BIOE 109 EvolutionSummer 2009

Lecture 3- Part INatural 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: 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 

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 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   

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

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: 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

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

Lives > 4,000 m in Himalayas

Andean goose, Chloephaga melanoptera

Lives > 3,500 m in Andes

3D structure of hemoglobin

119 55

x

x

119 55

Bar-headed goose proline leucine alanine

x

x

119 55

Bar-headed goose proline leucine alanine

Andean goose proline leucine serine

x

x

Recap

• Concept of fitness: Darwinian and relative fitness

• Types of selection: Purifying selection

Directional selection

Balancing selection

over-dominance frequency-dependent spatially/ temporally varying

• Convergent and parallel evolution