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Natural Selection

Natural Selection. DNA encodes information that interacts with the environment to influence phenotype. Among The Traits That Can Be Influenced By Genetically Determined Responses to the Environment Are: The Viability in the Environment Given Alive, the Mating Success in the Environment

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Natural Selection

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  1. Natural Selection

  2. DNA encodes information that interacts with the environment to influence phenotype Among The Traits That Can Be Influenced By Genetically Determined Responses to the Environment Are: • The Viability in the Environment • Given Alive, the Mating Success in the Environment • Given Alive and Mated, Fertility or Fecundity in the Environment.

  3. Viability

  4. Mating Success Normal Diet Mentally Retarded Institutionalized Low Chance of Mating Low Phenylalanine Diet Normal Intelligence High Chance of Mating p/p Baby Born With Normal Brain p/p fetus develops in Low Phenylalanine in utereo Environment

  5. Fecundity/Fertility H/+ In A Society With No Birth Control, No Genetic Literacy, and Low Expected Lifespan: Normal Fecundity H/+ In A Society With Birth Control, Genetic Literacy, and High Expected Lifespan: Low Fecundity

  6. Why Are Viability, Mating Success, and Fecundity/Fertility Important Phenotypes in Evolution? Because All Of These Phenotypes Influence The Chances For Successful DNA Replication

  7. Physical Basis of Evolution • DNA can replicate • DNA can mutate and recombine • DNA encodes information that interacts with the environment to influence phenotype

  8. Physical Basis of Evolution • DNA can replicate • DNA can mutate and recombine • DNA encodes information that interacts with the environment to influence phenotype Viability Mating Success Fecundity/Fertility

  9. Physical Basis of Evolution • DNA can replicate • DNA can mutate and recombine • DNA encodes information that interacts with the environment to influence phenotype These Are Combined Into A Single Phenotype of Reproductive Success Or FITNESS Viability Mating Success Fecundity/Fertility

  10. DNA can mutate and recombine DNA can replicate Genotypic Variation In Demes and Species Heritable Variation In Fitness Phenotypic Variation In Fitness Environment

  11. Natural Selection Is Heritable Variation In Fitness That Is, The Genes Borne By A Gamete Influence The Probability of That Gamete Being Passed On To The Next Generation. THINK LIKE A GAMETE!

  12. It’s the Environment, stupid! NATURAL SELECTION IS NOT CIRCULAR DNA can mutate and recombine DNA can replicate Genotypic Variation In Demes and Species Heritable Variation In Fitness Phenotypic Variation In Fitness

  13. Environment VAA VAa Vaa Viabilities AA p2 VAA Aa 2pqVAa aa q2 Vaa Adult Frequencies  Environment CAA CAa Caa Mating Prob. AA p2 VAACAA Aa 2pqVAaCAa aa q2 VaaCaa Mated Adult Frequencies  Environment bAA bAa baa Ave. No. Offspring AA p2 VAACAAbAA Aa 2pqVAaCAabAa aa q2 VaaCaabaa Mated Adult Frequencies Weighted By No. of Off.  Natural Selection At A Single Locus in A Randomly Mating Deme Zygotic Frequencies

  14. Convert to Freq. By Dividing by  = W = p2WAA+2pqWAa+q2Waa AA p2 WAA/W Aa 2pqWAa/W aa q2Waa/W Mated Adult Frequencies 1 1/2 1/2 1 Meiosis A p’= p2 WAA/W + pqWAa/W a q’= q2 Waa/W + pqWAa/W Gene Pool Let WAA = VAACAAbAA; WAa = VAaCAabAa; Waa = VaaCaabaa Zygotic Frequencies Environment WAA WAa Waa Fitness Mated Adult Frequencies Weighted By No. of Off. 

  15. p’= p2 WAA/W + pqWAa/W =( p2 WAA+ pqWAa)/W p’ = p(pWAA+ qWAa)/W Gene Pool

  16. p = p’ - p = p(pWAA+ qWAa)/W - p = p[pWAA+ qWAa)/W - 1] p = p[pWAA+ qWAa- W]/W Does Evolution Occur?

  17. Note, W = W(p+q)=pW+qW p = p[pWAA+ qWAa- W]/W =p[p(WAA-W)+ q(WAa-W)]/W Since p and W are always > 0, This is the only part of the equation That Can Change Sign and Hence Determine the Direction of Evolution Under Natural Selection. Does Evolution Occur?

  18. What is: p(WAA-W)+ q(WAa-W)? Mean Phenotype of Fitness Does Evolution Occur?

  19. What is: p(WAA-W)+ q(WAa-W)? Does Evolution Occur? Genotypic Deviations for the Phenotype of Fitness

  20. What is: p(WAA-W)+ q(WAa-W)? Does Evolution Occur? This is the Average Excess of the A Allele for the Phenotype of Fitness

  21. Does Evolution Occur? p = paA/W

  22. Does Evolution Occur? p = paA/W Natural Selection is An Evolutionary Force Whenever p ≠ 0 or p ≠ 1 (that is, there is Genetic variation) and when aA ≠ 0 (that is, When there is heritable variation in the Phenotype of fitness).

  23. To Understand Natural SelectionTHINK LIKE A GAMETE!

  24. Sickle Cell Anemia In Africa An Example of Natural Selection

  25. The Sickle Cell Mutation

  26. Infection of a Red Blood Cell By a Malarial Parasite • Sickle-Cells Are Filtered Out Preferentially by the Spleen • Malaria Infected Cells Are Often Filtered Out Because of Sickling Before the Parasite Can Complete Its Life Cycle • The Sickle Cell Allele is Therefore an Autosomal, Dominant Allele for Malarial Resistance.

  27. The SickleCell Anemia Phenotype

  28. Most Deaths Due to Sickle Cell Anemiaand Due to Malaria Occur BeforeAdulthood. Viability Is The Phenotypeof Living To Adulthood • In a non-Malarial Environment, The S Allele is a Recessive Allele For Viability Because Only the Homozygotes Get Sickle Cell Anemia. • In a Malarial Environment, The S Allele is an Overdominant Allele For Viability Because Only the Heterozygotes Are Resistant to Malaria And Do Not Get Sickle Cell Anemia.

  29. Two Complications to This Simple Story in Africa: • Epidemic Malaria is Recent to Most of Wet, Tropical Africa and the Process of Adaptation to Malaria in Africa Is Still Not in Equilibrium. • There is a Third Allele, Hemoglobin C, Involved in the Adaptation to Malaria in Africa.

  30. About 2000 years ago, A Malayo-Indonesian Colony Was Established on Madagasgar Epidemic Malaria in Africa

  31. Epidemic Malaria in Africa This Colony Introduced The Malaysian Agricultural Complex into This Region

  32. Epidemic Malaria in Africa This Agricultural Complex Was Taken Up By Bantu-Speaking Peoples, Followed by A Large Expansion of the Bantu In Africa About 1500 years Ago.

  33. The Malaysian Agricultural Complex In Africa • Is associated with slash-and-burn agriculture: Provides habitat and breeding sites for Anopheles gambiae, the primary mosquito vector for falciparum malaria. • Results in the high local densities of human populations that are necessary to establish and maintain malaria as a common disease.

  34. Epidemic Malaria in Africa

  35. The Hemoglobin C Mutation Hb-S GTG Hb-A GAG Valine Glutamic Acid Hb-C AAG 6th Codon Lysine

  36. The Hemoglobin C Mutation Hb-C Is A “Recessive” Allele for Malarial Resistance

  37. Hb-A, S and C

  38. Hb-A, S and C The A and S Alleles Define An Autosomal Recessive Genetic Disease: Selection Will Insure it is Rare But Difficult to Eliminate in a Random Mating Population.

  39. Hb-A, S and C • The A and C Alleles Define A Set of Neutral Alleles in a Non-malarial Environment: Their Frequencies Are Determined by Genetic Drift and Mutation.

  40. Hb-A, S and C Observed Relative Viabilities In Western Tropical Africa

  41. Hb-A, S and C • CC is the Fittest Genotype By Far • If Natural Selection is “Survival of the Fittest”, Then Natural Selection Should Increase the Frequency of the C allele and the CC Genotype. • Contrary to Rumor, Natural Selection is Not “Survival of the Fittest.” • Natural Selection Is Heritable Variation in Fitness, so Think Like A Gamete: Which Gamete Has the Highest Average Excess of Fitness?

  42. Initial Gene Pool Before Malaria pC=.005 pS=.005

  43. Initial Ave. Fitness After Transition to Malaysian Agricultural Complex pC=.005 pS=.005 Under Random Mating, the Mean Phenotype = W = 0.901

  44. Initial Phenotypes After Transition to Malaysian Agricultural Complex pC=.005 pS=.005

  45. Initial Phenotypes After Transition to Malaysian Agricultural Complex aA = -0.0005 aS = 0.0935 aC = 0.0000

  46. Initial Phenotypes After Transition to Malaysian Agricultural Complex The Initial Adaptive Response To A Malarial Environment Mediated By Natural Selection Is To Decrease A, Increase S, and Leave C The Same px = px(ax)/W) aA = -0.0005 aS = 0.0935 aC = 0.0000

  47. Gene Pool After Several Generations of Selection Under A Malarial Environment pS = 0.045 W = 0.907 pC = 0.005

  48. Gene Pool After Several Generations of Selection Under A Malarial Environment pC=.005

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