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Deviations from HWE I. Mutation II. Migration III. Non-Random Mating IV. Genetic Drift

Deviations from HWE I. Mutation II. Migration III. Non-Random Mating IV. Genetic Drift V. The Neutral Theory. Deviations from HWE I. Mutation II. Migration III. Non-Random Mating IV. Genetic Drift V. The Neutral Theory A. Variation

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Deviations from HWE I. Mutation II. Migration III. Non-Random Mating IV. Genetic Drift

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  1. Deviations from HWE I. Mutation II. Migration III. Non-Random Mating IV. Genetic Drift V. The Neutral Theory

  2. Deviations from HWE I. Mutation II. Migration III. Non-Random Mating IV. Genetic Drift V. The Neutral Theory A. Variation 1. Phenotypic variation was often interpreted as having selective value; in fact, most studies confirmed that under one environmental condition or another, there was a difference in fitness among variations.Mayr (1963) "it is altogether unlikely that two genes would have identical selective value under all conditions under which they may coexist in a population. Cases of neutral polymorphism do not exist."

  3. Deviations from HWE I. Mutation II. Migration III. Non-Random Mating IV. Genetic Drift V. The Neutral Theory A. Variation 1.Phenotypic variation was often interpreted as having selective value; in fact, most studies confirmed that under one environmental condition or another, there was a difference in fitness among variations. Mayr (1963) "it is altogether unlikely that two genes would have identical selective value under all conditions under which they may coexist in a population. Cases of neutral polymorphism do not exist." 2. In the 1960's - lots of electrophoretic work revealed a vast amount of variability - variability at the gene or protein level that did not necessarily correlate with morphological variation. These are silent mutations in DNA, or even neutral substitution mutations.This variation results in heterozygosity.

  4. Deviations from HWE I. Mutation II. Migration III. Non-Random Mating IV. Genetic Drift V. The Neutral Theory A. Variation 3. Most populations showed mean heterozygosities across ALL loci of about 10%. - And, about 20-30% of all loci are polymorphic (have at least 2 alleles with frequencies over 1%). Drosophila has 10,000 loci, so 3000 are polymorphic. At these polymorphic loci, H = .33 Conclusion - lots of variation at a genetic level... is this also solely maintained by selection?

  5. Deviations from HWE I. Mutation II. Migration III. Non-Random Mating IV. Genetic Drift V. The Neutral Theory A. Variation B. Genetic Load

  6. Deviations from HWE I. Mutation II. Migration III. Non-Random Mating IV. Genetic Drift V. The Neutral Theory A. Variation B. Genetic Load 1. "HARD" Selection can 'cost' a population individuals:

  7. Deviations from HWE I. Mutation II. Migration III. Non-Random Mating IV. Genetic Drift V. The Neutral Theory A. Variation B. Genetic Load 1. "HARD" Selection can 'cost' a population individuals: - those that die as a consequence of differential fitness values.

  8. Deviations from HWE I. Mutation II. Migration III. Non-Random Mating IV. Genetic Drift V. The Neutral Theory A. Variation B. Genetic Load 1. "HARD" Selection can 'cost' a population individuals: - those that die as a consequence of differential fitness values. - the "breeding population" is smaller than the initial population.

  9. Deviations from HWE I. Mutation II. Migration III. Non-Random Mating IV. Genetic Drift V. The Neutral Theory A. Variation B. Genetic Load 1. "HARD" Selection can 'cost' a population individuals: - those that die as a consequence of differential fitness values. - the "breeding population" is smaller than the initial population. - Reproductive output must compensate for this loss of individuals

  10. Deviations from HWE I. Mutation II. Migration III. Non-Random Mating IV. Genetic Drift V. The Neutral Theory A. Variation B. Genetic Load 1. "HARD" Selection can 'cost' a population individuals: - those that die as a consequence of differential fitness values. - the "breeding population" is smaller than the initial population. - Reproductive output must compensate for this loss of individuals - The stronger the "hard" selection, the more individuals are lost and the higher the compensatory reproductive effort must be.

  11. Deviations from HWE I. Mutation II. Migration III. Non-Random Mating IV. Genetic Drift V. The Neutral Theory A. Variation B. Genetic Load 1. "HARD" Selection can 'cost' a population individuals: - those that die as a consequence of differential fitness values. - the "breeding population" is smaller than the initial population. - - Reproductive output must compensate for this loss of individuals - The stronger the "hard" selection, the more individuals are lost and the higher the compensatory reproductive effort must be. - The 'cost' of replacing an allele with a new, adaptive allele = "Genetic Load" (L) L = (optimal fitness - mean fitness)/optimal fitness. Essentially, this is a measure of the proportion of individuals that will die as a consequence of this "hard" selection. The lower the mean fitness, the further the population is from the optimum, and the more deaths there will be.

  12. B. Genetic Load • 1. "HARD" Selection can 'cost' a population individuals: • 2. Why is this a problem?

  13. B. Genetic Load 1. "HARD" Selection can 'cost' a population individuals: 2. Why is this a problem? - If variation is maintained by selection, we are probably talking about "heterosis" - selection for the heterozygote where the heterozygote has the highest fitness (and both alleles are maintained). (Selection against the heterozygote can only maintain variation at equilibrium, and this is unstable).

  14. B. Genetic Load • 1. "HARD" Selection can 'cost' a population individuals: • 2. Why is this a problem? • - If variation is maintained by selection, we are probably talking about "heterosis" - selection for the heterozygote where the heterozygote has the highest fitness (and both alleles are maintained). • - The problem is that load can be high in this situation, because lots of homozygotes are produced each generation, just to die by selection.

  15. B. Genetic Load • 1. "HARD" Selection can 'cost' a population individuals: • 2. Why is this a problem? • - If variation is maintained by selection, we are probably talking about "heterosis" - selection for the heterozygote where the heterozygote has the highest fitness (and both alleles are maintained). • - The problem is that load can be high in this situation, because lots of homozygotes are produced each generation, just to die by selection. • - Let's consider even a "best case" scenario:

  16. B. Genetic Load • 1. "HARD" Selection can 'cost' a population individuals: • 2. Why is this a problem? • - If variation is maintained by selection, we are probably talking about "heterosis" - selection for the heterozygote where the heterozygote has the highest fitness (and both alleles are maintained). • - The problem is that load can be high in this situation, because lots of homozygotes are produced each generation, just to die by selection. • - Let's consider even a "best case" scenario: • - mean fitness = 1 - H((s+t)/2)

  17. B. Genetic Load • 1. "HARD" Selection can 'cost' a population individuals: • 2. Why is this a problem? • - If variation is maintained by selection, we are probably talking about "heterosis" - selection for the heterozygote where the heterozygote has the highest fitness (and both alleles are maintained). • - The problem is that load can be high in this situation, because lots of homozygotes are produced each generation, just to die by selection. • - Let's consider even a "best case" scenario: • - mean fitness = 1 - H((s+t)/2) • - If s and t = .1 (very weak), and H = .33 (average for Drosophila, • above), then the mean fitness = 0.967.

  18. B. Genetic Load 1. "HARD" Selection can 'cost' a population individuals: 2. Why is this a problem? - If variation is maintained by selection, we are probably talking about "heterosis" - selection for the heterozygote where the heterozygote has the highest fitness (and both alleles are maintained). - The problem is that load can be high in this situation, because lots of homozygotes are produced each generation, just to die by selection. - Let's consider even a "best case" scenario: - mean fitness = 1 - H((s+t)/2) - If s and t = .1 (very weak), and H = .33 (average for Drosophila, above), then the mean fitness = 0.967. - Not bad; not much death due to selection in this situation...

  19. B. Genetic Load 1. "HARD" Selection can 'cost' a population individuals: 2. Why is this a problem? - If variation is maintained by selection, we are probably talking about "heterosis" - selection for the heterozygote where the heterozygote has the highest fitness (and both alleles are maintained). - The problem is that load can be high in this situation, because lots of homozygotes are produced each generation, just to die by selection. - Let's consider even a "best case" scenario: - mean fitness = 1 - H((s+t)/2) - If s and t = .1 (very weak), and H = .33 (average for Drosophila, above), then the mean fitness = 0.967. - Not bad; not much death due to selection in this situation... - HOWEVER, there are 3000 polymorphic loci across the genome. So, mean fitness across the genome = (0.967)^3000!

  20. B. Genetic Load • 1. "HARD" Selection can 'cost' a population individuals: • 2. Why is this a problem? • - If variation is maintained by selection, we are probably talking about "heterosis" - selection for the heterozygote where the heterozygote has the highest fitness (and both alleles are maintained). • - The problem is that load can be high in this situation, because lots of homozygotes are produced each generation, just to die by selection. • - Let's consider even a "best case" scenario: • - mean fitness = 1 - H((s+t)/2) • - If s and t = .1 (very weak), and H = .33 (average for Drosophila, • above), then the mean fitness = 0.967. • - Not bad; not much death due to selection in this situation... • - HOWEVER, there are 3000 polymorphic loci across the genome. So, mean fitness across the genome = (0.967)^3000!This becomes ridiculously LOW (.19 x 10^-44) relative to the best case genome that is heterozygous at every one of the 3000 loci. - So, some individuals die because they are homozygous (and less fit) at A, others die because they are homozygous (and less fit) at B, other die because they are homozygous (and less fit) at C, and so forth...

  21. B. Genetic Load 1. "HARD" Selection can 'cost' a population individuals: 2. Why is this a problem? - If variation is maintained by selection, we are probably talking about "heterosis" - selection for the heterozygote where the heterozygote has the highest fitness (and both alleles are maintained). - The problem is that load can be high in this situation, because lots of homozygotes are produced each generation, just to die by selection. - Let's consider even a "best case" scenario: - mean fitness = 1 - H((s+t)/2) - If s and t = .1 (very weak), and H = .33 (average for Drosophila, above), then the mean fitness = 0.967. - Not bad; not much death due to selection in this situation... In this case, the load is SO GREAT across the genome that almost NOBODY lives to reproduce. And those that do can not possibly produce enough offspring to compensate for this amount of death.

  22. B. Genetic Load 1. "HARD" Selection can 'cost' a population individuals: 2. Why is this a problem? So, hard selection can not be SOLELY responsible for the variation we observe... a population could not sustain itself under this amount of genetic load...

  23. B. Genetic Load 1. "HARD" Selection can 'cost' a population individuals: 2. Why is this a problem? 3. Solutions a. Selectionists - Not all selection is "hard", imposing additional deaths above background mortality.

  24. B. Genetic Load 1. "HARD" Selection can 'cost' a population individuals: 2. Why is this a problem? 3. Solutions a. Selectionists - Not all selection is "hard", imposing additional deaths above background mortality. - There is also "soft" selection, in which the death due to selection occurs as a component of background mortality, not in addition to it.

  25. B. Genetic Load 1. "HARD" Selection can 'cost' a population individuals: 2. Why is this a problem? 3. Solutions a. Selectionists - Not all selection is "hard", imposing additional deaths above background mortality. - There is also "soft" selection, in which the death due to selection occurs as a component of background mortality, not in addition to it. - For instance, consider territoriality or competition for a resource. Suppose there is only enough food or space to support 50 individuals, but 60 offspring are produced each generation. Well, each generation there are 10 deaths and there are 50 "winners".

  26. B. Genetic Load 1. "HARD" Selection can 'cost' a population individuals: 2. Why is this a problem? 3. Solutions a. Selectionists - Suppose we have a population of aa homozygotes initially. All the territories are occupied by aa individuals and 10 individuals die.

  27. B. Genetic Load 1. "HARD" Selection can 'cost' a population individuals: 2. Why is this a problem? 3. Solutions a. Selectionists - Suppose we have a population of aa homozygotes initially. All the territories are occupied by aa individuals and 10 individuals die. - Well, If an 'A' allele is produce by mutation and heterozygotes have the highest relative fitness (probability of acquiring a territory), then the allele "A" increase in frequency to equilibrium....

  28. B. Genetic Load 1. "HARD" Selection can 'cost' a population individuals: 2. Why is this a problem? 3. Solutions a. Selectionists - Suppose we have a population of aa homozygotes initially. All the territories are occupied by aa individuals and 10 individuals die. - Well, If an 'A' allele is produce by mutation and heterozygotes have the highest relative fitness (probability of acquiring a territory), then the allele "A" increase in frequency to equilibrium.... - Selection occurs, BUT THERE ARE STILL ONLY 10 DEATHS PER GENERATION.

  29. B. Genetic Load 1. "HARD" Selection can 'cost' a population individuals: 2. Why is this a problem? 3. Solutions a. Selectionists - Suppose we have a population of aa homozygotes initially. All the territories are occupied by aa individuals and 10 individuals die. - Well, If an 'A' allele is produce by mutation and heterozygotes have the highest relative fitness (probability of acquiring a territory), then the allele "A" increase in frequency to equilibrium.... - Selection occurs, BUT THERE ARE STILL ONLY 10 DEATHS PER GENERATION. - In this case there is NO genetic load, as selection is NOT causing ADDITIONAL mortality. It is just changing the probability of who dies.

  30. B. Genetic Load 1. "HARD" Selection can 'cost' a population individuals: 2. Why is this a problem? 3. Solutions a. Selectionists - Suppose we have a population of aa homozygotes initially. All the territories are occupied by aa individuals and 10 individuals die. - Well, If an 'A' allele is produce by mutation and heterozygotes have the highest relative fitness (probability of acquiring a territory), then the allele "A" increase in frequency to equilibrium.... - Selection occurs, BUT THERE ARE STILL ONLY 10 DEATHS PER GENERATION. - In this case there is NO genetic load, as selection is NOT causing ADDITIONAL mortality. It is just changing the probability of who dies. - So, selection across lots of loci does not NECCESSARILY lead to impossible loads.... as long as it is SOFT SELECTION

  31. B. Genetic Load 1. "HARD" Selection can 'cost' a population individuals: 2. Why is this a problem? 3. Solutions a. Selectionists b. Neutralists

  32. B. Genetic Load 1. "HARD" Selection can 'cost' a population individuals: 2. Why is this a problem? 3. Solutions a. Selectionists b. Neutralists - Maybe MOST of this variation is NEUTRAL, and is simply maintained by drift as new mutant alleles sequentially replace one another.

  33. B. Genetic Load 1. "HARD" Selection can 'cost' a population individuals: 2. Why is this a problem? 3. Solutions a. Selectionists b. Neutralists - Maybe MOST of this variation is NEUTRAL, and is simply maintained by drift as new mutant alleles sequentially replace one another. c. In a sense, the argument is really about selection.

  34. B. Genetic Load 1. "HARD" Selection can 'cost' a population individuals: 2. Why is this a problem? 3. Solutions a. Selectionists b. Neutralists - Maybe MOST of this variation is NEUTRAL, as is simply maintained by drift as new mutant alleles sequentially replace one another. c. In a sense, the argument is really about selection. Selectionistsstate that selection is important for 2 reasons - it eliminates bad alleles and FAVORS advantageous alleles.

  35. B. Genetic Load 1. "HARD" Selection can 'cost' a population individuals: 2. Why is this a problem? 3. Solutions a. Selectionists b. Neutralists - Maybe MOST of this variation is NEUTRAL, as is simply maintained by drift as new mutant alleles sequentially replace one another. c. In a sense, the argument is really about selection. Selectionists state that selection is important for 2 reasons - it eliminates bad alleles and FAVORS advantageous alleles. Neutralists agree that selection weeds out deleterious alleles, but they claim that this leaves a set of alleles that are functionally equivalent - neutral - in relative value. And changes in these equivalent alleles occur as a consequence of drift.

  36. Deviations from HWE I. Mutation II. Migration III. Non-Random Mating IV. Genetic Drift V. The Neutral Theory C. Neutral Variation

  37. V. The Neutral Theory C. Neutral Variation - Variation occurs at many levels, from genes to proteins to physical and behavioral characteristics of organisms.

  38. V. The Neutral Theory C. Neutral Variation - Variation occurs at many levels, from genes to proteins to physical and behavioral characteristics of organisms. - adaptive phenotypic variation is due to selection.

  39. V. The Neutral Theory C. Neutral Variation - Variation occurs at many levels, from genes to proteins to physical and behavioral characteristics of organisms. - adaptive phenotypic variation is due to selection. - But is ALL genetic variation of selective value?

  40. V. The Neutral Theory C. Neutral Variation - Variation occurs at many levels, from genes to proteins to physical and behavioral characteristics of organisms. - adaptive phenotypic variation is due to selection. - But is ALL genetic variation of selective value? - "no"; obviously, silent mutations are not maintained by selection

  41. V. The Neutral Theory C. Neutral Variation - Variation occurs at many levels, from genes to proteins to physical and behavioral characteristics of organisms. - adaptive phenotypic variation is due to selection. - But is ALL genetic variation of selective value? - "no"; obviously, silent mutations are not maintained by selection - So, Kimura suggested that there is too much variation at the DNA level to be explained by selection... he suggested that MOST of the variation in DNA is of NO selective value - it is NEUTRAL VARIATION.

  42. V. The Neutral Theory C. Neutral Variation - Variation occurs at many levels, from genes to proteins to physical and behavioral characteristics of organisms. - adaptive phenotypic variation is due to selection. - But is ALL genetic variation of selective value? - "no"; obviously, silent mutations are not maintained by selection - So, Kimura suggested that there is too much variation at the DNA level to be explained by selection... he suggested that MOST of the variation in DNA is of NO selective value - it is NEUTRAL VARIATION. - Curiously, the rate of replacement by drift, alone = the rate of mutation:

  43. V. The Neutral Theory C. Neutral Variation - Variation occurs at many levels, from genes to proteins to physical and behavioral characteristics of organisms. - adaptive phenotypic variation is due to selection. - But is ALL genetic variation of selective value? - "no"; obviously, silent mutations are not maintained by selection - So, Kimura suggested that there is too much variation at the DNA level to be explained by selection... he suggested that MOST of the variation in DNA is of NO selective value - it is NEUTRAL VARIATION. - Curiously, the rate of replacement by drift, alone = the rate of mutation: 1) The number of new alleles produced at a locus = 2N(m), where m is the mutation rate.

  44. V. The Neutral Theory C. Neutral Variation - Variation occurs at many levels, from genes to proteins to physical and behavioral characteristics of organisms. - adaptive phenotypic variation is due to selection. - But is ALL genetic variation of selective value? - "no"; obviously, silent mutations are not maintained by selection - So, Kimura suggested that there is too much variation at the DNA level to be explained by selection... he suggested that MOST of the variation in DNA is of NO selective value - it is NEUTRAL VARIATION. - Curiously, the rate of replacement by drift, alone = the rate of mutation: 1) The number of new alleles produced at a locus = 2N(m), where m is the mutation rate. - So, if the average mutation rate is 1 in 10,000, but there are 20,000 individuals (2N = 40,000 alleles), then on average 4 new alleles will be produced by mutation every generation.

  45. V. The Neutral Theory C. Neutral Variation - Variation occurs at many levels, from genes to proteins to physical and behavioral characteristics of organisms. - adaptive phenotypic variation is due to selection. - But is ALL genetic variation of selective value? - "no"; obviously, silent mutations are not maintained by selection - So, Kimura suggested that there is too much variation at the DNA level to be explained by selection... he suggested that MOST of the variation in DNA is of NO selective value - it is NEUTRAL VARIATION. - Curiously, the rate of replacement by drift, alone = the rate of mutation: 1) The number of new alleles produced at a locus = 2N(m), where m is the mutation rate. - So, if the average mutation rate is 1 in 10,000, but there are 20,000 individuals (2N = 40,000 alleles), then on average 4 new alleles will be produced by mutation every generation. 2) Each allele has a probability of fixation = 1/2N.

  46. V. The Neutral Theory C. Neutral Variation - Variation occurs at many levels, from genes to proteins to physical and behavioral characteristics of organisms. - adaptive phenotypic variation is due to selection. - But is ALL genetic variation of selective value? - "no"; obviously, silent mutations are not maintained by selection - So, Kimura suggested that there is too much variation at the DNA level to be explained by selection... he suggested that MOST of the variation in DNA is of NO selective value - it is NEUTRAL VARIATION. - Curiously, the rate of replacement by drift, alone = the rate of mutation: 1) The number of new alleles produced at a locus = 2N(m), where m is the mutation rate. - So, if the average mutation rate is 1 in 10,000, but there are 20,000 individuals (2N = 40,000 alleles), then on average 4 new alleles will be produced by mutation every generation. 2) Each allele has a probability of fixation = 1/2N. 3) So, the rate of replacement = (number of new alleles formed) x (probability one become fixed) = 2N(m) x 1/2N = m per generation.

  47. V. The Neutral Theory C. Neutral Variation D. Predictions and Results

  48. V. The Neutral Theory C. Neutral Variation D. Predictions and Results 1. Rates of molecular evolution should vary in functional and non-functional regions

  49. V. The Neutral Theory C. Neutral Variation D. Predictions and Results 1. Rates of molecular evolution should vary in functional and non-functional regions - Rates should vary in different codon positions. Variation at the third position should be higher, because these are usually silent mutations. Mutations at the first two position change amino acids, and these changes are deleterious.

  50. V. The Neutral Theory C. Neutral Variation D. Predictions and Results 1. Rates of molecular evolution should vary in functional and non-functional regions - Rates should vary in different codon positions. Variation at the third position should be higher, because these are usually silent mutations. Mutations at the second position change amino acids, and these changes are deleterious.PATTERN CONFIRMED.

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