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Chapter 16
Evolution of Populations
Variation and Gene Pools
• Genetic variations (differences) are studied
in populations (group of individuals of the
same species that interbreed).
• A gene pool
is all of the
genes that
are present
in a
population.
• The relative frequency
of an allele is the # of
times that the allele
occurs in a gene pool
compared with the # of
times other alleles
occur (usually a %).
• In genetic terms,
evolution is any change
in the relative
frequency of alleles in a
population.
Sources of Genetic Variation
• There are two main
sources of genetic
variation:
o Mutations (any
change in a
sequence of DNA)
o The gene shuffling
that results from
sexual reproduction.
Single-Gene and Polygenic Traits
• The number of phenotypes produced for a
given trait depends on how many genes
control the trait.
Single-gene trait:
Polygenic trait:
Trait controlled by a single
gene with two alleles (ex:
widows peak)
Trait controlled by two or
more genes, each gene has
two or more alleles (ex:
height)
Single-Gene Traits
• Natural Selection on single-gene traits can
lead to changes in allele frequencies and thus
to evolution.
Polygenic Traits
• Natural selection can
affect the
distributions of
phenotypes in any of
three ways:
o stabilizing selection
o directional selection
o disruptive selection
Stabilizing Selection
• When individuals near the center of the curve
have higher fitness than individuals at either
end of the curve
Directional Selection
• When individuals at one end of the curve
have higher fitness than individuals in
the middle or at the other end
Disruptive Selection
• When individuals at the upper and lower
ends of the curve have higher fitness
than individuals near the middle
Genetic Drift
• A random change in allele frequency.
• In small populations, individuals that carry a
particular allele may leave more descendants
than other individuals do, just by chance.
• When allele frequencies change due to
migration of a small group of a population it is
known as the founder effect.
16-3 The Process of Speciation
• Remember: a species is a group of organisms
that breed with one another and produce
fertile offspring.
• Natural selection
and chance
events can
change the
relative
frequencies of
alleles in a
population, but
how do these
changes lead to
speciation (the
formation of
new species)?
• As new species evolve, populations become
reproductively isolated from each other.
• When the members of two populations
cannot interbreed and produce fertile
offspring, reproductive isolation has occurred.
Reproductive Isolation
Behavioral
Isolation
Geographic
Isolation
Temporal
Isolation
• Different
• Separated by • Reproduce at
courtship
geographic
different
rituals or
barriers such
times
other
as rivers or
reproductive
mountains
strategies
that involve
behavior
Testing Natural Selection in Nature
• Darwin
hypothesized
that the
finches he saw
on the
Galapagos
Islands had
descended
from a
common
ancestor.
• Darwin’s hypothesis relied on two testable
assumptions:
o For beak size and shape to evolve,
there must be enough heritable
variation in those traits to provide raw
material for natural selection.
o Differences in beak size and shape
must produce differences in fitness,
causing natural selection to occur.
• Peter and Rosemary
Grant tested these
hypotheses on the
medium ground finch
on Daphne Major, one
of the Galápagos
Islands.
• During droughts, food becomes scarce.
• Individual birds with different-sized beaks had
different chances of survival during a drought.
• When food was scarce, individuals with large
beaks were more likely to survive.
• More large
beaked birds
survived and
reproduced.
• The Grants
provided
evidence of
the process of
evolution.
• Beak size can
be changed by
natural
selection.
Speciation in Darwin’s Finches
• Speciation in the Galápagos
finches has occurred by:
1. founding of a new
population
2. geographic isolation
3. changes in new
population's gene pool
4. reproductive isolation
5. ecological competition
1. Founders Arrive
• A few finches from
the South American
mainland (species
A) travel to one of
the Galápagos
Islands.
• There, they survive
and reproduce.
2. Geographic Isolation
• Some birds from
species A cross
to a second
island.
• The two
populations no
longer share a
gene pool.
3. Changes in the Gene Pool
• Seed sizes on the
second island favor
birds with large
beaks.
• The population on
the second island
evolves into
population B, with
larger beaks.
4. Reproductive Isolation
• Finches mate with
birds that have the
same-sized beak they
do.
• If population B birds
cross back to the first
island, they will not
mate with birds from
population A.
• Populations A and B
are separate species.
No mating!
5. Ecological Competition
• As these two new
species live together in
the same environment
(on the first island),
they compete with
each other for seeds.
• More specialized birds
have less competition.
• Over time, the species
evolve in a way that
increases the
differences between
them.
Chapter 16 Vocab
•
•
•
•
• Speciation
• Reproductive
Isolation
• Behavioral Isolation
o stabilizing selection • Geographic Isolation
o directional selection • Temporal Isolation
Gene Pool
Relative frequency
Single-gene trait
Polygenic trait
o disruptive selection
• Genetic drift
• Founder effect