Title: Topic 7 Evolution of Populations
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BIOL 108 Winter 2025
2025 Neil Harris
Topic 7: Evolution of populations
Natural selection acts on
individuals, but only populations
evolve via natural selection.
A population is the smallest unit of
evolutionary change.
A population is a group of individuals
of the same species that live in the
same area and interbreed, producing
fertile offspring.
Microevolution is the change in
allele frequencies in populations
over generations.
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Genetics terminology: a brief review
A gene (or more accurately, genetic locus)
consists of two alleles in diploid individuals.
Alleles are different forms of a gene, corresponding
to different DNA sequences in each form.
One allele per chromosome.
In sexually reproducing diploid species, one chromosome
from each parent.
Each allele remains separate and does not blend.
Apparent blending is the result of incomplete
dominance.
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Microevolution & allele frequencies
Microevolution is the change in
allele frequencies in populations
over generations.
A population is a localized group of
individuals of a single species that
interbreed (share alleles) and produce
fertile offspring.
Individuals represent different
combinations of alleles drawn from
the gene pool, that is, from all the
alleles present in all individuals in the
population.
Different populations of the same species
may be geographically isolated and, thus,
have distinct gene pools (e.g. Fig 23.6
Yukon caribou populations).
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Generation 1 Generation 2
The gene pool consists of all alleles for all individuals in the populationBIOL 108 Winter 2025
2025 Neil Harris
Genetic variation makes evolution possible
Variation in heritable traits is a
prerequisite for evolution.
Genetic diversity (variation) is one of
the three main components of
biodiversity.
Genetic variation is advantageous to a
population because it enables the
population to adapt to the
environment via natural selection.
Genetic variation among individuals is
caused by differences in genes or
other DNA segments.
Individuals have a specific genotype
(combination of alleles).
Phenotype is a product of inherited
genotype and environmental
influences.
Natural selection can only act on variation
with a genetic component.
Not all phenotypic variation is heritable.
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Fig 23.5 Nonheritable variation: Moth caterpillars
have different appearances due to chemicals in
their diet (oak flowers (a) vs. oak twigs (b))BIOL 108 Winter 2025
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Sources of genetic variation
New genes and alleles arise by mutation or gene duplication.
1. Mutations are changes in an individuals DNA sequence.
Causes: i) small-scale (e.g. point mutation) or
chromosomal (e.g. insertion/deletion) errors in DNA
replication; ii) structural damage to DNA (e.g. radiation).
Only mutations in cells that produce gametes (eggs or
sperm) can be passed on to the next generation.
Mutations occur randomly.
Selection pressures do not cause beneficial mutations.
Mutations create new alleles.
Without mutations, there would be no new genetic material for
natural selection or other evolutionary processes to act upon.
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Cornish Rex cat breed arose
from one mutant kitten in 1950
(small, single-gene mutation)
WCBIOL 108 Winter 2025
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Sources of genetic variation
1. Mutations are changes in an individuals DNA sequence.
Chromosomal mutations that delete,
disrupt, or rearrange many loci are
typically harmful.
Duplication of small DNA segments
increases genome size and is usually
less harmful.
Duplication of DNA segments can result in
duplication of genes that can take on new
functions by further mutation.
Whole-genome duplication, a process
of genome doubling, is an important
driver of evolution by supplying
genetic material and increasing
genome complexity.
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A single mutation can have no effect
(no change in phenotype) or can have
a large effect (lethal).
Most mutations have negligible effect.
Mutations can be deleterious (bad),
neutral, or advantageous (good) in
the current environment.
e.g. a neutral mutation may later be
advantageous or disadvantageous if the
environment changes, i.e. selective
pressures change.
Most mutations within a given
environment are neutral or slightly
deleterious.BIOL 108 Winter 2025
2025 Neil Harris
Sources of genetic variation
While mutations are the source of
new alleles, mutations occur relatively
infrequently in populations.
Mutation rates are low in animals and
plants.
Human genomes accumulate ~60-
100 new mutations per generation.
Mutation rates are much lower in
prokaryotes.
However, mutations accumulate
quickly in prokaryotes because they
have very short generation times.
Although the immediate contribution
of mutations to genetic variation is
small, their long-term effects are
significant.
Most evolutionary change is based on the
long-term accumulation of many
mutations with small individual effects.
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1. Mutations are changes in an individuals DNA sequence.BIOL 108 Winter 2025
2025 Neil Harris
Sources of genetic variation
2. Sexual reproduction amplifies genetic variation by creating new
combinations of existing alleles.
Random mating between organisms; random fertilization.
Recombination of homologous chromosomes during meiosis shuffles existing
genetic material to create new combinations of alleles.
Recombination is often more impactful than mutation in generating
genetic variation within sexually reproducing populations over
short timescales.
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Chromosomes recombination is a source of variation
https://evolution.berkeley.edu/
Recombination shuffles
mutations to produce new
sequencesBIOL 108 Winter 2025
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Factors that alter allele frequencies in
populations
Natural selection, genetic drift, and gene flow are the mechanisms that
cause changes in allele frequencies in populations.
Non-adaptive evolution is any change
in allele frequency that does not lead
a population to become more adapted
to its environment.
Genetic drift and gene flow cause
non-adaptive evolution since their
effect on allele frequencies is
primarily random.
Mutations and recombination are also
non-adaptive since they dont impact
allele frequencies in populations.
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Natural selection causes adaptive
evolution, where a population
becomes more adapted to its
environment under natural selection.BIOL 108 Winter 2025
2025 Neil Harris
Factors that alter allele frequencies in
populations
1. Natural selection
Only natural selection causes
adaptive evolution (adaptation).
Adaptation: feature or trait (selected
through natural selection) that provides
an advantage (higher relative fitness) to
an individual possessing it.
Relative fitness is the contribution an
individual makes to the gene pool of the
next generation, relative to the
contributions of other individuals.
Natural selection acts on existing
variation.
Differential reproductive success of
individuals in a population results in the
alleles of those individuals being passed
to the next generation in greater
proportions.
Three modes of natural selection
(Fig 23.13):
Directional selection
Disruptive selection
Stabilizing selection
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Natural selection
Directional selection
Directional selection favours individuals that
differ from the current mean phenotype of a
population in one direction.
Under directional selection, a populations genetic
variance shifts toward a new phenotype with higher
relative fitness when exposed to selection
pressures.
Directional selection occurs in response to
consistent selective pressure, i.e. a steady change
in the environment.
The frequency distribution of phenotype shifts
under directional selection.
Directional shift in the mean of the population.
Directional selection does not change the range of genetic
variation in a population over short timescales. 11
Directional selection for greater beak depth in the
medium ground finch during drought on Daphne Major,
a small Galpagos island
1976: Before selection
N = 751 (all birds on island)
1977: After selection
N = 90 (survivors)
Beak depth (mm)
Number of finchesBIOL 108 Winter 2025
2025 Neil Harris
Natural selection
Disruptive selection
Disruptive selection favours
individuals at both extremes of the
phenotypic range.
Intermediate (average) phenotypes
have lower fitness than either extreme
phenotype.
Often results in two or more divergent
phenotypes.
Disruptive selection maintains genetic
variation in populations.
Example: disruptive selection for
beak size in the black-bellied
seedcracker (Pyrenestes ostrinus)
of West Africa.
The black-bellied seedcracker
population has divergent diets and
feeding efficiencies on their principal
food, sedge seeds (Scleria spp.).
Small-beaked birds specialize on soft-
seeded species, while large-beaked birds
specialize on hard-seeded species.
Juvenile birds that survive have either
relatively small or relatively large beaks.
Birds with intermediate beak size cannot
use either seed type efficiently and
survive poorly.
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Small-beaked female
morph feeding on
soft-seeded sedge
Large-beaked male
morph feeding on
hard-seeded sedge
Photos: T. Smith
https://go.nature.com/2KeaOHs
Redrawn from: Smith (1993). Disruptive selection and the genetic basis of bill
size polymorphism in the African finch Pyrenestes. Nature 363: 618-620;
https://doi.org/10.1038/363618a0BIOL 108 Winter 2025
2025 Neil Harris
Natural selection
Stabilizing selection
Stabilizing selection favours intermediate or
common phenotypes by selecting against extreme
phenotypes that deviate from the current
population mean.
Individuals with intermediate phenotypes have higher
fitness.
Stabilizing selection reduces genetic variation in a
population and maintains the populations mean
phenotype.
Little or no evolutionary change in the population.
Stabilizing selection is very common: it removes
deleterious mutations (removes individuals with lower
fitness).
Stabilizing selection conserves functional genetic features by
selection against deleterious variants, which are typically
uncommon.
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Stabilizing selection for birth weight in
humans: mortality is higher for very small
babies and for very large babies
Selection against
extreme phenotypesBIOL 108 Winter 2025
2025 Neil Harris
Factors that alter allele frequencies in
populations
2. Genetic drift
Genetic drift is random changes in
allele frequencies in a population.
Allele frequencies drift (fluctuate
unpredictably) from one generation
to the next.
Genetic drift is more likely in small
populations.
The smaller the sample, the greater the
chance of deviation from a predicted
result.
Rare alleles are more likely to be lost
due to genetic drift.
Reduces genetic variation in a population
through loss of alleles.
Genetic drift causes evolutionary
change but does not create
adaptations (non-adaptive).
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Genetic drift has large effects
on small populations
Genetic drift of a neutral allele
(frequency = 0.5) in 3 populations
over 100 generations
0
0.2
0.4
0.6
0.8
1
0 20 40 60 80 100
Allele frequency
Generations
Population size = 4
0
0.2
0.4
0.6
0.8
1
0 20 40 60 80 100
Allele frequency
Generations
Population size = 40
0
0.2
0.4
0.6
0.8
1
0 20 40 60 80 100
Allele frequency
Generations
Population size = 400
Allele fixed (present in all
individuals in population)
Allele extinct
(lost from population)
Very small populations: alleles
lost or fixed in a few generations
Midsized populations:
drift is less dramatic
Large populations: no
alleles lost or fixed
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0
0.2
0.4
0.6
0.8
1
0 20 40 60 80 100
Allele frequency
Generations
Initial allele frequency = 0.5
Rare alleles are more likely to
be lost due to genetic drift
Genetic drift of a neutral allele in 3
populations (n = 40) over 100
generations
0
0.2
0.4
0.6
0.8
1
0 20 40 60 80 100
Allele frequency
Generations
Initial allele frequency = 0.1
Rare alleles lost in
a few generations
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Genetic drift examples
Bottleneck effect
The bottleneck effect is a sudden reduction in
population size due to a change in the
environment.
Population size is reduced within its natural range.
e.g. a catastrophe destroys most of the populations habitat.
Large population few individuals.
Only these few individuals contribute alleles to the next
generation.
Allele frequency in the next generation is
different than the previous generation.
If the population remains small, it may be further
affected by genetic drift.
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Fig 23.10 The bottleneck effect
Low genetic diversity in Cheetahs was caused by a bottleneck
in last ice age (~10k years ago) and by more recent poaching
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Genetic drift examples
Bottleneck effect
Greater prairie chickens once lived throughout
Canadian and US prairies.
Extirpated in Canada.
Loss of prairie habitat caused a severe reduction
in the population of greater prairie chickens
(bottleneck).
Some populations remain throughout US prairie states.
The bottleneck reduced genetic variation in the
surviving populations (loss of genetic diversity),
resulting in loss of fertility (<50% of eggs hatched).
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Fig 23.11 Genetic drift and loss of genetic variation
Greater prairie chicken WCBIOL 108 Winter 2025
2025 Neil Harris
Genetic drift examples
Founder effect
The founder effect occurs when a
few individuals become isolated
from a larger population.
Allele frequency in the original large
population is unchanged.
The small founding population has
a small fraction of the total gene
pool present in the original
population.
Allele frequencies in the small
founder population differ from those
in the larger originating population.
e.g. increased frequency of
Huntingtons Disease in human
populations founded by small groups
of European migrants.
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founder
effect
continent
1 : 1 : 1
3 : 1 : 0BIOL 108 Winter 2025
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0
0.2
0.4
0.6
0.8
1
0 20 40 60 80 100
Allele frequency
Generations
Summary: effects of genetic drift
1. Genetic drift has the largest impact on small populations.
2. Genetic drift causes allele frequencies to change at random.
3. Genetic drift can lead to a loss of genetic variation within populations.
4. Genetic drift can cause harmful alleles to become fixed in small
populations.
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0
0.2
0.4
0.6
0.8
1
0 20 40 60 80 100
Allele frequency
Generations
Fixation of rare
alleles in 4 / 100
populations
Fixation of rare
alleles in 0 / 100
populations
Population size = 40
Initial allele frequency = 0.1
Population size = 400
Initial allele frequency = 0.1BIOL 108 Winter 2025
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Factors that alter allele frequencies in
populations
3. Gene flow
Gene flow is the movement of alleles
between populations of a species.
Alleles can be transferred through the
movement of fertile individuals (e.g.
dispersal of animals or seeds) or
gametes (e.g. pollen).
Gene flow can introduce new variation
into the receiving population.
Counteracts genetic drift but may slow
adaptation of the receiving population.
Gene flow reduces variation between
populations over time.
Populations become more similar
(homogenous) to each other.
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Gene flow is an
important agent
of evolutionary
change in human
populationsBIOL 108 Winter 2025
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Gene flow
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4 alleles4 alleles
6 alleles
Gene flow increases genetic
variation within receiving population
0/4 alleles shared with A0/4 alleles shared with B
Population A Population B
4/6 alleles
shared with B
4/6 alleles
shared with A
Gene flow decreases genetic variation between populations
wing colour
Large population with
four equally common
alleles for wing colour
wing colourBIOL 108 Winter 2025
2025 Neil Harris
Gene flow and local adaptation
Gene flow can decrease the fitness
of a receiving population.
Occurs when the immigration of
alleles that decrease fitness is more
rapid than natural selection for alleles
that increase fitness
Gene flow continuously introduces
maladaptive alleles into local populations.
e.g. Fig 23.12: gene flow in Lake Erie
water snake populations.
Gene flow can increase the fitness
of a receiving population.
e.g. gene flow supplies new alleles to
populations with limited genetic
variation.
Example: the spread of alleles for
resistance to insecticides.
Insecticides are used to target
mosquitoes carrying diseases (e.g.
malaria).
Mosquito populations become
insecticide-resistant via natural
selection.
The flow of insecticide resistance alleles
into new populations causes an increase
in fitness in the receiving population.
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Evolutionary change
Factors that alter allele frequencies in populations and bring
about the most evolutionary change:
1. Natural selection (adaptive)
2. Genetic drift (non-adaptive)
3. Gene flow (non-adaptive)
Other factors that bring about evolutionary change:
Extirpation of populations reduces a species genetic diversity.
Global extinction of a species (complete loss of genetic diversity).
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Black-footed ferret:
extirpated but reintroduced
into Canada
WC
Extinct New Zealand Moa
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How populations maintain genetic variation
in spite of natural selection and genetic drift
Neutral variation is genetic variation that does not confer a selective
advantage or disadvantage.
Natural selection does not affect the frequency of neutral mutations.
Genetic variation in populations is maintained by:
Mutation.
Recombination (crossing-over of chromosomes during meiosis).
Independent assortment (of alleles) during meiosis.
Random mating between individuals (sexual reproduction).
Random fertilization (sexual reproduction).
Recessive alleles are hidden from selection in heterozygote individuals.
Disruptive selection (natural selection).
Gene flow (between populations).
Balancing selection. 25BIOL 108 Winter 2025
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Balancing selection
Balancing selection is a form of
natural selection that maintains
genetic diversity by favouring
stable frequencies of multiple
alleles in the gene pool of a
population.
Mechanisms of balancing selection
include:
Temporal or spatial variation
Environmental conditions may change
over time or vary across different
geographic locations over the range of a
population, e.g. dry areas vs wet areas.
Different alleles may be favoured at
different times or in different locations,
promoting maintenance of alleles at
intermediate frequencies.
Heterozygote advantage
Frequency-dependent selection
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Balancing selection: heterozygote advantage
Heterozygote advantage occurs when
an organism with two different alleles
of a particular gene (heterozygote)
has a fitness advantage over an
organism with two identical copies of
either allele (homozygote).
Natural selection will tend to maintain two
or more alleles at that locus.
The combination of different alleles
provides an optimal balance, enhancing
an individuals ability to adapt to diverse
environmental conditions.
Heterozygote advantage can result from
stabilizing or directional selection.
Example: Sickle cell disease
A allele: normal hemoglobin and
functional, round red blood cells.
S allele: mutation induces hemoglobin
aggregation, causing sickle cells, which
causes chronic anemia.
Sickle cell (S) allele causes mutations in
hemoglobin but also confers malaria
resistance.
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Heterozygote advantage
Sickle cell disease
Heterozygous individuals (AS) have
an advantage over homozygous
individuals (AA and SS) in malarial
regions.
Relative fitness:
Non-malarial regions: AA > AS > SS
Malarial regions: AS > AA or SS
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Result in presence of malariaPhenotypeGenotype
Susceptible to malariaNormalAA
Mild sickle cell disease, but
protected from malaria
Sickle cell traitAS
Severe sickle cell diseaseSickle cell diseaseSS
The body destroys
sickleshaped red
blood cells, along
with parasites
contained within
Fig 23.18 Mapping malaria and the sickle-cell allele
Plasmodium falciparum-infected
red blood cell
WCBIOL 108 Winter 2025
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Balancing selection: frequency-dependent
selection
Under frequency-dependent selection, the fitness of an
allele depends on its frequency in the population.
If the fitness of an allele increases as it becomes rarer or more
common, this can lead to a dynamic equilibrium where no
single allele dominates the population.
Frequency-dependent selection usually results from
interactions between species (predation, parasitism, or
competition), or between individuals within species.
Negative frequency-dependent selection: the fitness of
an allele declines if it becomes too abundant in the
population.
When an allele becomes common due to selection, the fitness
of that allele is reduced and selection favours rarer alleles.
Alleles oscillate around equilibrium frequency to maintain genetic
diversity in the population.
Example: selection for approx. equal numbers of right-
mouthed and left-mouthed phenotypes of scale-eating fish.
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Fig 23.17 Frequency-dependent selection in scale-
eating fish (Perissodus microlepis) in Lake TanganyikaBIOL 108 Winter 2025
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Why natural selection does not produce perfect
organisms
1. Adaptations and the genes responsible for them only need to be good
enough to enable reproduction.
Natural selection increases fitness, but it produces systems that function no better than they
must. It yields adequacy of adaptation rather than perfection. Bartholomew, 1986
2. Natural selection has no goals.
Natural selection is not striving to produce progress or specific outcomes.
3. Natural selection only acts on existing variation.
Natural selection does not create new optimal variations.
Genetic variation (mutations) arises randomly.
4. Historical constraints limit natural selection.
Prior adaptation may limit future adaptability when selective pressures change.
5. Adaptations are compromises.
Populations are subject to multiple simultaneous, potentially opposing, selective pressures.
6. Chance events (genetic drift, gene flow) and environmental variability limit
natural selection.
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