Mendelian Genetics in Populations – Hardy Weinberg equilibrium The Hardy-Weinberg equilibrium...

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Transcript of Mendelian Genetics in Populations – Hardy Weinberg equilibrium The Hardy-Weinberg equilibrium...

  • Slide 1
  • Mendelian Genetics in Populations Hardy Weinberg equilibrium The Hardy-Weinberg equilibrium principle yields two fundamental conclusions: Conclusion 1: The allele frequencies in a population will not change, generation after generation. Conclusion 2: If the allele frequencies in a population are given by p and q, the genotype frequencies will be given by p 2, 2pq, q 2.
  • Slide 2
  • Mendelian Genetics in Populations Hardy Weinberg equilibrium The Hardy-Weinberg equilibrium has 5 assumptions 1: There is no selection 2: There is no mutation 3: There is no migration (gene flow) 4: There are no chance events (Chance events are unlikely to occur in a very large population. In a large population, eggs and sperm collide at their actual frequencies of p and q.) 5: Individuals choose their mates at random. If all of these assumptions are met, then nothing happens! The population does not change.
  • Slide 3
  • Mendelian Genetics in Populations - Selection Patterns of Selection Selection for Recessive and Dominant alleles Selection for Heterozygotes and Homozygotes Frequency Dependent Selection
  • Slide 4
  • Selection on Recessive and Dominant alleles Trilobium castaneum (flour beetles) Gene called - l locus Genotypes: +/+ - phenotypically normal +/l - phenotypically normal l/l - lethal Peter Dawson (1970) starts 2 experimental populations with heterozygotes ( +/l) as founders. Frequency of + and l are: The frequency of an allele in the next generation can be calculated by: From Box 6.5 From Box 6.3
  • Slide 5
  • Fitness What is fitness, w ? Your text book: Fitness is the extent to which an individual contributes genes to future generations. -an individuals score on a measure of performance expected to correlated with genetic contribution to future generations. e.g. lifetime reproductive success chance of survival to adulthood (reproduction) Absolute fitness - is the total number of surviving offspring an individual produces during its lifetime (lifetime reproductive success). Survivorship is a component of fitness. Absolute fitness = the chance of survival * the # of offspring.
  • Slide 6
  • Relative Fitness Absolute fitness is standardize to obtain Relative fitness. The genotype with the highest absolute fitness has a relative fitness of 1.0. For every other genotype, their relative fitness is:
  • Slide 7
  • Relative Fitness Example A beetle is polymorphic for color. It comes in black, brown and yellow morphs. Birds and lizards prey on them, so that due to differences in survivorship, the fitness of the color morphs differ. C B C B black - 67% chance of survival to adulthood C B C Y brown - 93% chance of survival to adulthood C Y C Y yellow - 11% chance of survival to adulthood Assuming they have identical numbers of offspring, what is the relative fitness of each genotype? w C B C B -.67/.93 = 0.72 w C B C Y = 1.0 w C Y C Y -.11/.93 = 0.12
  • Slide 8
  • Absolute Fitness Problem: A species of snake has two forms: brown and green. They are determined by a single locus with two alleles. GG = brown, Gg = brown, gg = green Brown phenotypes are cryptic and have a 80% chance of surviving to reproduce. Green phenotypes have a 60% chance of surviving to reproduce. The brown phenotype produces an average of 13 offspring. The green phenotype produces an average of 20 offspring. What are the absolute and relative fitnesses of each genotype?
  • Slide 9
  • Absolute Fitness Problem: A species of snake has two forms: brown and green. They are determined by a single locus with two alleles. GG = brown, Gg = brown, gg = green Brown phenotypes have a 80% chance of surviving to reproduce. Green phenotypes have a 60% chance of surviving to reproduce. The brown phenotype produces an average of 13 offspring. The green phenotype produces an average of 20 offspring. What are the absolute and relative finesses of each genotype? Absolute fitness of Brown phenotypes GG and Gg.80 chance of survival x 13 offspring = 10.4 expected offspring. Absolute fitness of Green phenotype gg.60 chance of survival x 20 offspring = 12 expected offspring. w (gg) = 1 w (Gg) = w (GG) = 10.4/12 = 0.87
  • Slide 10
  • Selection on Recessive and Dominant alleles 1)When a recessive allele is common (and a dominant allele is rare), evolution is rapid. Why? 2) When a dominant allele is common (and a lethal recessive allele is rare), evolution is slow. If the dominant and recessive allele differ in fitness, then there will be a rapid reduction in the recessive allele if it has lower fitness. Even lethal recessive alleles will persist because they hide in heterozygotes. They are virtually impossible to eliminate. If the dominant allele is lethal, what happens?That allele is eliminated in one generation.
  • Slide 11
  • Selection on Recessive and Dominant alleles 2) When a dominant allele is common (and a recessive allele is rare), evolution is slow. Even lethal recessive alleles will persist because they hide in heterozygotes. They are virtually impossible to eliminate.
  • Slide 12
  • Selection on Recessive and Dominant alleles When selection favors a recessive allele, evolution is slow at first. Why? Most recessive alleles are in heterozygotes and cannot be selected for. Once recessive alleles become present as homozygotes, the rate of evolution increases dramatically. The recessive allele goes to fixation (= 1.0)
  • Slide 13
  • Selection for Heterozygotes and Homozygotes Heterozygote superiority = heterosis = overdominance Fitness of the heterozygote is greater than that of either homozygote.
  • Slide 14
  • Selection for Heterozygotes Mukai and Burdick (1959) Single locus, two alleles VV, VL viable LL - lethal 2 experimental populations V = 0.5 L = 0.5 Results begin to look like Dawsons data with flour beetles. But viable allele does not go close to fixation, as Dawsons data did.
  • Slide 15
  • Selection on Homozygotes Mukai and Burdick (1959) Using model from Box 6.5 and the following fitnesses: VVVLLL 0.7351.00 Starting with V = 0.975, both experimental data support the model and previous experiments.
  • Slide 16
  • Selection for Heterozygotes Generalizations about heterozygote superiority: 1. Alleles will reach equilibrium at frequencies other than those predicted by H-W 2. Because selection is favoring heterozygotes, both alleles will be maintained over time instead of one allele being fixed and one eliminated (selection is maintaining genetic variability). 3. Because both alleles are actively maintained over time, all three genotypes will be actively maintained over time. 4. The general term for population with multiple phenotypes = balanced polymorphism heterosis is one mechanism that can produce this pattern (there are others)
  • Slide 17
  • Selection for Homozygotes Heterozygote inferiority = underdominance Heterozygotes have lower fitness than do either homozygote. Because heterozygotes have low fitness, most matings will take place among homozygotes. Whichever allele (dominant or recessive doesnt matter) is most common in a population will be selectively favored and will be fixed.
  • Slide 18
  • Selection for Homozygotes Do thought experiment: imagine population with two alleles A and a; Selection acts against heterozygotes (assume no heterozygotes survive) a.If A is more common than a, population will have high frequency of AA and lower frequency of aa. b. Assume random mating: who mates with whom? i. Because theyre more common, AA individuals will most likely mate with AA. ii. Because theyre rare, aa individuals are more likely to mate with AA than with aa. iii. Because most aa mate with AA, they produce heterozygous offspring. c. What happens to a alleles? a alleles will be found in heterozygotes and eliminated. d. What happens to the A allele? A becomes fixed. e. Same will happen if a more common than A. Note that fixation only depends on initial frequency, not on dominance or recessiveness.
  • Slide 19
  • Selection on Homozygotes Foster et al. (1972) Dont get caught up with how researchers created the alleles. Frequencies of 0.5 for each allele is unstable. As soon as frequency shifts above or below 0.5, the allele rapidly goes to fixation or 0. (Consider outcome of thought experiment).
  • Slide 20
  • Selection on Homozygotes Foster et al. (1972) If fitness of homozygotes are unequal, then unstable equilibrium point shifts in favor of the allele with greater fitness. Heterozygote inferiority leads to a loss of genetic diversity within a population. Heterozygote inferiority may help maintain genetic diversity among populations. How? A: By driving different alleles to fixation in different populations.
  • Slide 21
  • Frequency Dependent Selection What happens if the direction of selection changes over time? 1. When selection constantly favors one allele over another, that can result in fixation of that allele. 2. A stable equilibrium would be achieved if the heterozygotes were favored. 3. Negative frequency-dependent selection creates a condition of stable equilibrium by favoring one allele over another, but the favoritism alternates and is mediated by behavior! The punch line: In negative frequency dependent selection, rare alleles are selectively favored.
  • Slide 22
  • Frequency Dependent Selection Nave bumblebees visit stands of orchids to sample flowers. If a bee visits a purple flower and receives no reward, i