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Microevolution and Hardy-Weinberg Equilibrium: Population Genetics

Explore the evidence supporting the theory of evolution and learn about genetic variation, forces of evolution, and the Hardy-Weinberg principle. Understand how changes in allele frequencies and genetic drift affect populations.

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Microevolution and Hardy-Weinberg Equilibrium: Population Genetics

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  1. Chapter 16 Microevolution: Population Genetics and Hardy-Weinberg Equilibrium Mrs. Stewart AP Biology

  2. Bell Work List the evidence (at least 3) that supports the theory of evolution.

  3. Standard • CLE 3210.5.3 Explain how genetic variation in a population and changing environmental conditions are associated with adaptation and the emergence of new species.

  4. Objective • List examples of the forces that act upon populations to alter the allele frequencies • Explain how these forces affect allele frequencies in a population • Calculate Hardy-Weinberg problems accurately

  5. Variation Within a Population • Populations show variations in their phenotypes: • Different shaped beaks • Different colors • Different athletic abilities • Different immune system responses

  6. Sources of Variation Within a Population • Variations in the genotypes of a population arise by: • mutation – changes in genes that occur either naturally or influenced by environment • Passed to offspring if occurs in gametes • Recombination– the law of independent assortment (chromosomes) and crossing over during meiosis • random pairing of gametes (sexual reproduction) – organisms produce large numbers of gametes, so the union of a particular pair is strictly by chance.

  7. Population Genetics • The study of evolution from a genetic point of view • Microevolution: change in the allele frequencies of a population

  8. Reminder: • Phenotypes = physical appearance of traits • Genotypes = Which “variations” (alleles) a person inherited • Genes = the section of DNA that codes for a protein. Expression of these genes means those proteins are produced, which directly leads to the physical appearance of the trait (phenotype)

  9. Review: Remember: Genes are alleles R = non-red r = red Question: What color hair would a Rr person have?

  10. Review: • Classify each of the following examples a phenotype or a genotype. • Huntington’s disease • Able to taste PTC (bitter) • Carrier of Cystic Fibrosis

  11. Genotype Expression Review: • F = freckles, f = no freckles • If a gene is inherited as complete dominance, and a person has FF, what is the only protein expressed? • If a person has Ff, what protein(s) are expressed?

  12. Genotype Expression Review Part II • L = normal lactase; l = abnormal lactase • Why is a person who is Ll still able to break down lactose?

  13. Question: • How do we know if a person showing the dominant phenotype has two dominant alleles or if they are heterozygous?

  14. The Gene Pool • A collection of all the alleles in a population is called the GENE POOL. • In other words: All of the possible alleles (variations) that are present, for each gene, within a population • Frogs in a pond • Trees in the forest • People in a town

  15. Allele Frequency • Allele frequencyis the number of times a specific allele occurs in the gene pool • This is in comparison to how often the other alleles occur too

  16. Relative Allele Frequencies • determined by dividing the total number of a certain allele by the total number of alleles of all types in the population • Total number of a certain allele___ total number of all alleles in a population • Expressed as a percentage or a decimal.

  17. Calculate allele frequency

  18. Hardy – Weinberg Allele Frequencies • If all diploid organisms have 2 alleles, then we can calculate their frequencies as such: • p = frequency of dominant allele • q = frequency of recessive allele • The frequency of all alleles in a population will add up to 1 • p + q = 1

  19. Microevolution is measured by any change in the relative frequency of alleles in a population. • Remember: Populations, not individual organisms, evolve over time.

  20. Hardy - Weinberg • Allele frequencies in the gene pool do not change unless acted upon by certain forces. • Genetic mutations • Gene flow • Genetic drift • Nonrandom mating • Natural selection

  21. Hardy – Weinberg Principle • The Hardy-Weinberg principle describes a population that is not evolving • If a population does not meet the criteria of the Hardy-Weinberg principle, it can be concluded that the population is evolving

  22. Five conditions that affect the relative frequency of alleles • This is known as the Hardy Weinberg Genetic Equilibrium model • used to determine and understand the forces that act upon genetic equilibrium

  23. 1. Mutations • Mutations are changes in the DNA.

  24. 2. Gene Flow • The flow of genes between populations • Emigration and immigration cause gene flow between populations and can thus affect gene frequencies.

  25. Immigration vs. Emigration • Immigration: Gene flow INTO a population • Emigration: Gene flow OUT of a population

  26. Human Evolution from Gene Flow

  27. 3. Genetic Drift • Genetic drift is a change in allele frequencies due to random events. • Genetic drift operates most strongly in small populations.

  28. Think – pair - share • What are some other random events that could affect allele frequencies in a population?

  29. Random Mating • Do humans randomly mate? • No. • Random mating: happens more by chance and not by choice (has less effect on allele frequencies)

  30. 4. Nonrandom Mating: Sexual Selection • Mating is nonrandom whenever individuals may choose partners. • Sexual selection occurs when certain traits increase an individual’s success at mating. • Sexual selection affects the allele frequencies of a population. • Courtship ritual

  31. 5. Natural Selection • The ongoing process in nature where the presence or absence of certain factors in the environment “select” which traits/variations within a population are most successful • Most traits are polygenic = many variations

  32. Without Natural Selection, polygenic traits maintain a bell curve

  33. With Natural Selection… • Three general patterns • Stabilizing Selection • favors the formation of average traits. • Disruptive Selection • favors extreme traits rather than average traits. • Directional Selection • favors the formation of one of the extreme traits.

  34. Think – Pair – Share • How does evolution by natural selection depend on variation within a population?

  35. Bellwork – which natural selection pattern is represented?

  36. HW Calculations • If p and q represent the relative frequencies of the only two possible alleles in a population at a particular locus, then… • p2 + 2pq + q2 = 1 • p2 = # homozygous dominant individuals • q2 = # homozygous recessive individuals • 2pq = # heterozygous individuals

  37. Applying the HW principle: • We can assume the locus that causes phenylketonuria (PKU) is in Hardy-Weinberg equilibrium given that: • The PKU gene mutation rate is low • Mate selection is random with respect to whether or not an individual is a carrier for the PKU allele

  38. Natural selection can only act on rare homozygous individuals who do not follow dietary restrictions • The population is large • Migration has no effect as many other populations have similar allele frequencies

  39. PKU Problem • If the number of babies born with PKU is 1 out of 10,000, then how many people in the population are heterozygous for this trait? • p + q = 1 • p2 + 2pq + q2 = 1

  40. What do you know? • The problem gave you a phenotype which we know is the recessive phenotype. • So, 1 in 10,000 = the homozygous recessive = q2

  41. Practice: • The occurrence of PKU is 1 per 10,000 births • q2 = 0.0001 • q = 0.01 • The frequency of normal alleles is • p = 1 – q = 1 – 0.01 = 0.99 • The frequency of carriers is • 2pq = 2 x 0.99 x 0.01 = 0.0198 • or approximately 2% of the U.S. population

  42. Red Hair Practice • R = non red, r = red hair • If 2% of humans on the planet have red hair, then what percent of humans are heterozygous for this trait?

  43. Red Hair • Red hair = q2 • q2 = .02 … q = .14 • p + q = 1 … p = 1 - .14 = .86 • 2pq = 2 (.86)(.14) = .24 or 24%

  44. Practice problems

  45. HW Goldfish Lab • Each group should obtain the following items: • 10 pretzel goldfish • 10 cheddar goldfish

  46. Activity • Discover the allele frequency for brown (pretzel) fish. • Add up the total number of all alleles in your population of fish • Brown fish (pretzel) = BB • Orange fish (regular) = Bb • Yellow fish (parmesan) = bb • Divide the number of “brown” alleles by the total number of alleles.

  47. Activity • Immigrate: 10 yellow fish move in from a neighboring pond • Emigrate: 5 brown fish move out to another pond • Recalculate the brown allele frequency • Question: How does immigration or emigration affect allele frequencies in a gene pool?

  48. Activity • Genetic Drift:Perform a random act to your population • Recalculate the brown allele frequency • Question: How does genetic drift affect allele frequencies in a gene pool?

  49. Activity • What if every fish in your “pond” only wants to mate with an brown fish? • What would that do to your allele frequencies?

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