1. Lecture 8: Selection in Real Populations September 18, 2015

2. Announcements uMinor schedule changes  Added a selection lecture (today)  Delayed drift lecture (next Friday)  Merged population structure lectures uExam 1 is Wednesday, Sept 23 in computer lab. uReview session on Monday: bring questions uSample exam and key are posted on website

3. Last Time uDominance and types of selection  Why do lethal recessives stick around? uEquilibrium under selection  Stable equilibrium: overdominance  Unstable equilibrium: underdominance

4. Today uUnderdominance and Overdominance revisited uOverview of advanced topics in selection uIntroduction to Genetic Drift

5. Why does “nontrivial” equilibrium occur with underdominance? uWhy doesn’t A 1 allele always go to fixation if A 1 A 1 is most fit genotype? q (pq+p 2 ) pq = Proportion of A 1 alleles in heterozygous state: A1A1A1A1 ω A1A2A1A2 A2A2A2A2

6. ω A1A1A1A1 A1A2A1A2 A2A2A2A2 What determines the equilibrium point with underdominance? uWhy does equilibrium point of A 1 allele frequency increase when selection coefficient decreases? A1A1A1A1 A1A2A1A2 A2A2A2A2 ω ω 11 =1; ω 12 =0.8; ω 22 =1 s 1 =0.2; s 2 =0.2 Allele Frequency (q) ω 11 =0.85; ω 12 =0.8; ω 22 =1 s 1 =0.2; s 2 =0.05

7. Example: Kuru in Fore Tribespeople uPrion disease in Fore tribesmen uTransmitted by cannibalism of relatives by women/children uCannibalism stopped in 1950’s  Older people exposed to selection, younger are ‘controls’ uIdentified 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

8. SelectionMMMVVV No Cannibalism313037 Cannibalism4233 Are populations in Hardy-Weinberg Equilibrium without selection? How about with selection? What type of selection appears to be occurring? Genotype Counts at Postion 129 of the PRNP Gene

9. Effects of Kuru on Viability of Different Genotypes uFor overdominance, relative viability can be estimated as the proportion of a genotype that survives to adulthood, relative to survival of the heterozygote (see Hedrick p. 144 for derivation)

10. Kuru and Heterozygote Advantage uBalancing selection maintains polymorphism in human populations Selection coefficient 0.2985 0.373 0.403 (only females) incorrect in text!

11. Directional selection predominates for most loci Why doesn’t selection quickly wipe out most variation?

12. Antagonistic Pleiotropy uIndividual alleles affect multiple traits with opposing effects on fitness components uAspen and elk herbivory in Rocky Mountain National Park uAspen can inhibit herbivory with protective compounds: phenolic glycosides uTradeoff with growth Phenolic glycosides (%) Osier and Lindroth, Oecologia, in press

13. How does selection work in a variable environment? uSpatial versus temporal variation  Spatial variation maintains diversity, especially if habitat choice occurs  Temporal variation less effective at maintaining diversity, except for perennials uExample: plants that rely upon flooding for establishment often have large variation in flowering phenology. Early or late flowering can be favorable depending on timing of snow melt and spring floods.

14. Industrial Melanism uPeppered moth (Biston betularia) has dominant dark morph uElevated frequency in polluted areas uFrequency of dominant morph declining with environmental cleanup uRate of decline modeled with basic selection model, s=0.153 http://www.leps.it/in dexjs.htm?Speci esPages/BistoBet ul.htm

15. Frequency Dependent Selection uRelative fitness is a function of frequency in the population uNegative 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 uPositive 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

16. Frequency Dependent Selection in an Orchid uDactylorhiza sambucina has yellow and purple morphs uNo nectar or pollen reward for pollinators uNaive pollinators switch to different flower color if no reward provided uRare color morphs favored http://www.treknature.com/gallery/Euro pe/Czech_Republic/photo9844.htm

17. Frequency Dependent Selection in a Fish uPerissodus microlepis is scale-eating cichlid fish from Lake Tanganyika in central Africa uAssymetrical jaw causes feeding on alternate sides of prey uFrequency of left-and right jawed morphs fluctuates around 0.5 uPrey are on lookout for more common morph http://bio.research.ucsc.edu/~barrylab/ classes/evolution/Image61.gif

18. Coevolution uOrganisms exert selection pressure on each other, evolve in response to each other  Pest and pathogen  Predator and prey  Competitors  Mutualists uMaintains variation in both species through time uRed Queen Hypothesis http://en.wikipedia.org

19. Coevolution and the Importance of Sex uEvolution of sex is mystifying: sacrifices 50% of fitness and breaks up adaptive genotypes uRed Queen hypothesis is one explanation: stay ahead of the pathogens by generating variation uExperiment in C. elegans:  Infect populations with pathogen Serratia marcescens  Selfing populations have higher mortality long-term Morran et al. 2011 Science 333: 216-218

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

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

22. Genetic Drift uRelaxing another assumption: infinite populations uGenetic drift is a consequence of having small populations uDefinition: chance changes in allele frequency that result from the sampling of gametes from generation to generation in a finite population uAssume (for now) Hardy-Weinberg conditions  Random mating  No selection, mutation, or gene flow

23. Genetic Drift A sampling problem: some alleles lost by random chance due to sampling "error" during reproduction