Forces of Evolutionary
Change
LSC-10032 Lecture 14
Intended Learning Outcomes
• Describe the combined effects of
mutation on population genetics
Next Lecture
• Understand how migration, genetic
drift & gene flow influence evolution
• Understand which forces drive
evolution
H-W principle defines the following
evolutionary forces acting on genotype
and allele frequencies:
 Random mating
 Mutation
 Migration
 Random Genetic Drift
 Selection
What is their mode of action and
impact on evolution?
Question
Which of the following evolutionary forces DOES
NOT act on allelic frequencies?
A.
B.
C.
D.
E.
Mutation
Migration
Random genetic drift
Selection
Inbreeding
E.
Inbreeding
Mutation is a change in DNA, the hereditary material of life.
An organism's DNA affects how it looks, how it behaves, and its
physiology and all aspects of its life.
So a change in an organism's DNA can cause changes in all
aspects of its life.
Mutations are random, can be beneficial, neutral, or harmful for
the organism, but mutations do not "try" to supply what the
organism “needs”
Since all cells in our body contain DNA, there are lots of places for
mutations to occur; however, not all mutations matter for
evolution.
Somatic mutations occur in non-reproductive cells and won't be
passed onto offspring.
Mutation
• Mutations are raw material of evolution.
• No variation means no evolution and
mutations are the ultimate source of variation.
• Genes mutate and chromosomes can mutate
• Mutations are the source of variation, but the
process of mutation does not itself drive
genetic change in populations.
Mutations
Gene mutation
– Point Mutations
– Substitutions
– Insertions/Deletions
– Frame-shift
Chromosomal mutation
– Deletion
– Translocation
– Nondisjunction
– Inversion
Chromosomal mutation
Point mutation:
A mutation that alters a single base position in a
DNA molecule by converting it to a different base
or by the insert/deletion of a single base in a DNA
molecule.
Point mutations are classified in molecular terms in
Figure 16-2, which shows the main types of DNA
changes and their effects on protein function
when they occur within the protein-coding region of
a gene.
Types of mutations
Gene mutations can arise naturally or they can be
induced.
Spontaneous mutations are naturally occurring
mutations and arise in all cells. It occur in the absence
of exposure to mutagens.
Induced mutations arise through the action of certain
agents called mutagens that increase the rate at which
mutations occur.
Mutagen: An agent capable of increasing the mutation
rate e.g. chemicals/UV
Mutation rates
• The mutation rate is the probability that a copy of
an allele changes to some other allelic form in
one generation.
• Thus, the increase in the frequency of a mutant
allele will be the product of the mutation rate
times the frequency of the non-mutant allele.
• The number of observed mutations per genome
per generation provides an estimate of the
mutation rate.
The Fate of New Mutations
Probability of loss
100
0
0
15
30
Generations
• Majority lost due to either:
– natural selection: since
they are harmful, or
– random genetic drift:
since they are rare
• Likelihood of loss depends
on interaction with other
evolutionary forces
Mutation
• Mutation adds variation to population
• It is, by itself not a powerful force
• Imagine we have the following freq:
A=0.9 and a=0.1
• If A is mutated to a at a rate of
1 copy/10,000 generations
• Back mutations rarely happen
• Observe
Normally, they would produce alleles at the freq: A=0.9 and a=0.1.
Mutation has converted them to: A=0.9-(0.9x0.0001)=0.8999
and a=0.1+(0.0001x0.9)=0.10009
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
Mutation and Selection
• Mutation is a potent evolutionary force when
tied to selection
• How did Mycobacterium tuberculosis become
resistant to antibiotics?
• Tuberculosis (TB) is a disease caused by bacteria
that are spread from person to person through the
air.
• TB usually affects the lungs, but it can also affect
other parts of the body, such as the brain, the
kidneys, or the spine.
• Sometimes drug-resistant TB occurs when
bacteria become resistant to the drugs used to treat
TB.
• This means that the drug can no longer kill the TB
bacteria.
Evolution of Drug resistance
TB History
• 1900s showed decrease
– Nutrition
– Antibiotic rifampin
• 1993 global emergency
• Rifampin resistance
• Single point mutation
Bacterial mutation
• Point mutation in rpoB gene (RNA polymerase)
• Changed serine-leucine at 153rd amino acid
• Mutant forms not as efficient
• Rifampin binds to RNA polymerase
• Bound rifampin reduces protein synthesis
Events chain
• Mutation of rpoB by chance
• Not as efficient would have remained low
frequency
• Rifampin therapy
• Patient appears cured
• Mutated TB now predominant – advantage
• Mutated TB proliferates
• Drug resistant TB now dominates
Intended learning Outcomes
• Describe the combined effects of
mutation on population genetics