Biology 206: Evolutionary Genetics Lecture 8 PDF

Biology 206: Evolutionary Genetics Lecture 8 Forms of Selection Readings: E&Z Ch.6 –6.7  Patterns of natural selection All that algebra was fun, but what good did it do us? It shows us that allele frequencies are likely to change as a result of selectio...

Biology 206: Evolutionary Genetics Lecture 8 Forms of Selection Readings: E&Z Ch.6 –6.7  Patterns of natural selection All that algebra was fun, but what good did it do us? It shows us that allele frequencies are likely to change as a result of selection, and it gives us a nice little formula we could plug into a computer to figure out exactly how. It allows us to make predictions about the consequences of natural selection simply by knowing the pattern of viability differences.  Demonstration of directional selection against a recessive lethal in lab experiment Dawson (1970) Genetica flour beetle (Tribolium castaneum) Why doesn’t the lethal allele go to 0 after 12 generations?  Demonstration of directional selection against lethal alleles in lab experiment Dawson (1970) Genetica Lethal alleles at two loci “l” and “Sa” Initial populations all heterozygous (+/ l or +/ Sa) Freq of l gene  Demonstration of directional selection against lethal alleles in lab experiment Dawson (1970) Genetica Lethal alleles at two loci “l” and “Sa” Initial populations all heterozygous (+/ l or +/ Sa) Freq of l gene Freq of Sa gene “l” allele is “Sa” allele is recessive partially additive (expressed in het)  Effects of BENEFICIAL selection on x̅ different types of alleles as takes longer c Ettein see to increase dominant 4 EEition unless hes to fixdtion goes some drift selection can see no hiding time E&Z Fig. 6.18 Note: This is selection for a beneficial allele, selection against a harmful allele would be the inverse of this  Natural selection is more powerful in large populations (because drift is weaker) E&Z Fig. 6.12 Remember, drift is weaker in large populations. This means that there is less ‘randomness’ and selection can do it’s thing con lount on it In small populations, selection can be overwhelmed by drift (ie. a beneficial allele may not increase in frequency because by chance it isn’t passed on)  Patterns of natural selection Our algebra from Lecture 7 (Monday) allows us to express different types of selection in terms of the relative fitness of the genotypes. Pattern Description w11> w12 > w22 Directional selection or w11 < w12 < w22 w11 > w12 Heterozygote disadvantage and w22 > w12 w11 < w12 Heterozygote advantage and w22 < w12  Directional Selection Directional selection means that one of the two alleles has higher fitness than the other one The response to selection will depend on the pairing of the allele The allele will increase in frequency in the population, with the rate and final frequency depending on the selection coefficient and dominance:  Directional Selection Directional selection means that one of the two alleles has higher fitness than the other one. The allele will increase in frequency in the population, with the rate and final frequency depending on the selection coefficient and dominance: Genotype A1A1 A1A2 A2A2 Relative fitness (w) w11 w12 w22 5 1 lethal w 1 1-hs 1-s worst one Selection Coefficient (s) = Fitness disadvantage to genotype Dominance Coefficient (h) = Proportion of s applied to the het. genotype  Directional Selection Genotype A1A1 A1A2 A2A2 Relative fitness (w) w11 w12 w22 w 1 1-hs 1-s Selection Coefficient (s) = Fitness disadvantage to genotype Dominance Coefficient (h) = Proportion of s applied to the het. Genotype h =0: A1 is dominant h =1: A2 is dominant h =0.5 exactly additive (or codominant) SINGLE LOCUS SELECTION Hopi Hoekstra Case Study: coat colour in deer mice Peromyscus maniculatus The mice have evolved cryptically coloured fur. The mice typically occur on dark soil in Nebraska. The Sand Hills were formed from light- coloured quartz about 10,000 years ago, and the mice populations in these habitats have evolved light colours, to avoid predators. SINGLE LOCUS SELECTION Case Study: coat colour in deer mice Peromyscus maniculatus Conducted a massive field mark-recapture experiment, where they started sites with both light mice and dark mice and sampled them over time to look at proportion of mice surviving At the start, lots Almost no dark of dark mice on mice left after 14 light site months Rowan Barrett Barrett et al. Science 2019  Directional Selection: selection and dominance coefficients Single locus (Agouti) causes the coat colour difference (another locus – Mc1r - also involved, but controlled for that in this experiment) Wildtype A allele is mostly dominant to a allele (it is partially additive) Example survival data on the dark coloured sites: Genotype AA Aa aa w (on dark) 1.00 0.87 0.34 w 1 1-hs 1-s  Directional Selection: selection and dominance coefficients Single locus (Agouti) causes the coat colour difference (another locus – Mc1r - also involved, but controlled for that in this experiment) Wildtype A allele is mostly dominant to a allele (it is partially additive) Example survival data on the dark coloured sites: Genotype AA Aa aa w (on dark) 1.00 0.87 0.34 w 1 1-hs 1-s p 1 ste Selection Coefficient (s): waa = 1 – s 0.34 = 1 – s s = 1 – 0.34 s = 0.66  Directional Selection: selection and dominance coefficients Single locus (Agouti) causes the coat colour difference (another locus – Mc1r - also involved, but controlled for that in this experiment) Wildtype A allele is mostly dominant to a allele (it is partially additive) Example survival data on the dark coloured sites: Genotype AA Aa aa w (on dark) 1.00 0.87 0.34 w 1 1-hs 1-s p 2 p 1 ste Dominance Coefficient (h): ste Selection Coefficient (s): wAa = 1 – hs waa = 1 – s 0.87 = 1 – hs 0.34 = 1 – s hs = 1 – 0.87 s = 1 – 0.34 h = 0.13 / s s = 0.66 h = 0.13 / 0.66 = 0.20 You are studying crickets that either have wings or are wingless. Crickets with genotype WW have a survival of 1, crickets with genotype Ww have a survival of 0.6, crickets with genotype ww have a survival of 0.2. What is the value of h (dominance coefficient) and what is the value of s (selection coefficient)? 1 0.02 0.8 S A. h=0.6, s=0.2 B. h=0.4, s=0.8 45 1 0.6 C. h=1, s=0.8 he g D. h=0.5, s=0.2 50.5 E. h=0.5, s=0.8  2. Heterozygote Advantage (or Overdominance) The heterozygote has the highest fitness Genotype B1 B1 B1 B2 B2 B2 w 1-s1 1 1-s2  2. Heterozygote Advantage (or Overdominance) The heterozygote has the highest fitness Genotype B1 B1 B1 B2 B2 B2 w 1-s1 1 1-s2 natural selection can maintain deleterious Equilibrium allele alleles (e.g. sickle-cell anemia) frequencies s1=s2 p* = q* s1 > s2 p* < q* s1 < s2 p* > q*  Heterozygote advantage and sickle-cell anemia B-globin mutation E&Z Fig 6.20, also see Box 6.7 Genotype AA AS SS Final allele frequencies depend on the w 1-s1 1 1-s2 relative fitness of the homozygotes w 0.9 1 0.2 Both alleles are maintained (promotes genetic diversity) Geographic variation in S allele frequency Piel et al 2010 Nature Comm 3. Heterozygote Disadvantage (or Underdominance) The heterozygote has the lowest fitness Genotype B1 B1 B1 B2 B2 B2 Examples in w 1 1-s 1 nature ????  3. Heterozygote Disadvantage (or Underdominance) The heterozygote has the lowest fitness Genotype B1 B1 B1 B2 B2 B2 Examples in w 1 1-s 1 nature ???? can't it Prove Unstable Equilibrium If the B1 allele (p) starts at higher frequency, (depends on start) then it will go to fixation (because heterozygotes do worse, and there will be p> q qà0 more B1B1 than B2B2). The opposite will be P< q pà0 true if the B2 allele (q) starts at higher frequency. If p=q=0.5, adding drift will push it one way or the other.  4. Negative frequency-dependent selection An allele becomes less fit as it becomes more common As beards become more common, they become less attractive  4. Negative frequency-dependent selection An allele becomes less fit as it becomes more common As beards become more common, they become less attractive Multiple alleles will be maintained in a stable polymorphism (both alleles are maintained), promotes genetic diversity When would this occur? e.g. pathogens, mating, resource use, predation  Negative frequency-dependent selection E.g.: deceptive pollination in orchid. Bees learn to avoid unrewarding flowers Gigord et al. 2001 PNAS Fitness is greatest when the phenotype is rare. Next generation, alternate phenotype becomes more common, etc....  Negative frequency-dependent selection E&Z Fig 6.19 E.g.: deceptive pollination in orchid. Bees learn to avoid unrewarding flowers Gigord et al. 2001 PNAS Fitness of Fitness is greatest when the yellow morph phenotype is rare. Next high generation, alternate fitness phenotype becomes more common, etc.... Equilibrium occurs when w(yellow)=w(purple) (line on graph) In this example, it occurs at more freq(Y) = 0.7 (dotted line) common polinotorboid  4. Positive frequency-dependent selection An allele becomes more fit as it becomes more common Positive frequency-dependence leads to unstable polymorphism (ie. maintaining both alleles is unstable) Fitness of a genotype increases with its frequency Unstable equilibrium at p = q As soon as p > q, A goes to fixation As soon as p < q, a goes to fixation Difficult to study don't see it just end point When would this mode of selection occur?  4. Positive frequency-dependent selection Demonstration that fitness increases with morph frequency tiger-patterned butterflies Score predation damage on artificial models Common phenotype (C) suffers less predation than exotic (E) Predators learn to avoid toxic butterflies Chouteau et al. 2016 PNAS Consider a locus with two alleles (A, a). Which one of the following sets of relative fitnesses (w) will consistently lead to the highest frequency of the “a” allele? A. wAA = 1.00, wAa = 0.90, waa = 0.80 Enes'shest I B. wAA = 1.00, wAa = 0.87, waa = 1.00 m sight to C. wAA = 0.01, wAa = 1.00, waa = 0.10 both fixation I g alleles D. wAA = 0.90, wAa = 1.00, waa = 0.90 revenge malntained Fixation  Selection Model Round-Up Fitness Both alleles Model differences Variation maintained? constant? No (one goes to Directional Yes Removes fixation) Heterozygous advantage Yes Maintains Yes Heterozygous No Yes Removes disadvantage (unstable p=q) Negative Frequency- No Maintains Yes Dependent Positive Frequency- No No Removes Dependent (unstable p=q)  Key points 1. Selection is more powerful in large populations 2. Directional selection: an allele has a selective advantage, will increase in frequency 3. Heterozygote advantage (overdominance) leads to stable polymorphism – both alleles maintained 4. Heterozygote disadvantage (underdominance) leads to unstable polymorphism 5. Negative frequency-dependent selection occurs when fitness declines with increasing frequency 6. Positive frequency-dependent selection occurs when fitness increases with increasing frequency