Transcript Lecture 3: (Part 1) Natural selection

BIOE 109 Evolution
Summer 2009
Lecture 3- Part I
Natural selection – theory and definitions
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.
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.
5. Product of selection is a “compromise”.
What is fitness?
What is fitness?
Natural selection and the concept of
fitness
Darwinian fitness: the number of gene copies
(i.e., offspring) a phenotype places into the next
generation.
Natural selection and the concept of
fitness
Darwinian fitness: the number of gene copies
(i.e., offspring) a phenotype places into the next
generation.
Relative fitness: a phenotype’s Darwinian
fitness relative to other phenotypes.
http://www.blackwellpublishing.com/ridley/video_gallery/LP_What_is_fitness.asp
What is fitness?
1. Fitness is a description not an explanation
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:
Total fitness = viability + fecundity + longevity +
mating success
Natural selection at a single locus
1. Purifying selection
Natural selection at a single locus
1. Purifying selection
• a form of selection acting against deleterious
(harmful) alleles.
Natural selection at a single locus
1. Purifying selection
• a form of selection acting against deleterious
(harmful) alleles.
• the majority of deleterious alleles are recessive.
Natural selection at a single locus
1. 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:
Natural selection at a single locus
1. 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
2. Directional selection
Natural selection at a single locus
2. Directional selection
• a form of selection acting on advantageous mutations.
Natural selection at a single locus
2. 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).
Natural selection at a single locus
2. 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:
AA
Aa
aa
Natural selection at a single locus
2. 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
Aa
wAa
aa
waa
Natural selection at a single locus
2. 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
Natural selection at a single locus
2. 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
3. Balancing selection
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.
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.
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
1. Overdominance
Forms of balancing selection
1. Overdominance
- occurs when the heterozygote is more fit than either
alternate homozygote.
Forms of balancing selection
1. Overdominance
- occurs when the heterozygote is more fit than either
alternate homozygote.
Genotype:
AA
Aa
aa
Forms of balancing selection
1. Overdominance
- occurs when the heterozygote is more fit than either
alternate homozygote.
Genotype:
AA
Aa
aa
Fitness:
wAA
wAa
waa
Forms of balancing selection
1. Overdominance
- occurs when the heterozygote is more fit than either
alternate homozygote.
Genotype:
AA
Aa
aa
Fitness:
wAA
wAa
waa
0.88
1
0.14
Forms of balancing selection
1. Overdominance
- occurs when the heterozygote is more fit than either
alternate homozygote.
Genotype:
AA
Aa
aa
Fitness:
wAA
wAa
waa
0.88
1
0.14
Example: Sickle cell hemoglobin in west-central Africa
Example: Sickle cell hemoglobin in west-central
Africa
Alleles:
HbA = normal allele
HbS = sickle cell allele
Example: Sickle cell hemoglobin in west-central
Africa
Alleles:
HbA = normal allele
HbS = sickle cell allele
Genotypes:
HbAHbA: susceptible to malaria
HbAHbS: resistant to malaria, mild anemia
HbSHbS: susceptible to severe anemia
Example: Sickle cell hemoglobin in west-central
Africa
Alleles:
HbA = normal allele
HbS = sickle cell allele
Genotypes:
HbAHbA: susceptible to malaria
HbAHbS: resistant to malaria, mild anemia
HbSHbS: susceptible to severe anemia
• results in stable polymorphic equilibrium where HbA = 0.89
and HbS = 0.11
Forms of balancing selection
2. Frequency-dependent selection
Forms of balancing selection
2. Frequency-dependent selection
• the relative fitness 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 out-crossing
S loci in flowering plants
● leads to obligate out-crossing
● 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
2-year female morph cycle: Uta stansburiana
Orange females
• small eggs
• large clutches
Yellow females
• large eggs
• small clutches
2-year female cycle: Uta stansburiana
Orange female
frequency
Number of Breeding
Females
0.55
0.50
0.45
0.40
0.35
0.30
0.25
0.20
0.15
0.10
Population status
0.05
0.00
8990 91 92 93 94 95 96 97 98 99 00 01
Year
(Modified from Sinervo et al., 2000)
Orange common
Yellow common
Fitness of rare O
strategy
1.00
1.61
Fitness of rare Y
strategy
1.60
1.00
Convergent evolution
Parallel evolution
Convergent evolution
Parallel evolution
Convergent evolution
Parallel evolution
Convergent evolution: the evolution of similar traits
independently in distantly related taxa from different
ancestral features or from different developmental
pathways
Example: marsupial and placental mammals
(common ancestor ~ 170 mya)
Convergent evolution between
placental and marsupial mammals
Convergent evolution in crocodiles and hippos
Both have eyes on top of their heads
Parallel evolution: the evolution of similar traits
independently in closely related taxa involving the
same genes or developmental pathways
Parallel evolution: the evolution of similar traits
independently in closely related taxa involving the
same genes or developmental pathways
Example: hemoglobins in high altitude geese
Bar-headed goose, Anser indicus
Andean goose, Chloephaga melanoptera
Lives > 4,000 m in Himalayas
Lives > 3,500 m in Andes
3D structure of hemoglobin
x
x
119
55
x
x
119
Bar-headed goose
proline

alanine
55
leucine
x
x
119
55
Bar-headed goose
proline

alanine
leucine
Andean goose
proline
leucine

serine
Recap
• Concept of fitness: Darwinian and relative fitness
• Types of selection: Purifying selection
Directional selection
Balancing selection
over-dominance
frequency-dependent
• Convergent and parallel evolution
spatially/ temporally varying