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 A2aaA2
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
s
hs
ω
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