BIOB51 Evolutionary Bio Lecture 11 Final Exam Notes UTSC

BIOB51 – Lecture 11 Patterns of Selection:


• Selection can happen at any time and is the removal of alleles from a population because the expression of the alleles in the phenotype (in the adults, gametes, or zygote)
Point mutations and pain:

• They looked at the pedigree of individuals who are affected by the point mutations.
  
• In India, there was a child who placed knives in his arms and walked on burning coal but
  didn’t experience any pain o Squares = males
  o Circles = females
  o Black = affected
  o White = not affected
  
• This trait is found in many individuals
  
• All the individuals who had this trait had injuries to their lips or tongue and also had
  suffered fractures, limping etc
  o This shows it is a deleterious mutation
  ▪ However, deleterious mutations are eliminated by selection, yet this one isn’t.
  Patterns of Selection: has two broad categories:
  
  • Patterns that maintain genetic diversity
    
  • Patterns that reduce genetic diversity
    Negative frequency dependence: occurs when the fitness of a genotype depends on the
    frequency in the population
    
    • Selection favours first one allele and then the other
      
    • The allele frequency fluctuates around some equilibrium level
      
    • Fitness increases as an allele becomes rare
      

o The benefit of the trait is high if it is rare
o This is called rare phenotype advantage, which is
when individuals who are uncommon do better in their
lifetime reproductive success
• Fitness decreases as an allele becomes common
o The benefit of the trait is low if it is common
• The allele frequencies fluctuate around a stable equilibrium,
which is shown by ****
o A stable equilibrium point is where any deviations won’t
be maintained for very long because the population will move back to equilibrium

• This type of dependence leads to a balanced polymorphism, which is when there is a stable equilibrium with more than one allele
  
• This is a type of a pattern that maintains genetic diversity Heterozygote Advantage/Overdominance:
  
  • Heterozygotes have the highest fitness in the population
    o Either homozygote can be higher than the other, but the highest will always be the
    heterozygote
    
  • The allele frequency is often at stable
    equilibrium since both alleles are being
    selected
    
  • Heterozygotes are at higher frequency than
    expected under H-W
    
  • This is a type of pattern that maintains
    genetic diversity, which leads to a balanced polymorphism
    One allele favoured (directional selection): this arises when one allele is beneficial
    
    • This often leads to the fixation of favoured alleles and the loss of other alleles o An allele is fixed when its frequency = 1.0
      o An allele is lost when its frequency = 0.0
      
    • The rate of change depends on:
      o The initial frequency of each allele o Dominance
      
    • There are three types of favoured alleles: o Dominant
      o Recessive
      o Co-dominant (incomplete dominance)
      
    • This leads to a pattern that reduced genetic diversity
      Heterozygote inferiority (underdominance): the heterozygous form has the lowest frequency in the population
      
      • This often leads to the fixation of one allele and the loss of another
        
      • The loss of an allele depends on: o The initial frequencies
        o The relative fitness of homozygotes
        
      • This leads to patterns that maintain genetic
        diversity among populations
        
      • This also leads to patterns that reduce genetic diversity within populations
        Do these population genetic models of allele frequency change match reality?
        

• Negative frequency dependent selection examples: o The cichlid fish, which is a scale eating fish

• ▪  They eat scales taken from the side of a victim
  
• ▪  20% of their attacks are successful
  
• ▪  It is one species, but there are 2 types of fish:
  
  • Right handed fish:
    o Which has a genotype of RR and Rr o It attacks the left side of the victim
    
  • Left handed fish:
    

o It has a genotype of rr
o It attacks the right side of the victim
o The frequency of the left handed fish over time fluctuates
near a value of 0.50
▪ Prediction: rarer type in each year should have
higher fitness than the more common type
▪ This is because victims are vigilant to the most
common type of attack
▪ The phenotype with high frequency in the population has a lower
frequency of breeding adults
• The rarer type has a higher fitness, which means negative
frequency dependent selection is occurring
o What other traits are under frequency dependent selection? ▪ For example, rovers and sitters in Drosophila
▪ Another example, is left handedness in humans

• The frequency of left handedness in north America = 10-15%
  
• It is a polygenic trait, which is a trait that is associated with
  multiple genes that affect left-right body asymmetry
  
• Other primates in evolutionary history how handedness
  o Such as chimpanzees
  
• The dominant right handedness evolved about 500 000 years ago
  o It was among hominins, such as homo neanderthalnsis
  
• It is associated with decreased longevity and schizophrenia
  
• Why does left handedness occur in humans?
  

o Hypothesis: This is because of a rare phenotype advantage in hand to hand combat
▪ Humans are considered to be violent beings
o Prediction: increased benefit of LH when combat is common
▪ Common combat =
higher equilibrium frequency of LH