Natural selection, continued…
Modes of selection
Modes of selection…
1. Directional selection: selection favors higher or lower
value of a character
disfavored
favored
— Mean value of trait is shifted (if heritable)
— Decreases variation
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Examples of directional selection
Antibiotic resistance:
Maize oil content:
Drosophila bristle number (lab):
Mouse behavior (lab):
% oil
19%
0%
0
Generations
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Modes of selection…
2. Stabilizing selection: selection against phenotypes that deviate in
either direction from the optimal value for a character
favored
disfavored
disfavored
— Mean value of trait maintained
— Decreases variation (if heritable)
— predominant selective force for
many traits
2
Examples of stabilizing selection
Human birth weight
Gecko body size:
gain territory/mates
vs. exposure to owls
— Often involves 2 opposing
Aristelliger praesignis
directional selection forces
Gall size in gall flies
Eurosta solidaginis
Parasitoid wasps
Bird predation
3
Modes of selection…
3. Diversifying (disruptive): Selection for two or more modal
phenotypes and against those intermediate between them
disfavored
favored
favored
— Increases variation (if heritable)
— Mean value of trait may not change
Diversifying selection
Black-bellied seedcracker
(Pyrenestes ostrinus)
4
Why is there still genetic variation for traits directly correlated with fitness?
Possibilities …
1. ‘Fisher’s Fundamental Theorem hypothesis’:
(Recall, FFT: rate at which mean fitness of population increases is
proportional to the additive genetic variance for fitness.)
New, favorable mutations are constantly arising, creating genetic
variation for fitness-related traits. Nonequilibrium.
2. Balance between deleterious mutations and selection:
Most of the genetic variation is deleterious, but not so
deleterious to be purged.
3. Diversifying selection, and/or selection favoring different
traits in different times/places…
Frequency-dependent selection:
fitness depends on phenotype frequency
1. Negative (inverse) frequency-dependent selection: rare phenotype favored
Scale eating cichlid (Perissodus microlepis)
Futuyma, Fig. 12.16
5
Negative frequency-dependent selection:
e.g., Self-incompatibility alleles in plants: S locus
Pollen allele must differ from ovule-parent’s genotype for fertilization
Rare allele can fertilize more plants — until it becomes common
— maintains/increases genetic variation
expect increase to 1/k, where k= # alleles
(gametophytic self-incompatibility)
Positive frequency-dependent selection: common phenotype favored, fixed
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Positive frequency-dependent selection: common phenotype favored, fixed
Different populations may become fixed for different phenotypes
Müllerian mimicry:
several unpalatable
species share a
similar warning
pattern
In some cases, there is
convergence to different
phenotypes in different
regions, due to positive
frequency-dependent
selection.
Demonstrated by
transplant experiments
Region:
A
B
C
D
E
F
G
Selection with environmental heterogeneity
1. Spatial heterogeneity: fine scale (within population)
e.g., Copper tolerance in bentgrass
Bentgrass (Agrostis tenuis)
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Selection with environmental heterogeneity
1. Spatial heterogeneity: large scale (across species range)
Presence/absence of HCN
production (cyanogenesis)
2 compounds required,
controlled by 2 genes:
Ac/ac: presence/absence of cyanogenic
glucoside (= linamarin)
White clover (Trifolium repens)
[vacuole]
Li/li: presence/absence of hydrolyzing
enzyme = linamarase [cell wall]
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Need at least one functional allele at both genes to be cyanogenic.
Linamarase
Li
Linamarin
+

Ac
Ac __
Li __
Li __
ac ac
Ac __
li li
li li
ac ac
HCN
+
—
—
—
Individuals:
HCN color assay:
A) Leaf alone
A
B) Exogenous linamarin
B
C) Exogenous linamarase
C
Acli
AcLi
Acli acLi
AcLi
acli
acLi AcLi
Change in live weight
Cyanogenesis protects clover plants from herbivores
ac li
Ac Li
Days
Dirzo and Harper 1982.
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Frequency of cyanogenesis decreases with colder temperatures
Altitude
AcLi
acli
ac
Winter temperature
li
% Ac allele
6400
ft
Mean January temperature (F)
1900 ft
Ac
Li
Cline: gradual change in genotype/trait over a geographical continuum
Frequency of cyanogenesis and colder climate…
Hypothesis 1: Cyanogenesis is disadvantageous in colder climates
because freezing causes cell rupture and HCN autotoxicity.
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Frequency of cyanogenesis and colder climate…
Hypothesis 1: Cyanogenesis is disadvantageous in colder climates
because freezing causes cell rupture and HCN autotoxicity.
Testing freeze damage to under controlled conditions
(30 genotypes, 8 replicates apiece, randomized)…
Frequency of cyanogenesis and colder climate…
For a sample of clover plants from across Europe, cyanogenic
plants show lower freezing tolerance (18 genotypes):
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Frequency of cyanogenesis and colder climate…
For a sample of clover plants from across Europe, cyanogenic
plants show lower freezing tolerance (18 genotypes):
But: cyanogenic plants tend to come from warmer climates
Frequency of cyanogenesis and colder climate…
For a sample of clover plants from polymorphic populations
(12 genotypes):
No significant difference in freezing tolerance for
plants that come from the same climate
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Frequency of cyanogenesis and colder climate…
Hypothesis 2: If there are fewer herbivores in colder climates…
then plants investing in growth rather than
cyanogenesis may be at a competitive advantage.
Frequency of cyanogenesis and colder climate…
Hypothesis 2: If there are fewer herbivores in colder climates…
then plants investing in growth rather than
cyanogenesis may be at a competitive advantage.
• Acyanogenic plants show
Floral mass (g)
greater reproductive output:
AcLi
acli
(Kakes 1989)
Polymorphism may be maintained due to resource
allocation trade-offs, favored in different environments.
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Selection with environmental heterogeneity
2. Temporal heterogeneity
Beak depth
e.g., Darwin’s finch beak size in drought vs. wet years
dry
dry
dry
wet
Seed morphology
large, hard
small, soft
Heterozygote advantage (overdominance): heterozygote
fitness exceeds homozygotes
—No longer expect fixation of one allele.
e.g., hemoglobin β: AAAS heterozygote
β2
α2
β1
AS: β-chain: Glu (6) —> Val
α1
Sickle cell anemia (ASAS homozygote)
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AAAS heterozygote advantage in areas of malaria:
AS frequency is correlated with
regions of Malaria
AAAA: no anemia, susceptible to malaria
AAAS: slight anemia, resistance to malaria
ASAS: severe anemia and early death
AAAA
W:
0.89
AA AS
ASAS
1.0
0.2
Plasmodium falciparum
In Africa
Heterozygote advantage: cystic fibrosis
CFTR protein: Cystic fibrosis
transmembrane conductance
regulator
‘Homozygote’ recessive: cystic fibrosis; susceptible to Pseudomonas
aeruginosa lung infection; early death.
• Loss-of-function allele frequency: ~2-5% in Europeans
• ΔF508 (70%), plus ~1000 other loss-of-function haplotypes identified
15
Heterozygotes are protected against Salmonella typhi cell infiltration in the
gut: 86% fewer bacteria.
Across 11 European countries, severity of typhoid outbreaks is correlated
with frequency of ΔF508 in the next generation.
(But not all high-frequency disease alleles reflect heterozygote advantage)
Methods for documenting natural selection:
1. Longitudinal studies: follow a group of individuals for some
or all of their lives; observe variation in phenotypes and fitness
e.g., Darwin’s finches
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Methods for documenting natural selection:
1. Longitudinal studies: follow a group of individuals for some
or all of their lives; observe variation in phenotypes and fitness
e.g., Darwin’s finches
2. Experimental manipulation (transplants, mark/re-capture, etc.); look for
associations between phenotypes and fitness in different environments
e.g., predation on peppered moths; Müllerian mimicry
Methods for documenting natural selection:
1. Longitudinal studies: follow a group of individuals for some
or all of their lives; observe variation in phenotypes and fitness
e.g., Darwin’s finches
2. Experimental manipulation (transplants, mark/re-capture, etc.); look for
associations between phenotypes and fitness in different environments
e.g., predation on peppered moths; Müllerian mimicry
3. Comparison among age classes. Look for shifts in phenotypes.
e.g., copper tolerance in grass
17
Methods for documenting natural selection:
1. Longitudinal studies: follow a group of individuals for some
or all of their lives; observe variation in phenotypes and fitness
e.g., Darwin’s finches
2. Experimental manipulation (transplants, mark/re-capture, etc.); look for
associations between phenotypes and fitness in different environments
e.g., predation on peppered moths; Müllerian mimicry
3. Comparison among age classes. Look for shifts in phenotypes.
e.g., copper tolerance in grass
4. Long-term studies of trait distributions (over many generations)
e.g., Darwin’s finches
Better for detecting directional selection than stabilizing
Methods for documenting natural selection:
1. Longitudinal studies: follow a group of individuals for some
or all of their lives; observe variation in phenotypes and fitness
e.g., Darwin’s finches
2. Experimental manipulation (transplants, mark/re-capture, etc.); look for
associations between phenotypes and fitness in different environments
e.g., predation on peppered moths; Müllerian mimicry
3. Comparison among age classes. Look for shifts in phenotypes.
e.g., copper tolerance in grass
4. Long-term studies of trait distributions (over many generations)
e.g., Darwin’s finches
Better for detecting directional selection than stabilizing
5. Environmental perturbations
e.g., antibiotic resistance, pesticide resistance, Galapagos drought
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Methods for documenting natural selection:
1. Longitudinal studies: follow a group of individuals for some
or all of their lives; observe variation in phenotypes and fitness
e.g., Darwin’s finches
2. Experimental manipulation (transplants, mark/re-capture, etc.); look for
associations between phenotypes and fitness in different environments
e.g., predation on peppered moths; Müllerian mimicry
3. Comparison among age classes. Look for shifts in phenotypes.
e.g., copper tolerance in grass
4. Long-term studies of trait distributions (over many generations)
e.g., Darwin’s finches
Better for detecting directional selection than stabilizing
5. Environmental perturbations
e.g., antibiotic resistance, pesticide resistance
6. Correlation between environment and phenotype
e.g., cyanogenesis clines; caveat: correlation is not necessarily causation
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