advantage Fig. 18-13a, p.293

  33. less than 1 in 1,600 1 in 400-1,600 1 in 180-400 1 in 100-180 1 in 64-100 more than 1 in 64 Sickle-Cell Trait: Heterozygote Advantage Fig. 18-13b, p.293

  34. Sickle-Cell Trait: Heterozygote Advantage Fig. 18-13c, p.293

  35. Genetic Drift • Random change in allele frequencies brought about by chance • Effect is most pronounced in small populations • Sampling error - Fewer times an event occurs, greater the variance in outcome

  36. Computer Simulation Fig. 18-14a, p.294

  37. Computer Simulation Fig. 18-14b, p.294

  38. Bottleneck • A severe reduction in population size • Causes pronounced drift • Example • Elephant seal population hunted down to just 20 individuals • Population rebounded to 30,000 • Electrophoresis revealed there is now no allele variation at 24 genes

  39. Founder Effect • Effect of drift when a small number of individuals starts a new population • By chance, allele frequencies of founders may not be same as those in original population • Effect is pronounced on isolated islands

  40. Founder Effect phenotypes of mainland population phenotype of island population Fig. 18-15, p.295

  41. Inbreeding • Nonrandom mating between related individuals • Leads to increased homozygosity • Can lower fitness when deleterious recessive alleles are expressed • Amish, cheetahs

  42. Gene Flow • Physical flow of alleles into a population • Tends to keep the gene pools of populations similar • Counters the differences that result from mutation, natural selection, and genetic drift

  43. Speciation & Natural Selection • Natural selection can lead to speciation • Speciation can also occur as a result of other microevolutionary processes • Genetic drift • Mutation

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