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Lecture 8: Selection in Real
Populations
September 18, 2015
Announcements
u Minor schedule changes
 Added a selection lecture (today)
 Delayed drift lecture (next Friday)
 Merged population structure lectures
u Exam 1 is Wednesday, Sept 23 in computer
lab.
u Review session on Monday: bring questions
u 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?
ω
Proportion of A1 alleles in
heterozygous state:
pq
(pq+p2)
= q
A1A1
A1A2 A2A2
What determines the equilibrium
point with underdominance?
s1=0.2; s2=0.2
ω11=1; ω12=0.8; ω22=1
ω
s1=0.2; s2=0.05
ω11=0.85; ω12=0.8; ω22=1
Allele Frequency (q)
Why does equilibrium
point of A1 allele
frequency increase
when selection
coefficient decreases?
A 1A 1
A1A2
A2A2
s1
qeq =
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
Genotype Counts at Postion 129 of the PRNP Gene
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?
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)
PP
n11 =
PP
'
11 12
'
11 12
P P
n 22 =
P P
'
22 12
'
22 12
Kuru and Heterozygote Advantage
1 v
(s 
)
Selection coefficient
2 (only females)
0.403
0.2985
0.373
P P (0.133)(0.514)
n11 =
=
= 0.403 incorrect in
text!
PP
(0.221)(0.767)
'
11 12
'
11 12
sMM
qeq 
 0.483
sMM  sVV
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, 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/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 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
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