bioe 109 evolution summer 2009 lecture 3- part i natural selection – theory and definitions
Post on 19-Dec-2015
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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”.
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. 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
• 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
• 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: AA Aa aaFitness: wAA wAa 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: AA Aa aaFitness: wAA wAa waa
1.0 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: AA Aa aaFitness: wAA wAa waa
1.0 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
- 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
- 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 aaFitness: wAA wAa waa
Forms of balancing selection
1. Overdominance
- occurs when the heterozygote is more fit than either alternate homozygote.
Genotype: AA Aa aaFitness: 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 aaFitness: 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 alleleHbS = sickle cell allele
Example: Sickle cell hemoglobin in west-central Africa
Alleles:
HbA = normal alleleHbS = sickle cell allele
Genotypes:
HbAHbA: susceptible to malariaHbAHbS: resistant to malaria, mild anemiaHbSHbS: susceptible to severe anemia
Example: Sickle cell hemoglobin in west-central Africa
Alleles:
HbA = normal alleleHbS = sickle cell allele
Genotypes:
HbAHbA: susceptible to malariaHbAHbS: resistant to malaria, mild anemiaHbSHbS: 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
• 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: AA Aa aaFitness: wAA wAa 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: AA Aa aaFitness: wAA wAa waa
1-p2 1-2pq 1-q2
Example: Self-incompatibility (S) loci in flowering plants
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: AA Aa aaFitness: wAA wAa waa
1 0.95 0.91
gene flow
Environment B
Genotype: AA Aa aaFitness: wAA wAa waa
0.84 0.93 1
2-year female morph cycle: Uta stansburiana
Orange females• small eggs• large clutches
Yellow females• large eggs• small clutches
89 90 91 92 93 94 95 96 97 98
Year99
0
50
100
150Number ofBreedingFemales
2-year female cycle: Uta stansburiana
8990 919293949596 9798
Year
9900 01
0.10
0.40
0.50
0.55
0.45
0.35
0.30
0.25
0.20
0.15
0.00
0.05
Orange femalefrequency
Number of Breeding Females
(Modified from Sinervo et al., 2000)
Population status
Orange common Yellow common
Fitness of rare O strategy 1.00 1.61
Fitness of rare Y strategy 1.60 1.00
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)
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
Lives > 4,000 m in Himalayas
Andean goose, Chloephaga melanoptera
Lives > 3,500 m in Andes