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Transcript
Lecture 5
Natural selection – theory and definitions
Charles Darwin
Alfred Russel Wallace
Evolution by Natural Selection
1.  Variation among individuals
2.  Different survival and/or reproduction
3.  Change in genetic composition of population
4.  Evolution
Natural selection: Facts and inferences
Fact 1. Natural populations have large excess reproductive capacities.
Fact 2. Population sizes generally remain stable.
Fact 3. Resources are limiting.
Inference 1. A severe struggle for existence must occur.
Robert Thomas Malthus (1766-1834)
An Essay on the Principle of Population, 1798
Natural selection: Facts and inferences
Fact 1. Natural populations have large excess reproductive capacities.
Fact 2. Population sizes generally remain stable.
Fact 3. Resources are limiting.
Inference 1. A severe struggle for existence must occur.
Fact 4. An abundance of variation exists among individuals of a species.
Fact 5. Some of this variation is heritable.
Gregor Mendel (1822-1884)
Experiments with Plant Hybrids
(1865)
Natural selection: Facts and inferences
Fact 1. Natural populations have large excess reproductive capacities.
Fact 2. Population sizes generally remain stable.
Fact 3. Resources are limiting.
Inference 1. A severe struggle for existence must occur.
Fact 4. An abundance of variation exists among individuals of a species.
Fact 5. Some of this variation is heritable.
Inference 2. Genetically superior individuals outsurvive and outreproduce
others.
Inference 3. Over many generations, evolutionary change must occur in
the population.
A definition of natural selection:
“changes in the relative frequencies of
different genotypes (genes) in a
population because of differences in the
survivorship and/or reproduction of their
phenotypes”.
At what level does natural selection act?
- organisms may be decomposed into two components - the genotype
and the phenotype.
- genotype is the hereditary material, or set of genetic instructions, that
determine an organism’s structural, physiological, and behavioral
characteristics.
- the phenotype represents the physical expression of a particular
genotype.
- it results from an interaction between genotype and environment.
- a genotype may thus produce a number of different phenotypes
depending on the environmental conditions.
Some important principles of natural
selection
Some important principles of natural
selection
1. Natural selection (usually) acts at the level of
individuals, not populations.
Some important principles of natural
selection
1. Natural selection (usually) acts at the level of
individuals, not populations.
2. Populations, not individuals, evolve.
Some important principles of natural
selection
1. Natural selection (usually) acts at the level of
individuals, not populations.
2. Populations, not individuals, evolve.
3. Natural selection is retrospective and cannot
predict the future.
Some important principles of natural
selection
1. Natural selection (usually) acts at the level of
individuals, not populations.
2. Populations, not individuals, evolve.
3. Natural selection is retrospective and cannot
predict the future.
4. Natural selection is not necessarily
progressive.
What is the measure of the biological success of a genotype?
Darwinian fitness = the number of gene copies a phenotype places into the next
generation.
Relative fitness = a phenotype’s Darwinian fitness relative to other phenotypes.
- relative fitness is simply a standardized form of Darwinian fitness - we are usually
interested in quantifying how well a genotype does compared, or relative to, other
genotypes.
- the relative fitness of a genotype simply summarizes how well a genotypes does in
transmitting genes to the next generation compared to other genotypes.
- as an example, consider three genotypes A, B, and C.
Genotype
A
B
C
Generation 0
No. freq
10 0.33
10 0.33
10 0.33
30
Generation 1
No. freq
5
0.17
10 0.33
15 0.50
30
Relative fitness
5/15 = 0.33
10/15 = 0.67
15/15 = 1.0
What is fitness?
What is fitness?
1. Fitness is a description not an explanation
What is fitness?
1. Fitness is a description not an explanation
- the concept of fitness is not circular!
What is fitness?
1. Fitness is a description not an explanation
2. Fitness is an average property
What is fitness?
1. Fitness is a description not an explanation.
2. Fitness is an average property.
3. Total fitness is comprised of several individual
components:
What is fitness?
1. Fitness is a description not an explanation.
2. Fitness is an average property.
3. Total fitness is comprised of several individual
components:
• viability
• fecundity
• longevity
• mating success
Darwin’s finches
The vampire finch, Geospiza difficilis septentrionalis
Example:
Medium ground finches
on Daphne Major
Photo by Greg Lasley
Is the finch population variable?
Is some of the variation heritable?
Is there evidence for fitness differences?
Is there evidence for fitness differences?
Was there selective mortality?
Did the population evolve?
Natural selection at a single locus
1.  Directional selection
2. Purifying selection
3. Balancing selection
Natural selection at a single locus
1.  Directional selection
a form of selection where an advantageous allele
enters a population and displaces the previously
existing allele(s).
- 
example: AZT resistance in the HIV-1 virus.
Natural selection at a single locus
1. Directional selection
• a form of selection acting on advantageous mutations.
• the selectively favored allele “sweeps” through the
population to become fixed (i.e., reach a frequency of 1.0).
Example:
Genotype:
Fitness:
AA
wAA
1.0
Aa
wAa
aa
waa
1.005 1.010
• here, the small a allele would reach fixation in about 3,000
generations.
Natural selection at a single locus
2. Purifying selection
a form of selection acting to eliminate harmful
(deleterious) alleles from natural populations.
- 
example: human recessive diseases like TaySachs or porphyria.
Natural selection at a single locus
2. Purifying selection
• a form of selection acting against deleterious
(harmful) alleles.
• the majority of deleterious alleles are recessive.
• purifying selection drives deleterious recessives to
low frequencies where they are maintained at
mutation-selection balance:
rate of introduction = rate of removal
by mutation
by selection
e.g., Tay-Sachs disease, cystic fibrosis, etc.
Natural selection at a single locus
3.  Balancing selection
various forms of natural selection that actively maintain
genetic variation in natural populations.
-
example: human sickle cell anemia (an example of
overdominance).
Overdominance
- overdominance occurs when fitness of the heterozygote
exceeds either homozygote.
Alleles:
HbA = normal hemoglobin allele
HbS = sickle cell allele
Genotypes: Relative fitness
HbA HbA: susceptible to malaria 0.88
HbA HbS: resistant to malaria, 1, experiences mild anemia
HbS HbS: susceptible to severe anemia 0.12
Natural selection at a single locus
3. Balancing selection
- various forms of selection that lead to the active
maintenance of genetic variation in natural populations.
- alleles are said to be “balanced” because a stable
equilibrium state is reached.
- if allele frequencies are perturbed from this equilibrium,
selection will return them back to that state.
Forms of balancing selection
2. Frequency-dependent selection
Forms of balancing selection
2. Frequency-dependent selection
• the relative fitnesses of genotypes are not constant but vary
with their frequencies in the population.
Forms of balancing selection
2. Frequency-dependent selection
• the relative fitnesses of genotypes are not constant but vary
with their frequencies in the population.
Genotype:
Fitness:
AA
wAA
Aa
wAa
aa
waa
1-p2
1-2pq
1-q2
Forms of balancing selection
2. Frequency-dependent selection
• the relative fitnesses of genotypes are not constant but vary
with their frequencies in the population.
Genotype:
Fitness:
AA
wAA
Aa
wAa
aa
waa
1-p2
1-2pq
1-q2
Example: Self-incompatibility (S) loci in flowering plants
S loci in flowering plants
S loci in flowering plants
● leads to obligate outcrossing
S loci in flowering plants
● leads to obligate outcrossing
● at equilibrium, all S alleles occur at equal frequencies
Forms of balancing selection
3. Spatially or temporally varying selection
- some genotypes are more fit than others in some
habitats, or under some environmental conditions, than
others.
Environment A
Genotype:
Fitness:
AA
wAA
1
Aa
wAa
0.95
aa
waa
0.91
↑ ↓ gene flow
Environment B
Genotype:
Fitness:
AA
wAA
0.84
Aa
wAa
0.93
aa
waa
1
Environment A
Genotype:
Fitness:
AA
wAA
1
Aa
wAa
0.95
aa
waa
0.91
↑ ↓ gene flow
Environment B
Genotype:
Fitness:
AA
wAA
0.84
Aa
wAa
0.93
aa
waa
1
Example: The Lap locus in the mussel, Mytilus edulis
Lap cline in Mytilus edulis in Long
Island Sound