Evolution Lecture 7: Mutation and Genetic Variation II

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Evolution Lecture 7 Mutation and Genetic
Variation II
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Mutation Rates

• How do we determine these?
• Old waysexamine loss of function and determine
  of odd phenotypes/total normal and odd phenotypes
• Example Appearance of dwarfism when parents are
  normal.
• Report mutation rate/gene/generation
• If total genes known, then we might extrapolate
  to mutation rate/genome/generation
• Then we can calculate the of mutations/individua
  l/generation

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Variation in mutation rates among species

• Much lower in single-celled organisms as
  mutations/genome/generation than in multicellular
  organisms
• Mutation rates vary as a function of the number
  of cell divisions prior to gamete formation
• A sperm produced by a human undergoes 400 cell
  divisions prior to the formation of the gamete,
  whereas fruit flies undergo only 30 divisions
• If we correct for the number of cell divisions
  per generation, then we see that it is equivalent
  across multi and single celled organisms

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Estimating mutation rates in plants relative to
age

• Mangroves are long-lived compared to barley.
• The mutation rate that creates albinos is 25X
  greater in Mangroves
• These long-lived plants require many cell
  divisions prior to the differentiation of germ
  line cells from somatic cells

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How many new alleles occur in an average
individual

• 2-3 single celled organisms/1000 have a variable
  phenotype
• Problem with this analysis?
• We miss silent mutations
• How can we determine if this the number of new
  alleles per individual

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C. elegans experiment to determine actual rate

• Dee Denver sequenced 10,428 bp of mtDNA in this
  species
• Established 74 lines from this individual and
  each was propagated for 214 generations
• Descendant individuals were sequenced for 771,672
  bps and compared to original individual
• 26 mutations 13 transitions, 3 transversions, 10
  indels
• 1.6 x10-7 mutations per site per generation
• Extrapolating to the nuclear genome (97 million
  bps), then every individual hatched would have 15
  new mutations
• What is wrong with this calculation?
• 1 incorrect base inserted for every 10 million bps

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Phylogenetic methods for mutation rate

• Use phylogeny
• Estimate rates of substitution along branches
• Calibrate tree with fossils or geological dates
• Examples

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Mutation rate and natural selection

• DNA Polymerases and repair enzymes vary in
  accuracy
• Mutation in repair enzymes in bacteria can result
  in 100 to 1,000 X higher mutation rate
• When entered into novel environments it was found
  that they had elevated mutation rates
• When environment becomes normal, mutation rate
  decreases
• High mutation rate may be adaptive when organisms
  colonize new environments. However, when
  organisms become adapted to the environment, new
  mutations are usually deleterious

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Survivability decreases as the number of
mutations are allowed to increase per
generations as compared to control population
where mutations are selected out in C.
elegans Most mutations are mildly deleterious
and reduce fitness by 2 in heterozygous
condition
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• Experimental bacteria have inserted random DNA
  sequences
• throughout the genome. Controls have no
  insertions
• 2. Selection coefficient measures the difference
  in the rate of growth
• between exp. and control
• 3. Cumulative frequencythe number of inserts
• 4. In bacteria it takes a great number of changes
  to decrease fitness

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Neutral Mutations

• Most mutations are neutral and have no effect on
  fitness
• Many may be silent
• Some may only have a slightly deleterious effect

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The origin of new genes

• Gene duplication by unequal crossing over
• Unequal crossing over occurs during synapsis in
  prophase of Meiosis I
• Non-homologous areas synapse

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Unequal Crossing Over
Genes duplicate
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Gene Duplications

• These duplicate genes are free to mutate, given
  that the original gene exists and functions
  properly
• The new gene may change function over time
• Hemoglobin is a gene family resulting from
  duplication
• The alpha cluster is on chromosome 16 and the
  beta cluster is on chromosome 11
• All members have slightly different functions,
  but share incredible homology

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Various functions in hemoglobin family members
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Striking Homology in Hemoglobin Families
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Chromosomal Inversions
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Chromosomal Inversions

• This breakage and re-annealing usually leads to
  duplication or loss of chromosomal regions and
  dysfunctional gametes
• Some genes appear linked...that is, they are so
  close together on the chromosome that there is
  low probability for breakage and separation
• Examination of fruit flies have determined that
  there are combinations of genes that increase
  fitness in various habitats. These combinations
  exist in various inversions.
• They also occur convergently

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Clines exist where larger flies occur in wetter
colder environments associated with a type of
inversion
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Polyploidy and speciation
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Determining Genotypes and Calculating Allele
Frequencies (p.129-130)
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Mean Heterozygosity average frequency of
het across loci or the fraction of genes that are
het in an individual
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