Lecture 8: Selection in Real Populations

September 18, 2015

Announcements Minor schedule changes

Added a selection lecture (today)

Delayed drift lecture (next Friday)

Merged population structure lectures

Exam 1 is Wednesday, Sept 23 in computer lab.

Review session on Monday: bring questions

Sample exam and key are posted on website

Last Time Dominance and types of selection

Why do lethal recessives stick around?

Equilibrium under selection

Stable equilibrium: overdominance

Unstable equilibrium: underdominance

Today Underdominance and

Overdominance revisited

Overview of advanced topics in selection

Introduction to Genetic Drift

Why does “nontrivial” equilibrium occur with underdominance? Why doesn’t A1

allele always go to fixation if A1A1 is most fit genotype?

q(pq+p2)

pq=

Proportion of A1 alleles in heterozygous state:

A1A1

ω

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?

A1A1 A1A2 A2A2

ω

eqeq qsps 21

ω11=1; ω12=0.8; ω22=1 s1=0.2; s2=0.2

Allele Frequency (q)

ω11=0.85; ω12=0.8; ω22=1 s1=0.2; s2=0.05

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

Selection MM MV VV

No Cannibalism

31 30 37

Cannibalism 4 23 3

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

Effects of Kuru on Viability of Different Genotypes For 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)

Kuru and Heterozygote Advantage

Balancing selection maintains polymorphism in human populations

483.0

VVMM

MMeq ss

sq

Selection coefficient 0.2985 0.373

0.403)

2

1(

vs

(only females)

incorrect in text!

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

Phenolic glycosides (%)

Osier and Lindroth, Oecologia, in press

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, except for perennials

Example: 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.

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/indexjs.htm?SpeciesPages/BistoBetul.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, 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 and the Importance of Sex

Evolution of sex is mystifying: sacrifices 50% of fitness and breaks up adaptive genotypes

Red Queen hypothesis is one explanation: stay ahead of the pathogens by generating variation

Experiment in C. elegans:

Infect populations with pathogen Serratia marcescens

Selfing populations have higher mortality long-term

Morran et al. 2011 Science 333: 216-218

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

Diversity

Mutation+

Drift-

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 DriftA sampling problem: some alleles lost by random chance due to sampling "error" during reproduction