Ch. 22/23 Warm-up

1. Ch. 22/23 Warm-up • List 5 different pieces of evidence for evolution. • What did Darwin do when he returned home? • How did Lamarck’s theory help Darwin?

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: ___

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. 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

11. 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!

12. 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)

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

14. Allele frequencies

15. Genotypic frequencies

16. 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!

17. 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 =

18. Hardy-weinberg practice problem #2 • If 9% of an African population is born with a severe form of sickle-cell anemia (ss), what percentage of the population will be more resistant to malaria because they are heterozygous(Ss) for the sickle-cell gene?

19. Step 1: Find f(ss) We are told 9% of the population is ss, the homozygous recessive genotype. Therefore, f(ss) = .09

20. Step 2: Find P or Q Since f(ss) = Q2 .09 = Q2 Q = .30

21. Step 3: Find the Other Frequency Since P + Q = 1 P + .30 = 1 P = .70

22. Step 4: Find Unknown Phenotype or Genotype Frequency • The question asks you to find the % of heterozygotes. • Since f(Ss) = 2PQ • P = .70 • Q = .30 • f(Ss) = 2(.70)(.30) • f(Ss) = .41 • 41% of the population is heterozygous (Ss) for the trait.

23. Causes of evolution

24. 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!

25. 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)

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

27. 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

28. Genetic Drift

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

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

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

32. How does natural selection bring about adaptive evolution?

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

34. 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

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

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

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

38. HHMI Video:Natural Selection in Humans Running Time: 14:03 min

39. Natural selection cannot fashion perfect organisms. • Selection can act only on existing variations. • Evolution is limited by historical constraints. • Adaptations are often compromises. • Chance, natural selection, and the environment interact.

40. Sample Problem Define the following examples as directional, disruptive, or stabilizing selection: • Tiger cubs usually weigh 2-3 lbs. at birth • Butterflies in 2 different colors each represent a species distasteful to birds • Brightly colored birds mate more frequently than drab birds of same species • Fossil evidence of horse size increasing over time