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Evolution of Populations
Variation and Gene Pools
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Genetic variation is studied in populations.
A population is a group of individuals of the same
species that interbreed.
A gene pool consists of all genes, including all the
different alleles, that are present in a population.
The relative frequency of an allele is the number
of times the allele occurs in a gene pool, compared
with the number of times other alleles for the same
gene occur.
Relative frequency is often expressed as a
percentage.
Gene Pools:
Allele Frequency:
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Gene Pool for Fur Color in Mice:
Microevolution:
Evolution as Genetic Change
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Natural selection affects which
individuals survive and reproduce
and which do not.
If an individual dies without
reproducing, it does not contribute its
alleles to the population’s gene pool.
If an individual produces many
offspring, its alleles stay in the gene
pool and may increase in frequency.
Genetic Drift:
– A random change in allele frequency is called
genetic drift
– Random change means that an allele might
become more common in a population by
chance, not because it provides an advantage.
– In small populations, individuals that carry a
particular allele may leave more descendants
than other individuals do, just by chance.
– Gene flow
The Founder & Bottleneck Effects
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If genetic drift occurs when a
small group of individuals
colonizes a new habitat it is
often called a founder effect; if
it occurs after a natural disaster
wipes out a large proportion of
the original population it is often
called a bottleneck effect.
Individuals may carry alleles in
different relative frequencies
than did the larger population
from which they came.
The new population will be
genetically different from the
parent population.
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Genetic Drift
Sources of Genetic Variation
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In genetic terms, evolution is any
change in the relative frequency of
alleles in a population.
Genetic Variation (2 or more alleles in
the gene pool) in a population is
essential for natural selection to work:
– mutations
– genetic shuffling that results from sexual
reproduction.
Mutations:
• Any change in a sequence
of DNA
• Occur because of
mistakes in DNA
replication or as a
result of radiation or
chemicals in the
environment
• Do not always affect an
organisms phenotype
Gene Shuffling:
 Most
heritable differences are due to gene
shuffling.
 Crossing-over increases the number of
genotypes that can appear in offspring.
 Sexual reproduction produces different
phenotypes, but it does not change the
relative frequency of alleles in a population.
Gene Shuffling:
Genetic Equilibrium
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A population is in genetic equilibrium if allele
frequencies are not changing from one
generation to the next
According to the Hardy-Weinberg theory, a
population is in genetic equilibrium if the
following conditions are met simultaneously:
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Large population size
Random mating
No mutations
No migration
No natural selection
The Hardy-Weinberg Equations
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Although genetic equilibrium cannot be
maintained, it can be assumed to be occurring
at a particular moment.
Based on this premise, the H-W equations can
be used to estimate allele frequency and/or
percentage of the population that is either
homozygous recessive, homozygous dominant
or heterozygous.
The equations:
– p + q = 1 (p = dominant allele frq; q = recessive allele frq.)
– p2 + 2pq + q2 = 1 (hom. dom.; hetero; hom. rec.)
Single-Gene and Polygenic Traits
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Many traits are controlled by two or more genes and
are called polygenic traits.
One polygenic trait can have many possible
genotypes and phenotypes.
Height in humans is a polygenic trait.
A bell-shaped curve is typical of polygenic traits.
A bell-shaped curve is also called normal
distribution.
Natural Selection on Polygenic Traits
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3 categories:
 Directional: favors one extreme
 Stabilizing: favors the middle
 Disruptive: favors both extremes
Types of Natural Selection
What type of selection?
Divergent v. Convergent Evolution
Convergent
Divergent
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One species gives rise
to many species
Also known as adaptive
radiation
Many species with
common ancestor
Many homologous
structures
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Similar looking species
that do not have a
common ancestor
Similar behavior and
appearance due to
environmental
similarities
Many analogous
structures
Convergent Evolution
Coevolution
The evolution of one species is directly influenced
by the evolution of another
Punctuated Equilibrium
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Slow background evolution (stasis) is interrupted
by rapid bursts of change
Rapid bursts of change usually occur after a mass
extinction
Speciation:
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Speciation is the formation of new species.
A species is a group of organisms that breed with
one another and produce fertile offspring.
The gene pools of two populations must become
separated for them to become new species
When the members of two populations cannot
interbreed and produce fertile offspring,
reproductive isolation has occurred and
speciation will result.
Types of Reproductive Isolation
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Behavioral Isolation – Different mating
rituals prevent reproduction
Geographic Isolation – barriers such as
rivers or mountains prevent
reproduction
Temporal Isolation – different mating
times (seasonal, nocturnal v. diurnal)
prevent reproduction