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The Evolution of Populations
Introduction
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One obstacle to understanding evolution is the
common misconception that organisms evolve, in a
Darwinian sense, in their lifetimes.
Natural selection does act on individuals by
impacting their chances of survival and their
reproductive success. However, the evolutionary
impact of natural selection is only apparent in
tracking how a population of organisms changes
over time.
It is the population, not its individuals, that evolves.
Shell diversity…
Sexual Reproduction Review

Mutation and Sexual recombination generate
genetic variation
◦ Mutation
 Only source of new genes (alleles)
 Point mutations
◦ Changes in one nucleotide base
◦ Can have significant impact (ie. hemoglobin)
 Chromosome mutation
◦ Deletion, disruption, duplication, rearrangement
◦ Almost always harmful
Sexual Reproduction Review
 Most genetic variation are due to Sexual
recombination
◦ Rearranges alleles into new combinations
◦ Three mechanisms that “shuffle” alleles
 Crossing-over – Prophase I meiosis
 Independent assortment during meiosis
 Fertilization
Population Genetics
The study of how populations change genetically over time.
Population Genetics
The biological sciences now generally define
evolution as being the sum total of the genetically
inherited changes in the individuals who are the
members of a population's gene pool.
 Population

◦ A group of individuals of the same species that live in the
same area and interbreed.
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Gene Pool
◦ All of the alleles at all loci in all the members of a
population
◦ If all members of a population are homozygous for the
same allele, the allele is said to be fixed.
Hardy-Weinberg
This definition of evolution was
developed largely as a result of
independent work in the early 20th
century by Godfrey Hardy, an
English mathematician, and Wilhelm
Weinberg, a German physician.
 Through mathematical modeling
based on probability, they concluded
in 1908 that gene pool frequencies
are inherently stable but that
evolution should be expected in all
populations virtually all of the time.
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Hardy-Weinberg Equilibrium
The H-W equation can be used to test whether
a population is evolving
 The Theorem is used to describe a population
that is not evolving.
 Under certain (unobtainable) conditions, allele
frequencies will remain constant over
generations unless they are acted upon by
forces other than Mendelian segregation and
recombination
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Hardy-Weinberg Theorem
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The fact is:
◦ allelic frequencies change.
◦ Populations evolve.
But not because of Mendelian genetics.
 The relative frequencies of alleles or genotypes
remain the same between one generation and
the next.
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5 conditions for Hardy-Weinberg
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By applying the equation we can test this, but first,
these conditions must be met
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No mutations
Random mating
No natural selection
The population size must be extremely large. (no genetic
drift)
◦ No gene flow (emigration, immigration)
Hardy-Weinberg Theorem
Obviously, the Hardy-Weinberg equilibrium cannot exist in
real life. Some or all of these types of forces all act on living
populations at various times and evolution at some level
occurs in all living organisms.
 There are two formulas that must be memorized:
 p2 + 2pq + q2 = 1
 p+q=1
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Hardy-Weinberg Theorem
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for a trait controlled by a pair of alleles (A and a)
◦ p is defined as the frequency of the dominant allele
◦ q as the frequency of the recessive allele.
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In other words, p equals all of the alleles in individuals
who are homozygous dominant (AA) and half of the alleles
in people who are heterozygous (Aa) for this trait in a
population.
In mathematical terms, p = AA + ½Aa
Hardy-Weinberg Theorem
Likewise, q equals all of the alleles in individuals who are
homozygous recessive (aa) and the other half of the alleles
in people who are heterozygous (Aa).
 q = aa + ½Aa
 Because there are only two alleles in this case, the
frequency of one plus the frequency of the other must
equal 100%, which is to say
 p+q=1
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Hardy-Weinberg Theorem
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All possible combinations of alleles occurring randomly
(p + q)² = 1
or more simply
p² + 2pq + q² = 1
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p2 = percentage of homozygous dominant individuals
q2 = percentage of homozygous recessive individuals
2pq = percentage of heterozygous individuals
Hardy-Weinberg Theorem
From observations of phenotypes, it is usually only
possible to know the frequency of homozygous recessive
people, or q² in the equation, since they will not have the
dominant trait.
 Those who express the trait in their phenotype could be
either homozygous dominant (p²) or heterozygous (2pq).
 The Hardy-Weinberg equation allows us to predict which
ones they are. Since p = 1 - q and q is known, it is possible
to calculate p as well.
 This then provides the predicted frequencies of all three
genotypes for the selected trait within the population.
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Try these problems
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http://www.kstate.edu/parasitology/biology198/hardwein.html
Microevolution
A generation-to-generation change in a
population’s frequencies of alleles.
Causes of Microevolution
in a population
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Four factors can alter the allele frequencies in a population:
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mutation (rare)
genetic drift
gene flow
natural selection
All represent departures from the conditions required for
the Hardy-Weinberg equilibrium.
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Natural selection is the only factor that generally adapts a population
to its environment.
Selection always favors the disproportionate propagation of
favorable traits.
The other three may effect populations in positive, negative, or
neutral ways.
Mutation
Change in an organism’s DNA.
 A new mutation that is transmitted in gametes
can immediately change the gene pool of a
population by substituting the mutated allele for
the older allele.
 Over the long term, mutation is a very important
to evolution because it is the original source of
genetic variation that serves as the raw material
for natural selection.
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Causes of Microevolution
in a population
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Genetic Drift: is the change in allele frequency
of a small population, due to chance.
 Unpredictable, random, nonadaptive
 The smaller the population, greater the
chance
Genetic Drift
Genetic Drift: Two Examples
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Bottleneck = an
environmental crisis
may reduce the size of
the original population
and the small surviving
population may not be
representative of the
original population’s
gene pool.
Genetic Drift: Two Examples
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Bottleneck - The few
survivors that “pass
through the
bottleneck” may have
a gene pool that no
longer reflects the
original population
gene pool.
The California
condor was reduced
to nine individuals.
Genetic Drift: Two Examples
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The cheetah
has reached
the brink of
extinction
twice
◦ Ice age 10, 000
years
◦ 19th century
hunting
Genetic Drift: Two Examples
• Founder Effect
• This occurs when a few members of a
population colonize an isolated island, lake , or
some other new habitat.
 The allele frequency may not represent the gene
pool of the larger population they left.
 Thus diseases of recessive genes, which require
two copies of the gene to cause the disease, will
show up more frequently than they would if the
population married outside the group.
Genetic Drift: Two Examples
•
Ellis-Van Creveld Syndrome
AMISH
relatively high frequency of
certain inherited disorders
among human populations
established by a small number
of colonists.
Gene Flow
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A population gains or looses alleles by genetic
additions to and/or subtractions from the
population.

Members of a
population are far
more likely to
breed with
members of the
same population
than with
members of other
populations.
Gene Flow
 Gene Flow tends to reduce the genetic
differences between populations
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Individuals near the
population’s center
are, on average,
more closely related
to one another than
to members of
other populations
Natural Selection
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Relative fitness refers to the
contribution an organism
makes to the gene pool of
the next generation relative
to the contributions of other
members.
It is measured by
reproductive success
Acts more directly on the
phenotype, changing the
allele frequency of the
population in three ways
Natural Selection
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Directional Selection shifts the overall makeup of the
population by favoring variants of one extreme over
another
1) artificial selection
(insecticide resistance)
2) Large black bears
survive periods of
extreme cold better
than small ones so
become more common
during glacial periods
Directional Selection
Directional selection
for beak size in a
Galápagos
population of the
medium ground finch
Directional Selection
Natural Selection
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Disruptive Selection occurs when
conditions favor individuals on both
extremes of a phenotypic range rather
than individuals with intermediate
phenotypes
African fire-bellied seed cracker finch
Natural Selection
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Stabilizing Selection acts against both extreme
phenotypes and favors intermediate variants
Siberian Husky
◦ Medium dog, males weighing 16-27kg
(35-60lbs).
◦ Strong pectoral and leg muscles,
allowing it to move through dense
snow.
◦ If heavier muscles, it would sink
deeper into the snow. They would
move slower or would sink and get
stuck.
◦ If lighter muscles, it would not be
strong enough to pull sleds and
equipment.
◦ So, stabilizing selection has chosen a
norm for the size of the Siberian
Husky.
Modes of
Selection
Original
population
Original population
Evolved
population
In this case, darker mice are favored
because they live among dark
rocks and a darker fur color conceals
Them from predators.
Phenotypes (fur color)
These mice have colonized a
patchy habitat made up of light
and dark rocks, with the result
that mice of an intermediate
color are at a disadvantage.
If the environment consists of
rocks of an intermediate color,
both light and dark mice will
be selected against.
Sexual Selection
leads to sexual dimorphisms
Two varieties of sexual selection:
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Intrasexual selection
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competition between the same sex- usually male
vs. male
Example: deer or rams butting heads, antlers,
horns, large stature or musculature
Intersexual selection
female mate choice based on appearance or
behavior of males
Example: peacock plumage, elaborate mating
behaviors
Sexual selection and the evolution of male appearance
Diploidy
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preserves variation in eukaryotes
◦ Two copies of every gene
◦ Prevents the elimination of recessive alleles via
selection because they do not impact the phenotype
in heterozygotes.
◦ Even recessive alleles that are unfavorable can
persist in a population through their propagation by
heterozygous individuals.
Both quantitative and discrete characters
contribute to variation within a population.
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Quantitative characters are those that vary along
a continuum within a population.
For example, plant height in our wildflower population
includes short and tall plants and everything in
between.
 Quantitative variation is usually due to polygenic
inheritance in which the additive effects of two or more
genes influence a single phenotypic character.
 Discrete characters, such as flower color, are usually
determined by a single locus with different alleles with
distinct impacts on the phenotype.
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Polymorphisms
Two or more distinct forms of a discrete
character in a population. (dominant and
recessive alleles)
 Allows for natural selection to act on phenotypes
changing the allele frequencies of the gene pool.
 It gives the heterozygous individual the ability to
have a better reproductive success. Called the
Heterozygote advantage.
 This maintains both alleles in the population.
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Heterozygote Advantage
 A situation in which a single
disadvantageous allele is not selected out of
a population, because, when a person is
heterozygous for that allele (the person has
one disadvantageous allele and one normal
allele), the person gains some sort of local
advantage by having the disadvantageous
allele.
Heterozygote Advantage
Plasmodium falciparum
AA = No sickle (Dead - malaria)
Aa = sickle trait
aa = sickle disease (Dead)
Most Species Exhibit Geographic
Variations
Variations in gene pools between populations
 Cline = a gradual change in a trait corresponding
to a graded change in some geographic axis. Some
of the variation has a genetic basis.
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yarrow plants in the Sierra
Nevada Mountains
decrease in size with
increasing elevation.
Why doesn’t natural selection produce
perfect organisms?
Selection can only edit existing variations.
 Evolution is limited by historical constraints.
 Adaptations are often compromises.
 Chance, natural selection, and the environment
interact.
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