Download Genetical theory of natural selection

Genetical theory of natural selection
Reminders
 Natural selection ≠ evolution
 Natural selection ≠ evolution by natural selection
 Natural selection can have no effect unless phenotypes differ in genotype
 Feature cannot evolve by natural selection unless it affects survival or
reproduction
Fitness
 Fitness of a genotype is the average lifetime contribution of individuals of that
genotype to the population
 Many times measured as reproductive success (number of offspring that
survive)
 Absolute fitness (R) versus relative fitness (W)
 Rate of genetic change under selection depends on relative fitness of
genotypes
Mean fitness
 Average fitness (w) of individuals in a population relative to the fittest genotype
 Suppose W A = 0.75, WB = 1.0, p = 0.2, q = 0.8
 Then w = (0.2)(0.75) + (0.8)(1.0) = 0.95
Coefficient of selection
 Amount by which the fitness of one genotype is reduced relative to the
reference genotype
 WA = 0.75, s = 0.25
Overall fitness
 Fitness depends not only on reproductive success, especially when species
reproduce sexually and have more than one reproductive event
 Age of reproduction
 Selection during sexual reproduction
Phenotype and fitness
 Relationship described by modes of selection
 Directional
 Stabilizing (normalizing)
 Diversifying (disruptive)
What are effects of selection?
 Fixation of one allele/loss of genetic variation (directional selection)
 Maintenance of genetic variation (balancing selection)
Assumptions for now
 Population is very large
 Mutation and gene flow do not occur
 Selection at each locus independent
 Natural section is differential survival
 No overlapping generations
Directional selection
 Assume fitness of heterozygote is intermediate between the two homozygotes
Fixation by natural selection
 Advantageous allele increases in frequency per generation according to:
Δp = 1/2spq
w
 Δp is positive whenever p and q are greater than zero
 Rate of evolutionary change increases as variation at locus increases
 Δp is positive as long as s is greater than zero
Number of generations to fixation
 Depends on:
 Initial frequency
 Selection coefficient
 Degree of dominance
Time to fixation
 If initial frequency low, recessive mutation increases very slowly (rarely
exposed in homozygous form); when common, recessive alleles go to fixation
quickly (selection against deleterious dominant fast)
 Dominant mutations increase in frequency rapidly, but approach fixation slowly
(selection against rare recessive is slow)
w increases as natural selection proceeds
Directional selection
Directional selection
Why do we see deleterious alleles in a population?
 Frequency of a deleterious allele is the balance between the rate at which it is
eliminated by natural selection and the rate at which it is introduced by
mutation or gene flow
 Directly proportional to mutation rate and inversely proportional to strength
of selection
Gene flow
Gene flow in mussels
Gene flow can reduce adaptiveness
How does selection maintain variation?
 Balancing selection
 Heterozygote advantage/disadvantage
 Varying selection
 Frequency dependent selection
If selection coefficient of two homozygotes are equal…
If selection coefficient of two homozygotes are not equal…
Overdominance
 Sickle cell anemia and malaria
 Single amino acid substitution leads to crystallization of hemoglobin at low
oxygen concentrations
 Heterozygotes have mild condition, homozygotes often don’t survive
Malaria
 Malaria caused by Plasmodium falciparum (a protist) that also infects rbc’s
 Individuals “normal” at the sickle cell locus get very bad malaria, heterozygotes
less bad, blood cells sickle preventing population growth of protist
 Heterozygote advantage arises from balance of opposing selective factors –
anemia and malaria (antagonistic selection)
Anemia/malaria
Heterozygote advantage (overdominance)
Heterozygote disadvantage (underdominance)
Varying selection
 Temporal variation
 Spatial variation
Multiple-niche polymorphism
Frequency-dependent selection
 The fitness of a genotype depends on the genotype frequencies in the
population
Inverse-frequency dependent selection
 Rarer the phenotype, the greater its fitness
Scale eating fish
Inverse-frequency dependent selection
Rewardless orchids
Positive frequency dependent selection
 Fitness of a genotype is greater the more frequent it is in a population
Mullerian mimicry
Adaptive landscapes
Adaptive landscapes