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Lecture 8: Selection in Real Populations February 7, 2014 Exam 1 Wednesday, February 12 in computer lab Review session on Monday: bring questions Sample exam and key are posted on website Conflicts and rescheduling Last Time Dominance and types of selection Why do lethal recessives stick around? Equilibrium under selection Stable equilibrium: overdominance Unstable equilibrium: underdominance Today Overdominance and Underdominance Overview of advanced topics in selection Introduction to Genetic Drift Equilibrium under Overdominance Allele frequency always approaches same value of q when perturbed away from equilibrium value Stable equilibrium Allele frequency change moves population toward maximum average fitness s1 qeq s1 s2 Heterozygote Disadvantage (Underdominance) 1 0.8 ω 0.6 0.4 0.2 0 AAA 1A1 Fitness Fitness in terms of s and h AAa1A2 Aaa 2A2 Genotype A1A1 A1A2 A2A2 ω11 ω12 ω22 1 + s1 1 1 + s2 s1 qeq s1 s2 Heterozygote Disadvantage (Underdominance) Fitness Fitness in terms of s and h Genotype A1A1 A1A2 A2A2 ω11 ω12 ω22 1 + s1 1 1 + s2 s1 = 0.1 s2 = 0.1 Equilibrium under Underdominance Allele frequency moves away from equilibrium point and to extremes when perturbed Unstable equilibrium Equilibrium point is at local minimum for average fitness Population approaches trivial equilibria: fixation of one allele Where are equilibrium points? ω11 =1.1 ω12 = 1 ω22 = 1.1 Underdominance Revisited Fitness Fitness in terms of s1 and s2 Fitness in terms of s and h h 1 s1 hs Genotype A1A1 A1A2 ω11 ω12 1 + s1 1 1 1-hs s1 hs ω s s2 s2 hs s s (h 1) A2A2 ω22 1 + s2 1-s A1A1 A1A2 A2A2 Why does “nontrivial” equilibrium occur with underdominance? Why doesn’t A1 allele always go to fixation if A1A1 is most fit genotype? ω A1A1 A1A2 A2A2 What determines the equilibrium point with underdominance? ω11=1; ω12=0.8; ω22=1 ω11=0.85; ω12=0.8; ω22=1 Why does equilibrium point of A1 allele frequency increase when selection coefficient decreases? ω A 1A 1 A1A2 A2A2 s2 peq s1 s2 s1 peq s2 qeq Example: Kuru in Fore Tribespeople Prion disease in Fore tribesmen Transmitted by cannibalism of relatives by women/children Cannibalism stopped in 1950’s Older people exposed to selection, younger are ‘controls’ Identified locus that causes susceptibility: Prion Protein Gene, PRNP MM and VV are susceptible, MV are resistant http://learn.genetics.utah.edu/features/prions/kuru.cfm Kuru and Heterozygote Advantage 1 v (s ) Selection coefficient 2 (only females) 0.403 0.2985 0.373 sMM qeq 0.483 sMM sVV Tremendous selective advantage in favor of heterozygotes Balancing selection maintains polymorphism in human populations Directional selection predominates for most loci Why doesn’t selection quickly wipe out most variation? Antagonistic Pleiotropy Individual alleles affect multiple traits with opposing effects on fitness components Aspen and elk herbivory in Rocky Mountain National Park Aspen can inhibit herbivory with protective compounds: phenolic glycosides Tradeoff with growth Osier and Lindroth, Oecologia, in press Phenolic glycosides (%) How does selection work in a variable environment? Spatial versus temporal variation Spatial variation maintains diversity, especially if habitat choice occurs Temporal variation less effective at maintaining diversity Conditions for stable equilibrium much more stringent for temporal variation Industrial Melanism Peppered moth (Biston betularia) has dominant dark morph Elevated frequency in polluted areas Frequency of dominant morph declining with environmental cleanup Rate of decline modeled with basic selection model, s=0.153 http://www.leps.it/in dexjs.htm?Speci esPages/BistoBet ul.htm Frequency Dependent Selection Relative fitness is a function of frequency in the population Negative frequency-dependence: fitness is negatively correlated with frequency Should maintain variation in the population Examples include predator-prey interactions, pollinatorfloral interactions, and differential use of nutrients by different genotypes Positive frequency-dependence: fitness is positively correlated with frequency Should drive alleles to fixation/loss more rapidly Examples include decreased pollination for rare flowers, or increased predation for unusual phenotypes Frequency Dependent Selection in an Orchid Dactylorhiza sambucina has yellow and purple morphs No nectar or pollen reward for pollinators Naive pollinators switch to different flower color if no reward provided Rare color morphs favored http://www.treknature.com/gallery/Euro pe/Czech_Republic/photo9844.htm Frequency Dependent Selection in a Fish Perissodus microlepis is scale-eating cichlid fish from Lake Tanganyika in central Africa Assymetrical jaw causes feeding on alternate sides of prey Frequency of left-and right jawed morphs fluctuates around 0.5 Prey are on lookout for more common morph http://bio.research.ucsc.edu/~barrylab/ classes/evolution/Image61.gif Coevolution Organisms exert selection pressure on each other, evolve in response to each other Pest and pathogen Predator and prey Competitors Mutualists Maintains variation in both species through time Red Queen Hypothesis http://en.wikipedia.org Coevolution of Rabbits and Myxomatosis Rabbits overrunning Australia in mid 20th century Introduced Myxoma virus to control population Kill rate declined over time Reduced virulence of virus % Mortality Wiped out up to 99% of rabbit population in some places Enhanced resistance of rabbits Virus now regaining high virulence 100 90 80 70 60 50 40 30 20 10 0 Unselected 1961 1967 1972 How will the frequency of a recessive lethal allele change through time in an infinite population? What will be the equilibrium allele frequency? What Controls Genetic Diversity Within Populations? 4 major evolutionary forces Mutation Drift + - Diversity +/- Selection + Migration Genetic Drift Relaxing another assumption: infinite populations Genetic drift is a consequence of having small populations Definition: chance changes in allele frequency that result from the sampling of gametes from generation to generation in a finite population Assume (for now) Hardy-Weinberg conditions Random mating No selection, mutation, or gene flow Genetic Drift A sampling problem: some alleles lost by random chance due to sampling "error" during reproduction
