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III. A Closer Look at Natural Selection
A. Natural selection, adaptation, and evolution
B. Patterns of natural selection
1. Directional selection
2. Stabilizing selection
3. Diversifying selection
C. Effects of selection on the distribution of phenotypes
Fig. 23. 12
Directional selection
Pattern
Effect
before
Frequency
Fitness (W)
1
0
Phenotype (trait)
Phenotype (trait)
Directional selection
Pattern
Effect
before
after
Frequency
Fitness (W)
1
0
Phenotype (trait)
Phenotype (trait)
Mallett, J. 1989. TREE 4: 336
Pesticide
Pesticide
Pesticide
Pesticide
Pesticide application
% resistant
Fig 22.13
Evolution of antibiotic resistance
Source: Otaya, 1971. Epidemiological study of erythromycin resistance in
Staphylococcus aureus in Japan. In Drug Resistance in Bacteria. 1. Macrolide
antibiotics. Edited by S. Mitsuhashi. University Park Press, Baltimore.
Guppies
Poecilia reticulata 5 cm
Size of adult
guppies
in pool
small
large
Size of predators in pool
Number of
guppies
Adult guppy size in each pool
Guppy size is variable
Guppy size
In lab without any predators….
Time (generations)
Guppy size is heritable (genetic)
Guppy size affects survival
to reproduce
Big predator
– Pike-Cichlid
Crenicichla alta
16 cm
eats large
guppies
Smaller predator – Killifish
Rivulus harti 10 cm
eats small guppies
Large predators eat large guppies,
more small size alleles passed on
Small predators eat small guppies,
more large size alleles passed on
B – large size
B’ - small size
Transplant to
Small predator
pool
Adult guppy
size
Control (stay in
Large predator
pool
Time (generations)
Mostly small alleles
At beginning
Stabilizing selection
Pattern
Effect
before
after
Frequency
Fitness (W)
1
0
Phenotype (trait)
Phenotype (trait)
Stabilizing selection in humans
Stabilizing selection on
number of eggs in starlings
Too many chicks?
Too few eggs?
Conflict and Cooperation
• Parent-offspring conflict
• Sibling conflict
• Inclusive fitness and kin selection
B r – C > 0 (benefits to recipient * relatedness –
costs to giver)
• Reciprocal Altruism: If costs are small, benefits are
large and altruistic acts are reciprocated – altruism can
evolve
Disruptive selection
Pattern
Effect
before
after
Frequency
Fitness (W)
1
0
Phenotype (trait)
Phenotype (trait)
Male Lazuli Buntings
showing variation
in the brightness
of their breeding
plumage
Low plumage
Brightness score
High plumage
Brightness score
Plumage brightness
From Greene et al 2000, Nature 407:1000-1003
Diversifying selection on coat color in deer mice
Dark color is favored on
rich soil
Light color is favored on
sandy soil
Frequency dependent selection
Why are there equal numbers of
males and females?
Sexual Selection
Fig. 23. 12
Effects of selection on phenotype distributions
Location of curve = mean
Width of curve = variance
Possible effect of continual directional selection
Eventually variance might decrease
after
before
after
Phenotype (trait)
before
after
Frequency
Frequency
Frequency
before
Phenotype (trait)
Phenotype (trait)
Possible effect of continual stabilizing selection
Variance decreases to none
Phenotype
(trait)
before
after
Frequency
before
after
Frequency
Frequency
before
Phenotype
(trait)
Phenotype
(trait)
after
Electrophoresis - separates
proteins based on differences in
size and electrical charge
Heterozygous
Homozygous slow
Homozygous fast
Survey of electrophoretic variation in natural populations
**
The Paradox of Variation:
Evolution requires variation, but natural
selection eliminates variation.
Dominant traits can hide recessive traits from selection
Fig 14.11
Epistasis can hide dominant alleles from natural selection
C= pigment
c = none
B = deposition of lots of
pigment (black)
b = less deposition
(brown)
If cc, fur is white
regardless of genotype
at B locus
Genetic hitchhiking: tight linkage to a favorable gene can
protect a less favorable gene from selection.
Good gene
Bad gene
Heterozygote advantage in Sickle cell anemia:
Hb+ = normal RBC
(co-dominant)
Hbs = sickled RBC
Susceptibility
Relative
Genotype
RBC
to malaria
fitness
HB+ Hb+
normal
highest
intermediate
Hb+ Hbs
normal*
lower
highest
Hbs Hbs
sickled
lower
lowest
Diversifying selection in time: Snow goose
Nesting habitat
Diversifying selection in space: Deer mice
Dark color is favored on
rich soil
Light color is favored on
sandy soil
Diversifying selection over density: sea urchins
Pair-wise Reproductive Success
0.12
No match
0.1
Partial match
Full match
0.08
0.06
0.04
0.02
0
Low Density
High Density
Spawning Density of Males (#/m2)