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Allele Frequencies, Types of Selection & Hardy-Weinberg

Allele Frequencies, Types of Selection & Hardy-Weinberg

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Allele Frequencies, Types of Selection & Hardy-Weinberg

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  1. Allele Frequencies, Types of Selection & Hardy-Weinberg Evolution of Populations: Chapter 16

  2. Evolution of Populations Notes Tuesday, 2/21/12 Take out Notebook Turn in: Natural Selection of Antibiotic Resistance Lab and Adaptation Project

  3. Evolution of Populations Notes

  4. How is evolution defined in genetic terms? Evolution is any change in the relative frequency of alleles in the gene pool of a population. (also called microevolution) Remember: Alleles are different forms of genes.

  5. Microevolution: The frequency of an allele in a gene pool of a population depends on many factors and can change over time. • Over long periods of time, microevolution can lead to macroevolution- change from one species to another.

  6. What is a GENE POOL? A gene pool is the combined genetic information of all members of a particular population.


  8. What is relative frequency? • The relative frequency of an allele is the number of times that allele occurs in a gene pool compared with the number of times other alleles occur. Expressed in percent.

  9. Relative Frequency Practice! • In a population of 50 students, there are 40 alleles for hitchhiker’s thumb and 60 alleles for a straight thumb. What is the relative frequency of alleles for hitchhiker’s thumb? • 40/100 = 0.40 = 40% • What is the relative frequency of alleles for straight thumb? • 60/100 = 0.60 = 60%

  10. How do allele frequencies change during evolution? •

  11. How do allele frequencies change during evolution? • Natural Selection which acts on the phenotype rather than the genotype of an organism. • Mutations (change in the DNA) which are constantly being generated in a gene pool. • Genetic Drift- Random change in allele frequency. • Gene Flow- The movement of alleles into or out of the gene pool. • Non-random Mating – when females prefer one phenotype over another. Also called sexual selection.

  12. Remind us: What is Natural Selection? Individuals with favorable genetic variations will survive and reproduce. Individuals that lack adaptations will most likely die before they ever reproduce. Thus, alleles that are favorable will become more common and those that are not will decrease.

  13. Remind us: What are Mutations? Changes in the DNA and therefore in the genes!

  14. Genetic Driftis random change in allele frequencies in small populations.

  15. Genetic Drift is usually caused by natural disasters like a fire or flood. • Genetic Drift can cause evolution due to chance rather than natural selection.

  16. Genetic Drift causes… the founder effect-when a migration of a small subgroup of a population causes a change in allele frequencies. (also called bottleneck affect)

  17. Imagine: The Island of Bio Students!

  18. Genetic Drift often leads to a decrease in variation. Based on this info and what you know about evolution, do you think genetic drift will help a population survive or will it cause the population to go extinct? Why?

  19. Remember: Populations with less variation are likely to go extinct! • Cheetahs in Africa are one of the most extreme examples of genetic drift. The Cheetahs alive today are the descendants of only a few cheetahs. This means that Cheetahs are VERY genetically similar. • They have less • resistance to • disease and are • more likely to go • extinct.

  20. Lizard Evolution on Islands shows genetic drift in action! •

  21. Gene Flow: the movement of alleles into or out of a population.Immigrants add new alleles.Emigrants take alleles awayGene Flow Animation:

  22. Non-random Mating – when females prefer one phenotype over another. Also called sexual selection. Sexual Selection can cause sexual dimorphism: when males and females of the same species look noticeably different from each other

  23. Example of sexual selection: Peacocks • Female peacocks prefer males with bright beautiful tails, so over time male peacocks have evolved to have very showy tails. • Scientists have linked female preference to important traits such as health, size, and strength. In peacocks, scientists have linked tail size and color to nutrition.

  24. This means that females do not just want to mate with a male with a pretty tail for the look of him- females want to mate with males with pretty tails because they could have only produced this tail if he was able to find nutritious food. The female wants a male who is healthy enough to find food and can provide her offspring with nutritious food too.

  25. Sexual Selection and natural selection may often operate in opposing directions Males have traits like showy plumage in spite of their potential costs such as increased visibility to predators.

  26. Reminder: When reading and answering questions out of the book…. READ CAREFULLY!!!!!!! Don’t just copy the sentences in bold. Try to understand what the question is asking and answer it!

  27. If evolution is change in allele frequency in a population, what if some alleles are lethal? Will the population survive? Will the alleles stay in the population?

  28. Allele Frequencies and Sickle Cell Anemia • Students know why alleles that are lethal in a homozygous individual may be carried in a heterozygote and thus maintained in a gene pool.

  29. How Sickle Cell Works

  30. Sickle Cell and Pain

  31. Malaria Infects Red Blood Cells

  32. A Mutation Story • What is the name of the deadly disease? • What causes Malaria? • How is having one mutated gene beneficial to Africans? • How many genes does an individual need to have to express sickle cell anemia? • How can a mutation be harmful in one environment and helpful in another? • Why would a mutation persist if it kills people? • Why are there more people with sickle cell anemia in one part of the world than in other parts?

  33. Allele Frequency and Sickle Cell Anemia LabWeds-Thurs, Feb. 2/22-2/23

  34. Allele Frequency and Sickle Cell Anemia Background Info Read popcorn style!

  35. Friday, 2/24/12 Warm-Up: Write down Table of Contents! Finish LAB!


  37. Why are alleles that are lethal in a homozygous individual maintained in the gene pool? (Use Sickle Cell and Malaria as an example.)

  38. Types of Selection and Hardy Weinberg Notes(Ch. 16.2) Monday, 2/27/12

  39. In Single- Gene Traits, … there are two phenotypes, whereas in polygenic traits, there are multiple phenotypes. Natural Selection acts differently on each of these.

  40. Is height a single-gene trait or a polygenic trait? Polygenic Trait!

  41. Is Sickle Cell Anemia a single gene trait or a polygenic trait? Single Gene Trait!

  42. Natural Selection on Single-Gene Traits can lead to… …changes in allele frequencies and therefore, evolution! This was shown in the Peppered Moth Simulation.

  43. Natural Selection on Polygenic Traits occurs in three ways: • Disruptive Selection • Stabilizing Selection • Directional Selection

  44. Directional Selection • When individuals at one end of the curve have a higher fitness than those at the other end. • Examples: Finch bills, peppered moths Animation of Directional Selection: •

  45. Stabilizing Selection • When individuals near the center of the curve have higher fitness than individuals at either end of the curve- selection against both extremes. • Examples: human baby size, lizard size, number of children Animation of Stabilizing Selection: •,6452,499573-,00.html

  46. Disruptive Selection • When individuals at the lower and upper ends of the curve have higher fitness than those in the middle. This could cause the population to split into two distinct subgroups. • Examples: duck bills, sexual dimorphism, sickle cell anemia Animation of Disruptive Selection: •

  47. CA BIO STANDARD- Evolution • Students know the conditions for Hardy-Weinberg equilibrium in a population and why these conditions are not likely to appear in nature.