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Chapter 3:
Evolutionary genetics
of natural populations
What is Evolution?
– Change in the frequency of an allele within a
population
– Evolution acts on DIVERSITY to cause adaptive
change
• Ex. Light vs. Dark wings in the pepper moth
– High levels of diversity are GOOD for
conservation biology!!!
Imagine you are an inventor and you can pick a
toolkit, which will you choose?
Having many diverse tools available to use allows you
to create novel things!
Genetic Diversity is Evolution’s Toolkit
• Genetic diversity gives evolution more to
select on and novel changes can result!
What is the goal of conservation
genetics..
• Preserve genetic diversity within species
• Keep species dynamic entities, capable of
evolving
• Here we address the questions how is genetic
diversity produced, and how quickly is it
regenerated if it is lost?
What can cause a change in genetic
diversity?
•
•
•
•
•
Mutation
Migration
Selection
Chance events
Fragmentation
Mutation
A sudden genetic change in an allele or chromosome
The source of all genetic diversity
Mutation can refer to…
– The process by which novel genetic variants arise
– The phenotypic products of the genetic changes
Central Dogma of Biology
DNA
ATGGTTTAA
Transcription
RNA
Translation
AUGGUUUAA
Proteins
(MET)(VAL)(STOP)
Tissue, body, etc
There are several types of mutations
• Point mutations
– Substitution
– Insertions
– Deletions
• Inversions
Point Mutations:
Substitution
Synonymous substitution
GUU
GUC
Nonsynonymous substitution
GUU
CUU
Synonymous: having the same meaning as another word or phrase in the same language.
Val
Leu
Point mutation - Insertions
Original Sequence
(ATG)(CGT)(GAG)(TCG)(AGA)
Amino Acid Product
(MET)(ARG)(GLU)(SER)(ARG)
Mutated Sequence
(ATG)(CGT)(AGA)(GTC)(GAG)A
Insertion
New Amino Acid Product
(MET)(ARG)(ARG)(VAL)(GLU)(...)
Point mutation - Deletions
Original Sequence
Amino Acid Product
Mutated Sequence
(ATG)(CGT)(TTG)(AAG)(AGA)
(MET)(ARG)(LEU)(LYS)(ARG)
(ATG)(CGT)-(TGA)(AGA)(GA
Deletion
New Amino Acid Product
(MET)(ARG)(STOP)
Frameshifts
The fat cat sat
The fat cat sat
Hef atc ats at
Other types of mutations
Inversion
• Is the mutation rate is equal across the
genome?
• Is the frequency of SNPs the same in all
regions of the genome?
What mutations will be under the strongest
selection pressures?
What will the most common mutations be?
What will the most rare mutations be?
How will mutations affect genes ?
Coding region vs promoter vs introns
• Only 1-2% of all mutations that occur in
coding DNA will be advantageous
• Deleterious alleles are selected against
• The balance between the formation of
deleterious alleles and their removal via
selection is called
Mutation Selection Balance
Neutral Mutations
• Do not effect the protein end product
– no strong selection pressure
• They are used as genetic markers
• Helpful used to compare individuals
Mutation load
• Low frequencies of deleterious alleles are
found in all naturally out breeding populations
• Inbreeding increases the probability of these
alleles being expressed in homozygous
genotypes
Can Mutations help restore genetic
diversity?
• To restore genetic diversity via mutation you need
hundreds to millions of generations
Mutations can’t restore genetic diversity
fast enough from a conservation
perspective.
What are other options???
What can cause a change in genetic
diversity?
•
•
•
•
•
Mutation
Migration
Selection
Chance events
Fragmentation
What can change allele frequency?
• Migration
• Why can migration can rapidly restore genetic
diversity to a population
Migration
• Partially isolated populations diverge over
time as a result of chance and selection
• Prior to Mongol invasions the B blood allele
was absent from Europe.
Cline: gradual
change in allele
frequency across a
geographical area
How can migration effect genetic diversity?
The genetic impact of migration depends on the
proportion of alleles contributed by immigrants
and on the difference in frequency between the
native population and the immigrants
q = m(qm − qo )
m – migration rate
qm - allele frequency in immigrants
qo- allele frequency in original population
Migration can be an effective
way to restore genetic
diversity
BUT
Is migration always beneficial
for conservation?
Introgression
Many endangered species can be
threatened by gene flow from related,
but not endangered, species
What is the overall accumulation of alleles from dogs in the Web Valley wolf population?
M=
ql - qo
qm - qo
M=
0.78 – 1.0
0 – 1.0
= 0.22
The Web Valley population derives about 22% of its genetic composition from
domestic dogs at this locus
THIS IS FOR A SINGLE LOCI – CONSERVATION GENETICS IS INTERESTED IN A GENOME WIDE
SCALE
What can cause a change in genetic
diversity?
•
•
•
•
•
Mutation
Migration
Selection
Chance events
Fragmentation
What can change allele frequency?
• Natural Selection
Organisms are better adapted to
their environment
• Physical and biotic
environments of virtually all
species are continually
changing
• Species must ADAPT to these
changes
What are environmental and
anthropogenic changes that
cause selection?
Pests, parasites, and diseases
Pollution
Rainfall / Drought
Temperature
Competitors
Habitat loss
Example of Adaptation
Introduced into Australia in 1859 for sport hunting
Rapidly increased in numbers and became serious pests.
Had a negative effect on the local species
Myxoma virus introduced in 1950 and caused 99% mortality
Strong directional selection resulted in rapid increases in genetic resistance of rabbits to the myx
The myxoma virus also evolved lower virulence, as this increased the probability of being transm
With every infection the mortality decreases.
The mortality to this virus strain dropped from around 90% to 25% in 1958.
Pg 39 in your textbook
Some other examples of adaptations
• Genetic
– change in allele frequency
• Physiological
– modifications in haemoglobin levels to cope with
altitude
• Behaviors
– avoidance behaviors
Illustrating Natural Selection:
California Condor
Natural Selection in Action
• Recessive lethal: an allele that does not effect
the fitness of a heterozygote but all
homozygotes die
• California condors homozygous for the dwdw
gene die shortly after hatching
Genotypes:
++ : normal homozygote dominant
+dw : normal heterozygote
dwdw : dwarf lethal homozygous recessive
How rapidly does the frequency of the recessive
lethal dwdw allele decline due to selection in
the endangered California condor?
q1 =
q
1+q
q = frequency of the lethal allele
If the dwdw allele has a frequency of 0.17 at fertilization and all homozygotes die.
What will be the expected frequency of the dwdw allele in adults as a result of
natural selection?
0.17
(1+0.17)
=
0.145
The frequency of the dwdw allele will drop from
17% to 14.5% in one generation
Conservation biology is not only
concerned with selection against
deleterious mutations, but also
selection on favorable mutations
The Pepper Moth
and the
Industrial Revolution
How quickly can allele frequency change?
• The melanic form was first recorded in 1848
• By 1900 99% of all moths living in polluted areas were
melanic
• The rate of change for a particular allele will depend on
the selection strength (s) and the allele frequencies (p
and q)
• What do you predict to happen if pollution controls
were put in place and trees became lighter in color
again?
Cook, 2003
Why did these changes occur
• Very strong predation pressure
Selection on quantitative characters
• So far we have discussed selection on a single loci
• But conservation genetics is primarily concerned
with reproductive fitness a quantitative trait
influenced by many loci
• We can determine the evolutionary potential of a
population by…
heritability
Heritability
• Heritability (h2) is a measure of how well a
quantitative trait is transmitted from one
generation to the next.
• Most easily measured by comparing the
trait among relatives
Human Height
• Quantitative trait
• 60 % - 80 % heritability – genetics
• This means that 40 % - 20 % of the variation is due
to the environment
• nutrition
:: Comparing Heritability ::
The slope is a direct
measure of the heritability
(h2 ) of a trait
Both parents and the
environment play a
role in the phenotype
of the offspring
There is no relationship
between parent and
offspring values.
The slope of the
relationship is 0.
• Complete heritability:
Fingerprint ridge count in humans
• Incomplete heritability:
Shell width in Partula snails
• Zero heritability:
Conception rate in cattle
Why is heritability important?
• Selection Response: predict how particular populations will
respond to selection for particular characters.
Heritability
h2 =
Vg
Vp
Vg = variation due to genetic
diversity
Vp = total phenotypic variation
Total Variation Among
Individuals
Vp = Vg + Ve
Vp = total variation in population
Vg = variation due to genetic
differences
Ve = variation due to the
environment
What contributes to genetic variation?
Vg = Va + Vd + Vi
Vg = Variation due to genetic diversity
Va = Variation due to the average effect of alleles.
Determines immediate evolutionary potential
Vd = Variation due to dominance
Reflects the susceptibility to inbreeding depression
Vi = Variation due to interactions among loci
Determines the impact of outcrossing
Genotype × environment interaction
•
Populations adapt to particular environmental conditions
• Survive and reproduce better in their native conditions than in other
environments.
Genotype × environment interaction
Genotype × environment interactions &
Management of endangered species
High performance in captivity does not guarantee success in the wild
Mixing of genetic material from different populations may generate underperforming
genotypes
Knowledge of genotype × environment interaction can strongly influence the choice of
populations for reintroduction
Ex. Disease resistance
What happens if the selection is to strong
and the organisms cannot adapt?!
Adaption is not a cure all!
It is limited by genetics and by time
The sixth extinction
• Mass extinction: a widespread and rapid decrease in the amount of
life on
• Changes are occurring to fast for the species to adapt
• Humans have also been responsible for…
– translocations of species
– extinction of food species
– introduction of novel chemicals to
the environment
– habitat fragmentation
What can cause a change in genetic
diversity?
•
•
•
•
•
Mutation
Migration
Selection
Chance events
Fragmentation
What can change allele frequency?
What can change allele frequency?
• Chance effects:
– natural disasters
What can cause a change in genetic
diversity?
•
•
•
•
•
Mutation
Migration
Selection
Chance events
Fragmentation
What can change allele frequency?
• Fragmentation
• Limits gene flow
• Random differences among subpopulations
Take Home Points
• Genetic diversity is GOOD!
• Diversity is key for maintaining a healthy
population
Take Home Points
• Factors that can effect evolution, aka the frequency
of alleles in a population are..
•
•
•
•
•
Mutation
Migration
Selection
Chance events
Fragmentation