Evolution Lecture 10: Population Genetics:Mutation, Migration and Drift

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Evolution Lecture 10 Population
GeneticsMutation, Migration and Drift
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Mutation

• Mutation adds variation to population
• It is, by itself not a powerful force
• Imagine we have the following freqa0.9 and
  A0.1
• Say, A is mutated to a at a rate of 1 copy/10,000
  generations
• Back mutations rarely happen
• Observe

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Normally, they would produce alleles at the
freqA0.9 and a0.1. Mutation has converted
them toA0.9-(0.9x0.0001)0.8999 and
a0.1(0.0001x0.9)0.10009
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Mutation and rate of change?

• Allele frequency change occurs slowly!!
• Not a big deal by itself!
• This example is at a quick rate of mutation

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Over long periods, mutation can change allele
frequencies
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Mutation and selection

• Mutation is a potent evolutionary force when tied
  to selection
• Lenski (1994) took 12 populations of cells and
  grew them on nutrient poor media (selective
  environment).
• He then took subsamples of each population daily
  for 1500 days and grew them in fresh media for
  10,000 generations
• Samples were frozen (still living) at regular
  intervals. This was so relative fitness of
  ancestors and descendants could be compared.
• He also measured cell size.
• Individuals grown in harsh environments produced
  mutations that allowed it to reproduce quicker
• The time from the appearance of a mutation to the
  fixation of that was so quick we can almost not
  see it on a graph

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Mutation-selection balance

• The rate at which deleterious alleles are being
  eliminated is equal to the rate at which new
  copies are made
• qsqrt(u/s), where q is the equilibrium
  frequency, u is the mutation rate and s is the
  selection coefficient.
• Ranges from 0-1. This tells us the degree of
  selection against the mutation. If selection
  coefficient is small and and mutation rate is
  high, then the equilibrium frequency of that
  allele will be high.

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Mutation-Selection Example

• Spinal muscular atrophy is a neurodegenerative
  disease and is caused by deletions in the gene
  telSMN on chromosome 5.
• Second most common autosomal recessive allele
• It has a freq of 0.01 in Caucasian population and
  has a selection coefficient of 0.9.
• You would expect this allele to become extinct,
  however, it occurs at 1/100
• If we substitute allele freq. for q and selection
  coefficient for s and solve for u, we get a
  number that is 9.0 x
• 10-5 mutations per telSMN allele per generation
• When we examine 340 individuals, it was found
  that 7 of the parents did not have this mutation
  (brand new mutation)
• This rate is 1.1 x 10-4
• Very close to estimate

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Is Cystic fibrosis maintained by
mutation-selection balance?

• Most common genetic disease
• LOF of CFTR gene. This is a cell surface protein
  that is expressed in the lungs and prevents
  bacterial (Pseudomonas) infection
• People of European ancestry seem to have this at
  a frequency of 0.02.
• Using the equation in Box 5.10, we find that the
  mutation rate creating the new allele would have
  to be very high (4 x 10-4) with a selection
  coefficient of 1.0 to maintain an allele
  frequency of 0.02.
• However, the real mutation rate is 6.7 x 10-7.
  Therefore, the frequency at 0.02 cannot be
  maintained by a steady supply of mutations
• Is it possible, the allele is being maintained by
  overdominanceheterozygote superiority?

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Pseudomonas
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Cystic fibrosis and het. superiority?

• It is possible that heterozygous individuals
  cystic fibrosis are resistant to typhoid fever?
• The CFTR protein is also found in the gut.
• Typhoid bacteria (Salmonella) exploit this
  protein to cross the gut and increase infection
• If you look at normal CFTR in homozyg., het., and
  homoz. with loss of both CFTR (F508) copies..you
  see

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Selection for cyst fibrosis gene after a typhoid
outbreak
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Migration

• The movement of alleles between populations
• Migration can be caused by anything that moves
  alleles. Dispersal of animals, pollen on the
  wind etc.

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Amounts of gene flow?
What about distance?
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Migration can obviously change allele
frequencies!!
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Migration as a mechanisms of evolution

• Water snakes (Nerodia sipedon) in Lake eerie come
  in two color phases banded and unbanded
• This is a two allele system
• Banded dominant to unbanded
• The mainland has really only banded
• The islands may have both

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Lake Erie water snakes

• It was found that, when basking on islands, the
  unbanded snakes are more cryptic and thus remain
  hidden better.
• Why wouldnt selection cause the unbanded pattern
  to go to fixation?
• Migration. Every year, banded snakes migrate
  from the mainland and introduce fresh banded
  alleles
• Therefore, migration offsets selection

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Unopposed migration

• Migration may be opposed by selection
• If not, migration tends to homogenize populations
• If gene flow from the mainland to the island was
  not opposed by selection, than the island would
  be homogenized by banded color patterns
• Fst statistics predict the amount of allelic
  variation from 0-1. High numbers indicate high
  variation

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Age of flower populations and diversity
Youngfounded from many alleles Intermediatemigr
ation homogenizes Pop OldCompetition and
disease leaves only few representatives, no new
migration
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Next time, genetic drift!!!