BIOB51 Lec 07 Mutation and Variation
1. Understanding check: how do
traits evolve?
-
Understanding
check hypothetical steps in the
origin of avian flight
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Bipedal
terrestrial: simple feathers thermoregulation & display
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Powered
flight: modern feathers allow efficient gliding and true flight
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F1 generation
increases diversity, on average, you look like your parents, but there is a
little bit of variation
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The
distribution will move upwards when we look at the individuals that survive
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This change in
trait value over generations required 3 things: phenotypic variation,
selection, genes (heritable traits)
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Not all new
traits will lead to evolutionary change
2. Sources of phenotypic variation
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Expressions of
PAV allele directly related to bitterness ratings in heterozygotes
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Can this type
of variation evolve?
o
Genetic
variation in type of encoded proteins, amount, timing or location of expression
Environmental
-
Monozygotic
twins
o
External
influence on amount of protein produced, fxn of proteins
-
Inducible
defenses in Daphnia clones
o
Growing in
presence predators: they develop barbed spines on their backs, or spine on head
capsule or tail
o
Not expected
to show an evolutionary change
o
All Daphnia
are able to induce defenses but the degree of which might change
-
Can this type
of variation evolve? No
o
No genetic
differences, can’t pass off these changes
Genotype by Environmental Interaction
-
Phenotypic
plasticity: development of different phenotypes in different environments
o
Getting
sunburned
-
Ex. tobacco
hornworms in textbook
o
Typical vs
black caterpillar
o
Black one
exposed to heat shock and will turn green
o
Others are
black and are insensitive to that change
o
This variation
is due to differences in their genetic make up
o
Effects how
they respond to temperature
o
Plasticity is
evolving here
-
Can this type
of variation evolve? Yes
o
There is a
genetic variation that effects how you respond to the environment
o
Extent to
which different genotypes show different degrees of phenotypic plasticity
(development of different phenotypes in different environments)
3. Genetic variation: mutation and
new alleles
-
If there is no
variation, the population can’t evolve
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Natural
selection is expected to reduce heritable variation in populations
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Evolution
cannot occur without heritable variation:
o
Historical
view:
§
Nat selection
eliminates variation
§
Little
variation present in current populations (evolution is a thing of the past!)
-
BUT we now
know that mutation is constantly generating new variants
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Does mutation
‘keep up’ with selection?
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Mutation:
change in the nucleotide sequence of DNA
Point mutation:
-
Single-base
change in DNA sequence = new allele
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Common source
of genetic variation
-
Can have large
effects on phenotypes
o
Even mutations
in introns can affect proteins
o
Introns are
non coding parts of DNA
-
Causes:
o
Random errors
in DNA replication
o
Damage or
chemical alteration to DNA
o
Lack of repair
(e.g by DNA polymerase)
Ex. Sickle-cell anemia
Ex. AVI -> PAV
Indels:
-
Inserted or
deletion of nucleotides in DNA sequence
-
Most common
source of genetic variation
-
Can shift
codon reading frame
o
Changes
meaning of all downstream codons
-
Causes:
o
Misalignment
of DNA during replication
§
Common in
regions w/ repeated base pairs
o
Lack of repair
(e.g by DNA polymerase)
Ex. Huntington’s disease
-
Progressive
brain disorder
-
Autosomal
dominant, manifests in adults
-
Caused by
insertion of many copies of CAG triplets in the HTT gene
2 Ways to measure genetic variation
1.
Polymorphism
-
Fraction of
genes in a population with at least 2 alleles
2.
Mean
heterozygosity (H)
-
Mean %
individuals heterozygous per locus
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Fraction of
genes that are heterozygous in genotype of average person
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Lower than
polymorphism
-
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CFTR gene =
1400 different mutations at this one locus cause the same phenotypic effect
(cystic fibrosis)
Conclusion: dozens to hundreds of different alleles
are present for most genes in most populations (high polymorphism)
-
Mutations
generate new alleles in every generation
-
Fate of a new
allele depends on its fitness effect
How can mutations be neutral to
fitness?
4. Mutation rates
5. Evolution of mutation rate?
