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Microevolution

Microevolution. Chapter 17. Selective Breeding & Evolution. Evolution is genetic change in a line of descent through successive generations Selective breeding practices yield evidence that heritable changes do occur. Domestication of Dogs. Began about 50,000 years ago

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Microevolution

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  1. Microevolution Chapter 17

  2. Selective Breeding & Evolution • Evolution is genetic change in a line of descent through successive generations • Selective breeding practices yield evidence that heritable changes do occur

  3. Domestication of Dogs • Began about 50,000 years ago • 14,000 years ago - artificial selection • Dogs with desired forms of traits were bred • Modern breeds are the result

  4. Results of Artificial Selection • Extremes in size • Great Dane and Chihuahua • Extremes in form • Short-legged dachshunds • English bulldog • Short snout and compressed face • Extreme traits lead to health problems

  5. Evolutionary Theories • Widely used to interpret the past and present, and even to predict the future • Reveal connections between the geological record, fossil record, and organism diversity

  6. Early Scientific Theories • Hippocrates - All aspects of nature can be traced to their underlying causes • Aristotle - Each organism is distinct from all the rest and nature is a continuum or organization

  7. Confounding Evidence • Biogeography • Comparative anatomy • Geologic discoveries

  8. Biogeography • Size of the known world expanded enormously in the 15th century • Discovery of new organisms in previously unknown places could not be explained by accepted beliefs • How did species get from center of creation to all these places?

  9. Comparative Morphology • Study of similarities and differences in body plans of major groups • Puzzling patterns: • Animals as different as whales and bats have similar bones in forelimbs • Some parts seem to have no function

  10. Geological Discoveries • Similar rock layers throughout world • Certain layers contain fossils • Deeper layers contain simpler fossils than shallow layers • Some fossils seem to be related to known species

  11. 19th Century - New Theories • Scientists attempt to reconcile evidence of change with traditional belief in a single creation event • Two examples • Georges Cuvier - multiple catastrophes • Jean Lamarck - inheritance of acquired characteristics

  12. The Theory of Uniformity • Lyell’s Principles of Geology • Subtle, repetitive processes of change had shaped Earth • Challenged the view that Earth was only 6,000 years old

  13. Darwin’s Voyage • At age 22, Charles Darwin began a five-year, round-the-world voyage aboard the Beagle • In his role as ship’s naturalist, he collected and examined the species that inhabited the regions the ship visited

  14. Voyage of the Beagle EQUATOR Galapagos Islands Figure 17.4ePage 275

  15. Darwin Wolf Pinta Genovesa Marchena Santiago Bartolomé Seymour Rabida Baltra Pinzon Fernandia Santa Cruz Santa Fe Tortuga San Cristobal Española Floreana GalapagosIslands Volcanic islands far off coast of Ecuador All inhabitants are descended from species that arrived on islands from elsewhere Isabela Figure 17.4dPage 275

  16. Malthus - Struggle to Survive • Thomas Malthus, a clergyman and economist, wrote essay that Darwin read on his return to England • Argued that as population size increases, resources dwindle, the struggle to live intensifies, and conflict increases

  17. Galapagos Finches • Darwin observed finches with a variety of lifestyles and body forms • On his return, he learned that there were 13 species • He attempted to correlate variations in their traits with environmental challenges

  18. Darwin’s Theory A population can change over time when individuals differ in one or more heritable traits that are responsible for differences in the ability to survive and reproduce.

  19. Alfred Wallace • Naturalist who arrived at the same conclusions Darwin did • Wrote to Darwin describing his views • Prompted Darwin to finally present his ideas in a formal paper

  20. 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

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

  22. 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

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

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

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

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

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

  28. 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

  29. a A q p AA(p2) Aa(pq) A p aa(q2) Aa(pq) a q Punnett Square In-text figurePage 280

  30. Conditions for Hardy-Weinberg • Single gene , there can be no sex-linkage or mutliple alleles • Mating must be random • No migration into or out of population • No gene changes through mutations • All genotypes must be viable, survive and produce the same number of offspring • Population must be of infinite size

  31. 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: In-text figurePage 280

  32. 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 Figure 17.9Page 281 90 aa butterflies NO CHANGE

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

  34. 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

  35. Variation in Populations • All individuals have the same genes that specify the same assortment of traits • Most genes occur in different forms (alleles) that produce different phenotypes • Some phenotypes compete better than others

  36. Change over Time • Over time, the alleles that produce the most successful phenotypes will increase in the population • Less successful alleles will become less common • Change leads to increased fitness • Increased adaptation to environment

  37. Results of Natural Selection Three possible outcomes: • A 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

  38. 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 Figure 17.10Page 282 Range of values for the trait at time 3

  39. Peppered Moths • Prior to industrial revolution, most common phenotype was light colored • After industrial revolution, dark phenotype became more common

  40. Pesticide Resistance • Pesticides kill susceptible insects • Resistant insects survive and reproduce • If resistance has heritable basis, it becomes more common with each generation

  41. Antibiotic Resistance • First came into use in the 1940s • Overuse has led to increase in resistant forms • Most susceptible cells died out and were replaced by resistant forms

  42. 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 Figure 17.12Page 284 Range of values for the trait at time 3

  43. Selection for Gall Size • Gall-making fly has two major predators • Wasps prey on larvae in small galls • Birds eat larvae in large galls • Flies that cause intermediate-sized galls have the highest fitness

  44. 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 Figure 17.14Page 285

  45. African Finches 60 nestlings • Selection favors birds with very large or very small bills • Birds with intermediate-sized bill are less effective feeders 50 drought survivors 40 Number of individuals 30 20 10 10 12.8 15.7 18.5 Widest part of lower bill (millimeters) Figure 17.15Page 285

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

  47. Balanced Polymorphism • Polymorphism - “having many forms” • Occurs when two or more alleles are maintained at frequencies greater than 1 percent

  48. 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 Figure 17.17Page 286-287

  49. 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

  50. 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

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