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Microevolution
The evolution of local populations or demes.
Individuals are selected
But only populations evolve
Three Levels of
Evolutionary Change
• Microevolution: evolution of local populations
• Speciation: Origin of new species (life forms)
• Macroevolution: long term consequences of
microevolution and speciation …patterns of evolution
The Modern Synthesis
• By 1900, biologists expected Darwin to be
vindicated …evolution by natural selection
would be verified
• But no one understood how reproduction
occurred
• 1900 - Mendelism rediscovered
At first, Mendelism was considered
to refute Darwinism!
• Darwinism
– requires pools of genetic variability upon which natural selection could
act, screening variants
• Early Mendelian geneticists
– relied on genes that they could detect because of big mutations that
created large phenotypic changes
– these mutations were invariably difficult to maintain
– concluded that natural selection is a cleansing force, not a creative
force
– Therefore, the creative force in evolution would be mutation pressure,
the origin of rare advantageous mutations
– By the 1920s Biology textbooks were being written that ignored Darwin
…alluded to the Mutation Theory of Evolution
The Modern Synthesis:
The Synthetic Theory of Evolution
Darwinism & Mendelism Reconciled
1930s through 1950
• But hidden variation was sought and
demonstrated, 1920s to 1960s
• And the new discipline of population
genetics provided the methods to study
populations, 1930s to 1950
Genetic variability is the currency of
evolutionary change
constant
environment
changing
environment
genetically invariant population
genetically variable population
Premise: organisms are so complicated and dependent on specific
environmental conditions that most genotypes will fail in a new environment
Genetic variability is the currency of
evolutionary change
constant
environment
changing
environment
genetically invariant population
yes
maybe
no
yes
maybe
no
genetically variable population
yes
maybe
no
yes
maybe
no
Premise: organisms are so complicated and dependent on specific
environmental conditions that most genotypes will fail in a new environment
Lottery Model
Winning the lottery is analogous to a genotype being
well suited to new environmental conditions
Which is a better chance of winning a lottery?
o Buying one ticket and copying it a thousand times?
o Buying a thousand tickets?
Microevolution
Evolution is a change in the
genetic composition of a population
Sources of Genetic Variability
(VG)
Population-Environment Interactions
Mutation
VG
Sexual
Recombination
gene
pool
Natural
Selection
Genetic
Drift
Changes in VG
Gene Flow
(migration)
adaptive
Evolution
non-adaptive
Microevolution's 5
Systemic Forces
Sources of Genetic Variability
1. Mutation (source of new alleles & genes)
2. Gene Flow (migration, cohesion mechanism)
3. Sexual Recombination (new gene combinations)
Population Environment Interactions
4. Genetic Drift (random differential reproduction)
5. Natural Selection (non-random differential reprod.)
Are Mutations
Adaptive
or
Adaptively Ambiguous?
Are advantageous mutations more likely to be produced in
environments where they will be useful or are they produced at
random?
Fluctuation Test
Subdivide growing bacterial colonies
Two hypotheses:
• Advantageous mutations will be more likely
when the environment changes
• Mutations occur constantly and are not more
likely when they will be useful
Observed:
Replica Plating
Replica Plating
can transfer a plate full of colonies without altering their
relative positions
the replica stamp transfers
colonies without altering
their relative positions
source plate with colonies
"a hospitable environment"
empty target
"a hostile environment"
Two Hypotheses
replica
plating
replica
plating
adaptive
mutation hypothesis
adaptively ambiguous
mutation hypothesis
Two Hypotheses
replica
plating
replica
plating
adaptive
mutation hypothesis
adaptively ambiguous
mutation hypothesis
Two Hypotheses
replica
plating
replica
plating
observed
adaptive
mutation hypothesis
adaptively ambiguous
mutation hypothesis
Migration - Gene Flow
1 migrant per 10,000 residents will prevent populations from
diverging except under the influence of strong selection
Mating
Mating
Small populations
can cause Genetic Drift
Industrial Melanism
Kinds of Natural Selection
What is selected?
Individuals …mostly
Levels of Selection
• Genic
• Individual Selection
• Group
• Kin
• Species
Darwinian Selection
Levels of Selection
• Genic
• Individual Selection
• Group
• Kin
• Species
Levels of Selection
• Genic Selection
• Individual
• Group
problem with
individual selection:
recombination destroys
fit genotypes
immediately
• Kin
• Species
so what replicates
with fidelity?
small sections of DNA
containing few genes
Levels of
Selection
Traits that seem to conflict with
individual selection:
short-term advantages
(individual selection)
versus
• Genic
• Individual
long-term advantages
(group selection)
• Group Selection
• Kin
• Species
e.g., paradoxes:
alarm calls
altruism
Problem: birth & death of
populations is slow compared to
individual selection
Levels of Selection
• Genic
• Individual
When does genetic sacrifice pay off?
• Group
• Kin Selection
1. reciprocal altruism ...rare
• Species
2. the beneficiaries of your sacrifice
carry your genes, i.e., they are kin
Levels of Selection
• Genic
• Individual
time
• Group
• Kin
• Species Selection
low speciation &
extinction rates
high speciation
& extinction rates
Other Kinds of Selection
•
Sexual Selection ...already discussed
• Balancing selection
• Heterozygote selection
•
Frequency-dependent Selection
”the advantage of being rare”
◊ Frequency-dependent Mating ...rare mate advantage
◊ Apostatic selection ...predator/pathogen pressure
◊ Selection for Ecological Combining Ability ...competition
A
F
B
G
E
C
ran dom
² Vg
H
no n-rand om
² Vg
D
I
Evolution
J
Microevolution
Evolution is a change in the
genetic composition of a population
Sources of Genetic Variability
(VG)
Population-Environment Interactions
Mutation
VG
Sexual
Recombination
gene
pool
Natural
Selection
Genetic
Drift
Changes in VG
Gene Flow
(migration)
adaptive
Evolution
non-adaptive
Calculating
Hardy-Weinberg Equilibrium
Calculating
Hardy-Weinberg Equilibrium
• Count the number of each diploid genotype
• Calculate allele frequencies
• Calculate Hardy-Weinberg Equilibrium
• Compare observed and predicted values
• Departures indicate evolution occurring
Assumptions of HWE:
Microevolution's 5 Systemic Forces Not Active
Sources of Genetic Variability
1. Mutation (source of new alleles & genes)
2. Gene Flow (migration, cohesion mechanism)
3. Sexual Recombination (new gene combinations)
Population Environment Interactions
4. Genetic Drift (random differential reproduction)
5. Natural Selection (non-random differential reprod.)
Isozyme Surveys
Monomeric
Enzymes
Genotype
current
Dimeric
Enzymes
current
Genotype
FF
FF, FS, or SS
FS
FF, FS, or SS
SS
FF, FS, or SS
Calculate allele frequency and
HWE expectations
• Count the number of each diploid genotype
• Calculate allele frequencies
p = (2*D + H) / 2N; q = (2*R + H) / 2N; note that p + q = 1
• Calculate Hardy-Weinberg Equilibrium
DHWE = p2; HHWE = 2pq; RHWE = q2
• Compare observed and predicted values
How will inbreeding affect the expected
number of genotypes, compared to HardyWeinberg Equilibrium?
Calculate allele frequency and
HWE expectations
• Count the number of each diploid genotype
• Calculate allele frequencies
• Calculate Hardy-Weinberg Equilibrium
• Compare observed and predicted values
Population Structures
• Polymorphic Genes
– Transient polymorphisms
– Balanced polymorphisms - equilibrium conditions
• Monomorphic Genes - Fixation for one allele
– Structural constraints may not permit allelic variation
– Fixation will result from selection when one allele has a higher
fitness than others
– Fixation will result from genetic drift
Population Size, Selection and Drift
C
p
D
population
size
B
A
generations
C
1.0
Transient polymorphisms
Balanced polymorphisms
B
p
Fixation - monomorphisms
A
0
Population Size, Selection and Drift
C
p
D
a population
system subject
to genetic drift
population
size
B
Distribution of p opulation allele frequencies
in a sampled geograp hical region
A
0.50
generations
0.40
Fre quen cy of
po pulation s
exhib iting given
allele fr equ encies
0.30
0.20
0.10
0.00
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
Allele fre quen cie s pe r local popu lation
1
Genetic Drift
Q: Assuming that you are studying a population system whose local populations
all begin with an allele frequency of about 0.5, what kinds of affects would
genetic drift have over time?
a. allele frequecies will tend to fixation of one allele or the other
b. balanced polymorphisms will be commmon
c. natural selection will work weekly if at all
d. transient polymorphisms will be common at first
e. transient polymorphisms will be common later in the population’s histories
f. mutations will tend to be lost rapidly in older populations
Identify examples of:
a. a lineage evolving (∆VG)
b. transient polymorphism
c. balanced polymorphism
d. monomorphism
e. genetic drift dominating
f. natural selection dominating
Species and Speciation
Are Species Real?
• natural selection occurs in demes (local populations)
• species are composed of numerous local populations
(population systems)
• over time, all populations accumulate genetic differences from
their ancestors (divergence)
• what would cause local populations to evolve in unison?
(coherent population systems, preventing divergence of their
populations)
The Role of Gene Flow
• Isolated populations inevitably accumulate genetic
differences of time (Divergence)
• Even low migration rates can prevent divergence (producing
evolutionary coherence of a population system, preventing
divergence of populations)
• Species composed of coherent population systems are more
than just names, they are cohesive lineages ...real species
Biological Species
Population systems that can interbreed, but
which are isolated from other populations.
Crucial Criterion:
Reproductive Isolation
Anagenesis versus Cladogenesis
Anagenesis
Cladogenesis
A Speciation Mechanism
Grand Canyon Squirrels: Ammospermophilus
Allopatric Speciation
A. leucurus
A. harrisi
Geographical (Allopatric) Speciation
Deme
Gene Flow
Extensive
Gene Flow
Reproductive
Isolation
Divergence
River
River
River
TIME
River
?
What Happens if Mating
Opportunities Resume?
•
Mating produces viable offspring? Offspring fertile:
– Hybridization & gene flow erase divergence
•
Mating produces viable offspring? Offspring not fertile:
– Hybridizing individuals waste reproductive effort, suffer low fitnesses
•
Mating produces inviable offspring?
– Hybridizing individuals waste reproductive effort, suffer low fitnesses
•
Mating avoided ...no hybridization?
– Already two different species
Reproductive Isolation
Reinforcement
• Postzygotic barriers
– reduced hybrid viability
– reduced hybrid fertility
Post-Z
higher
fitnesses
because
wastage
lessened
Pre-Z
– hybrid breakdown
• Prevents fertilization if mating occurs
– mechanical isolation
– gametic isolation
• Prevents Mating
– Habitat isolation
– Temporal isolation
– Behavioral isolation
Postzygotic
Reproductive Isolation
Horse
x
infertile
Mule
Donkey
Blue Boobies
behavioral isolation