Lecture 8: Selection in Real Populations

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

ω Heterozygote Disadvantage (Underdominance) A1A1 A1A2 A2A2

Heterozygote Disadvantage (Underdominance) 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 s s1 hs s2 ω 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

ω11=1; ω12=0.8;ω22=1 ω ω11=0.85; ω12=0.8;ω22=1 A1A1 A1A2 A2A2 ω A1A1 A1A2 A2A2 What determines the equilibrium point with underdominance? • Why does equilibrium point of A1 allele frequency increase when selection coefficient decreases?

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 0.403 0.373 0.2985 Selection coefficient (only females) • 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?

Phenolic glycosides (%) Osier and Lindroth, Oecologia, in press 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

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 http://www.leps.it/indexjs.htm?SpeciesPages/BistoBetul.htm • 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

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, pollinator-floral 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/Europe/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 • Wiped out up to 99% of rabbit population in some places • Kill rate declined over time • Reduced virulence of virus • Enhanced resistance of rabbits • Virus now regaining high virulence

How will the frequency of a recessive lethal allele change through time in an infinite population? What will be the equilibrium allele frequency?

Mutation Drift + - + +/- Selection Migration What Controls Genetic Diversity Within Populations? 4 major evolutionary forces Diversity

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

Lecture 8: Selection in Real Populations