One ScottFreemanJonC.Herron
EvolutionaryAnalysis
FourthEdition
Chapter5
MutationandGeneticVariation
Copyright2007PearsonPrenticeHall,Inc.
Mutations
significant criticism of Darwin was that he was
unable to explain the source of variation.
Melanistic zebra due to a mutation.
Mutations
Mutation = any change to the nucleotide
base sequence of DNA.
Are the ultimate source of new alleles and
new genes
Typically result from errors during DNA
copying
Mutations
Point Mutations = Substitution of one base
for another.
AUGCCUCUCGAG codes for Met, Pro,
Leu, Glu
AUGUCUCUCGAG codes for Met, Ser,
Leu, Glu
Mutations
Transition- Change between purines or
between pyrimidines.
Transversion- Changes from a purine to a
pyrimidine or vice versa.
Mutations
Point mutations in coding regions may be:
Replacement substitutions- results in a
replacement of the amino acid being coded.
Silent substitutions- results in no change in
amino acid sequence of protein (due to
redundancy of genetic code)
Most commonly involves 3rd codon position
Mutations
Frameshift mutations- Insertion or deletion of a
nucleotide to a DNA sequence.
AUGCCUCUCGAG codes for Met, Pro, Leu, Glu
AUGGCCUCUCGAG codes for Met, Ala, Ser, Arg
Disrupts the reading frame of the protein, nearly always
results in a non-functional protein (loss-of-function
mutation).
Mutation Rates
Mutation rates are low per gene.
With many genes/individuals the mutation
rate is increased.
10% of human gametes carry mutations
causing phenotype change
Most individuals probably have mutations
that form new alleles
Mutation Rates
Tracking mutations that cause major
phenotypic changes produces underestimates.
Misses silent substitutions completely.
Fails to detect replacement substitutions
producing minor changes
Misses what selection eliminates
Ex. When mutation is fatal
Mutation Rates
Sequencing studies produce more accurate results.
Caenorhabditis elegans (roundworm)- 2.1
mutations/genome/generation for nuclear DNA
If mutations are allowed to accumulate where
there is no natural selection, fitness levels drop.
Mutation Rates
Mutation rates vary among species and even
among populations of a species.
Dna polymerases vary in replication
accuracy
Dna repair mechanisms vary in efficiency
Fitness Effects of Mutations
Most mutations are either neutral or harmful
Beneficial mutations do occur, but are rare.
High mutation rates can be beneficial in
novel or changing environments.
Why?
Fitness Effects of Mutations
Low mutation rates are better in stable
environments.
Past natural selection has likely produced
adaptive phenotypes
Mutations produced are typically harmful
Gene Duplication
Several kinds of mutations can create
entirely new genes.
Gene duplication is the most important
source of new genes.
Unequal cross-over is most common type
of gene duplication.
Gene Duplication
Unequal cross-over occurs during meiosis I
and results in one chromosome acquiring an
extra copy of some regions of the DNA.
Gene Duplication
Extra copy is free to accumulate mutations.
Other copy maintains the original function.
Extra copy may become nonfunctional
(pseudogene) or form a new gene with a
new function.
Could provide also an additional copy of the
parent gene.
Gene Duplication
e.g. human globin genes (subunits of
hemoglobin) are found in 2 clusters.
8 functional forms of these genes expressed
at different times during development.
Gene Duplication
Duplicated genes that diverge in their
nucleotide sequence within a species are
paralogous genes.
Ex. Alpha and Beta-globin genes in humans
Gene Duplication
Duplicated genes that diverge in their nucleotide
seuqence in a different lineage after speciation are
orthologous genes.
Ex. Beta-globin genes in humans and mice
Chromosome Mutations
Mutations can also
occur at larger,
chromosome-level
scales.
Inversions-
Chromosome Mutations
Consequences of inversions:
Unless break disrupts a gene, may not be
any phenotypic effect.
Alleles on an inversion tend to be inherited
together and not disrupted by crossing over.
Inversions may be favored by natural
selection if they preserve favorable
combinations of alleles.
Polyploidy
Organisms that have more
than two chromosome sets
are said to be polyploid.
4N, 8N, etc.
Typically due to meiotic
errors that result in
diploid gametes
Most common in plants
Polyploidy
Usually results in genetic isolation and
speciation.
Mating 4N with 2N individuals produces
3N offspring which has low fertility due to
mechanical problems in meiosis.
Must mate with self or other polyploids.
Polyploidy
Evolutionary consequences of polyploidy?
Ray-finned fish may have experienced
polyploidy early in their evolution.
Additional genetic material may have led to
huge diversity in ray-finned fish.
Similar data for Angiosperms (grasses to
deciduous trees)
Genetic Variation
How do we measure genetic variation in
populations?
Direct observation of phenotypes; only
possible in limited situations (e.g.
incomplete dominance or codominance
Examine variations in proteins or in dna
Genetic Variation
Individuals who produce CCR5-^32 protein are
less susceptible or resistant to aids then people
with normal CCR5+ protein.
++ = susceptible, +^32 = susceptible but progress
to AIDS more slowly, ^32^32 = resistant.
CCR5-^32 protein shorter than normal CCR5+
protein.
Can be separated with gel electrophoresis.
Allele Frequencies
Allele Frequency is a measure of how
common one allele is relative to all other
alleles of the same gene in a population.
In diploid species, there would be 2X as
many alleles as individuals in the
population.
How did they determine the allele
frequencies in Table 5.3?
Genetic Variation
Most populations show
considerable genetic variation.
Typical values:
4-15% of average individuals
loci are heterozygous.
33-50% of enzymes are
polymorphic (>1 allele)
Genetic Variation
Even more genetic variation revealed at DNA
level: e.g. Cystic Fibrosis alleles do not all
represent same change in DNA.
30,000 cystic fibrosis alleles were sequenced,
result from >500 different mutations.
Genetic Variation
Natural selection should eliminate most
deleterious alleles, thus reducing variation.
Why then do populations have so much
variation?
Variation may be favored by selection
(heterozygote advantage)
Variations may be selected neutral; no
fitness differences between alleles