1 / 56

Evolution of Populations: Genetic Variation and Hardy-Weinberg Equilibrium

Learn about the evidence for evolution and how sexual reproduction and mutation contribute to genetic diversity. Understand the conditions for Hardy-Weinberg equilibrium and use the Hardy-Weinberg equation to calculate allele and genotype frequencies.

lewisf
Télécharger la présentation

Evolution of Populations: Genetic Variation and Hardy-Weinberg Equilibrium

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Ch. 22/23 Warm-up • List 5 different pieces of evidence for evolution. • (Review) What are the 3 ways that sexual reproduction produces genetic diversity? • What is 1 thing you are grateful for today?

  2. Ch. 23 Warm-up • In a population of 200 mice, 98 are homozygous dominant for brown coat color (BB), 84 are heterozygous (Bb), and 18 are homozygous (bb). • The allele frequencies of this population are: B allele: ___ b allele: ___ • The genotype frequencies are: BB: ___ Bb: ___ bb: ___ • Use the above info to determine the genotype frequencies of the next generation: B (p): ___ b (q): ___ BB (p2): ___ Bb (2pq): ___ bb (q2): ___

  3. The Evolution of Populations Chapter 23

  4. What you must know: • How mutation and sexual reproduction each produce genetic variation. • The conditions for Hardy-Weinberg equilibrium. • How to use the Hardy-Weinburg equation to calculate allelic frequencies and to test whether a population is evolving.

  5. Smallest unit of evolution Microevolution: change in the allele frequencies of a population over generations

  6. Darwin did not know how organisms passed traits to offspring 1866 - Mendel published his paper on genetics Mendelian genetics supports Darwin’s theory  Evolution is based on genetic variation

  7. Sources of Genetic Variation • Point mutations: changes in one base (eg. sickle cell) • Chromosomal mutations: delete, duplicate, disrupt, rearrange  usually harmful • Sexual recombination: contributes to most of genetic variation in a population • Crossing Over (Meiosis – Prophase I) • Independent Assortment of Chromosomes (during meiosis) • Random Fertilization (sperm + egg)

  8. Population genetics: study of how populations change genetically over time Population: group of individuals that live in the same area and interbreed, producing fertile offspring

  9. Gene pool: all of the alleles for all genes in all the members of the population • Diploid species: 2 alleles for a gene (homozygous/heterozygous) • Fixed allele: all members of a population only have 1 allele for a particular trait • The more fixed alleles a population has, the LOWER the species’ diversity

  10. Populations & gene pools • Concepts • a population is a localized group of interbreeding individuals • gene pool is collection of alleles in the population • remember difference between alleles & genes! • allele frequency is how common is that allele in the population • how many A vs. a in whole population

  11. Evolution of populations • Evolution = change in allele frequencies in a population • hypothetical: what conditions would cause allele frequencies to not change? • non-evolving population REMOVE all agents of evolutionary change • very large population size (no genetic drift) • no migration (no gene flow in or out) • no mutation (no genetic change) • random mating (no sexual selection) • no natural selection (everyone is equally fit)

  12. Hardy-Weinberg Principle Hardy-Weinberg Principle: The allele and genotype frequencies of a population will remain constant from generation to generation …UNLESS they are acted upon by forces other than Mendelian segregation and recombination of alleles Equilibrium= allele and genotype frequencies remain constant

  13. Hardy-Weinberg equilibrium • Hypothetical, non-evolving population • preserves allele frequencies • Serves as a model (null hypothesis) • natural populations rarely in H-W equilibrium • useful model to measure if forces are acting on a population • measuring evolutionary change G.H. Hardy mathematician W. Weinberg physician

  14. Conditions for Hardy-Weinberg equilibrium • No mutations. • Random mating. • No natural selection. • Extremely large population size. • No gene flow. If at least one of these conditions is NOT met, then the population is EVOLVING!

  15. p + q = 1 Note: 1 – p = q 1 – q = p Allele Frequencies: Gene with 2 alleles : p, q p = frequency of dominant allele (A) q = frequency of recessive allele (a)

  16. p2 + 2pq + q2 = 1 Genotypic Frequencies: • 3 genotypes (AA, Aa, aa) p2 = AA (homozygous dominant) 2pq = Aa (heterozygous) q2 = aa (homozygous recessive)

  17. Hardy-Weinberg theorem • Counting Alleles • assume 2 alleles = B, b • frequency of dominant allele (B) =p • frequency of recessive allele (b) = q • frequencies must add to 1 (100%), so: p + q = 1 BB Bb bb

  18. Hardy-Weinberg theorem • Counting Individuals • frequency of homozygous dominant: p x p = p2 • frequency of homozygous recessive:q x q = q2 • frequency of heterozygotes: (p x q) + (q x p) = 2pq • frequencies of all individuals must add to 1 (100%), so: p2 + 2pq + q2 = 1 BB Bb bb

  19. B b BB Bb bb H-W formulas • Alleles: p + q = 1 • Individuals: p2 + 2pq + q2 = 1 BB Bb bb

  20. Using Hardy-Weinberg equation population: 100 cats 84 black, 16 white How many of each genotype? q2 (bb): 16/100 = .16 q (b): √.16 = 0.4 p (B): 1 - 0.4 = 0.6 p2=.36 2pq=.48 q2=.16 BB Bb bb What are the genotype frequencies? Must assume population is in H-W equilibrium!

  21. BB Bb bb Using Hardy-Weinberg equation p2=.36 2pq=.48 q2=.16 Assuming H-W equilibrium BB Bb bb Null hypothesis p2=.20 p2=.74 2pq=.64 2pq=.10 q2=.16 q2=.16 Sampled data How do you explain the data? How do you explain the data?

  22. Allele frequencies

  23. Genotypic frequencies

  24. Strategies for solving H-W Problems: • If you are given the genotypes (AA, Aa, aa), calculate p and q by adding up the total # of A and a alleles. • If you know phenotypes, then use “aa” to find q2, and then q. (p = 1-q) • Use p2 + 2pq + q2 to find genotype frequencies. • If p and q are not constant from generation to generation, then the POPULATION IS EVOLVING!

  25. Hardy-weinberg practice problem #1 The scarlet tiger moth has the following genotypes. Calculate the allele and genotype frequencies (%) for a population of 1612 moths. AA = 1469 Aa= 138 aa = 5 Allele Frequencies: A = a = Genotypic Frequencies: AA = Aa = aa =

  26. Hardy-weinberg practice problem #2:PTC Tasters • Taster = AA or Aa Nontaster = aa • Tasters = ____ Nontasters = ___ q2 = q = p + q = 1 p = 1 – q = p2 + 2pq + q2 = 1

  27. Causes of evolution

  28. Conditions for Hardy-Weinberg equilibrium • No mutations. • Random mating. • No natural selection. • Extremely large population size. • No gene flow. If at least one of these conditions is NOT met, then the population is EVOLVING!

  29. Minor Causes of Evolution: #1 - Mutations • Rare, very small changes in allele frequencies #2 - Nonrandom mating • Affect genotypes, but not allele frequencies Major Causes of Evolution: • Natural selection, genetic drift, gene flow (#3-5)

  30. 1. Mutations Mutations can change the frequency of alleles in a population, but this is a very slow effect in humans. Bacteria - this is fast! Mutation is one of the sources of genetic variation that leads to natural selection.

  31. 2. Nonrandom Mating Inbreeding and assortive mating cause an increase in homozygotes. Allele frequencies will not change, but genotype frequencies will.

  32. .024 0.835 0.278 0.332 0.304 0.156 0.489 0.486 0.672 0.009 0.233 0.182 0.150 0.700 0.150 Nonrandom Mating M/M M/N N/N M/M M/N N/N Eskimo Egyptian Chinese Australian OBSERVED EXPECTED from Hardy-Weinberg Theorem

  33. Major Causes of Evolution #3 – Natural Selection • Individuals with variations better suited to environment pass more alleles to next generation

  34. Major Causes of Evolution #4 – Genetic Drift • Small populations have greater chance of fluctuations in allele frequencies from one generation to another • Examples: • Founder Effect • Bottleneck Effect

  35. 4. Genetic Drift = change in allele frequency due to CHANCE. Ex: Billy goat determines which plants survives by randomly chewing off some flowers. So the allele frequency may be not 0.5 R and 0.5r in each generation. 2 types of drift….

  36. Genetic Drift

  37. The Bottleneck Effect (skewed representation of alleles after disasters) can lead to genetic drift. ‘Bottle neck’ is the disaster! Alleles left after disaster may not be 0.5R and 0.5r….Ex: cheetahs and hunting

  38. Bottleneck Effect • Sudden change in environment drastically reduces population size Northern elephant seals hunted nearly to extinction in California

  39. Founder Effect • A few individuals isolated from larger population • Certain alleles under/over represented Polydactyly in Amish population

  40. The Founder Effect—a small number of individuals colonize a new, isolated area— this can lead to genetic drift. Ex: eye disease alleles have a high frequency in the founders of a colony                                                                      <> 1814 - Tristan da Cunha – colonized by 15 people!

  41. Major Causes of Evolution #5 – Gene Flow • Movement of fertile individuals between populations • Gain/lose alleles • Reduce genetic differences between populations

  42. 5.Gene Flow - Migration If populations aren’t completely isolated, individuals can migrate and introduce alleles into another population. Ex: wind… pollinators… This may cause a change in allele frequency in the next generation.

  43. How does natural selection bring about adaptive evolution?

  44. Natural selection can alter frequency distribution of heritable traits in 3 ways: • Directional selection • Disruptive (diversifying) selection • Stabilizing selection

  45. Modes of Natural Selection • Directional Selection • Favors individuals at one end of the phenotypic range • Most common during times of environmental change or when moving to new habitats • Disruptive selection • Favors extreme over intermediate phenotypes • Occurs when environmental change favors an extreme phenotype

  46. Modes of Natural Selection • Stabilizing Selection • Favors intermediate over extreme phenotypes • Reduces variation and maintains the current average • Example: Human birth weight

  47. Disruptive Selection: eg. small beaks for small seeds; large beaks for large seeds Stabilizing Selection: eg. narrow range of human birth weight Directional Selection: eg. larger black bears survive extreme cold better than small ones

  48. Sexual selection • Form of natural selection – certain individuals more likely to obtain mates • Sexual dimorphism: difference between 2 sexes • Size, color, ornamentation, behavior

  49. Sexual selection • Intrasexual– selection within same sex (eg. M compete with other M) • Intersexual– mate choice (eg. F choose showy M)

  50. Preserving genetic variation • Diploidy: hide recessive alleles that are less favorable • Heterozygote advantage: greater fitness than homozygotes • eg. Sickle cell disease

More Related