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
Examples of directional selection
Antibiotic resistance:
Drosophila bristle number (lab):
Modes of selection…
2. Stabilizing selection: selection against phenotypes that deviate in
either direction from the optimal value for a character
favored
disfavored
Maize oil content:
disfavored
Mouse behavior (lab):
% oil
19%
— Mean value of trait maintained
— Decreases variation (if heritable)
— predominant selective force for
many traits
0%
0
Generations
80
1
Examples of stabilizing selection
Gall size in gall flies
Human birth weight
Gecko body size:
gain territory/mates
vs. exposure to owls
Eurosta solidaginis
Bird predation
Parasitoid wasps
— Often involves 2 opposing
Aristelliger praesignis
directional selection forces
Modes of selection…
Diversifying 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
Black-bellied seedcracker
(Pyrenestes ostrinus)
2
Why is there still genetic variation for traits directly correlated with fitness?
Frequency-dependent selection:
fitness depends on phenotype frequency
1. Negative (inverse) frequency-dependent selection: rare phenotype favored
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.
Scale eating cichlid (Perissodus microlepis)
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…
Futuyma, Fig. 12.16
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
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
expect increase to 1/k, where k= # alleles
In some cases, there is
convergence to different
phenotypes in different
regions, due to positive
frequency-dependent
selection.
(gametophytic self-incompatibility)
Demonstrated by
transplant experiments
Region:
A
B
C
D
E
F
G
3
Selection with environmental heterogeneity
1. Spatial heterogeneity: fine scale (within population)
e.g., Copper tolerance in bentgrass
Bentgrass (Agrostis tenuis)
Selection with environmental heterogeneity
1. Spatial heterogeneity: large scale (across species range)
Need at least one functional allele at both genes to be cyanogenic.
Linamarase
Li
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)
Linamarin
+
Li __
Li __
Ac
Ac __
li li
ac ac
Ac __
li li
ac ac

HCN
+
—
—
—
[vacuole]
Li/li: presence/absence of hydrolyzing
enzyme = linamarase [cell wall]
Individuals:
HCN color assay:
A) Leaf alone
A
B) Exogenous linamarin
B
C) Exogenous linamarase
C
Acli
AcLi
Acli acLi
AcLi
acli
acLi AcLi
4
Cyanogenesis protects clover plants from herbivores
Frequency of cyanogenesis decreases with colder temperatures
Altitude
AcLi
acli
ac
Change in live weight
Winter temperature
li
ac li
Ac Li
% Ac allele
6400
ft
Days
Dirzo and Harper 1982.
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.
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):
But: cyanogenic plants tend to come from warmer climates
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Frequency of cyanogenesis and colder climate…
Frequency of cyanogenesis and colder climate…
For a sample of clover plants from polymorphic populations
(12 genotypes):
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
No significant difference in freezing tolerance for
plants that come from the same climate
Polymorphism may be maintained due to resource
allocation trade-offs, favored in different environments.
Selection with environmental heterogeneity
Heterozygote advantage (overdominance): heterozygote
fitness exceeds homozygotes
2. Temporal heterogeneity
—No longer expect fixation of one allele.
e.g., Darwin’s finch beak size in drought vs. wet years
e.g., hemoglobin β: AAAS heterozygote
dry
dry
dry
β2
β1
AS: β-chain: Glu (6) —> Val
wet
large, hard
Seed morphology
Beak depth
acli
(Kakes 1989)
α2
α1
small, soft
Sickle cell anemia (ASAS homozygote)
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AAAS heterozygote advantage in areas of malaria:
Heterozygote advantage: cystic fibrosis
AS frequency is correlated with
regions of Malaria
CFTR protein: Cystic fibrosis
transmembrane conductance
regulator
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
‘Homozygote’ recessive: cystic fibrosis; susceptible to Pseudomonas
aeruginosa lung infection; early death.
• Loss-of-function allele frequency: ~2-5% in Europeans
In Africa
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 Δ F508 in the next generation.
• Δ F508 (70%), plus ~1000 other loss-of-function haplotypes identified
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
(But not all high-frequency disease alleles reflect heterozygote advantage)
6. Correlation between environment and phenotype
e.g., cyanogenesis clines; caveat: correlation is not necessarily causation
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