Download Evolution of Populations

 Variations
are inherited from one generation
to the next leading to natural selection
 Differences that help organisms survive and
reproduce become more common and
differences that are not beneficial become
less common
 It changes the genes!!!
 Darwin
did not know about how things are
inherited yet (Mendel).
 We can use our knowledge from Mendel’s
study of heredity and combine it with
Darwin’s study of evolution to explain how
inheritable variation appears and how
natural selection effects that variation.
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Biologists studying evolution focus on a
population = collection of individuals of the
same species in a given area
A population shares a common group of
genes (genetic information) = gene pool
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Contain two or more alleles (forms of a gene) for
each inheritable trait
Relative frequency (%) = the number of
times an allele occurs in a gene pool
compared to the number of times other
alleles occur
Section 16-1
Sample Population
48%
heterozygous
black
36%
homozygous
brown
16%
homozygous
black
Frequency of Alleles
allele for
brown fur
allele for
black fur
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Mutations = any change in a sequence of DNA
Remember: mutations result as a mistake during
replication or toxin (chemicals/radiation)
– Some mutations effect phenotypes (physical
characteristics), which can effect an organism’s
fitness (ability to survive)
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Gene shuffling = different gene combinations
inherited during gamete production creating
different genotypes (genetic makeup), different
phenotypes and more variation
Crossing over increases number of different genotypes
– Does not change the relative frequency of alleles in a
population
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If it is a trait controlled by a
single-gene with 2 alleles,
there will be only 2
phenotypes.
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Frequency of
Phenotype
(%)
The number of
phenotypes produced
for a given trait depends
on how many genes
control the trait.
Phenotypes of a single-gene is
represented by a bar graph
If it is a trait is polygenic
with 2 or more alleles, there
will be many genotypes and
even more phenotypes.
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Phenotype
Phenotypes of a polygenic trait
is represented by a normal
distribution (bell curve)
Frequency of
Phenotype
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Phenotype (height)
 Natural
selection acts on phenotypes,
survival and reproduction determine which
alleles are inherited, changing relative
frequencies of alleles in a population over
time.
 Thus evolution is any change in the relative
frequencies of alleles in a population’s gene
pool and acts on populations, not individuals.
 Natural
selection on single-gene traits can
lead to changes in allele frequencies and
thus to evolution.

One of the two phenotypes may make an
organisms better fit, thus under pressure from
natural selection and its relative frequency will
increase
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Natural selection can affect the distributions
of phenotypes in any of three ways:
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Directional selection: When the entire bell moves
left/right because there’s a higher fitness and
increase in the number of individuals with the
trait at one end of the curve
Stabilizing selection: When the bell becomes
more narrow, because there’s a higher fitness
and increase in the number of individuals with
the trait in the center of the curve
Disruptive selection: The bell can split into two,
because there’s a higher fitness and increase in
the number of individuals at both ends of the
curve
Section 16-2
Key
Directional Selection
Food becomes scarce.
Low mortality,
high fitness
High mortality,
low fitness
Section 16-2
Stabilizing Selection
Key
Low mortality,
high fitness
High mortality,
low fitness
Birth Weight
Selection against
both extremes
keep curve
narrow and in
same place.
Section 16-2
Disruptive Selection
Low mortality,
high fitness
High mortality,
low fitness
Population splits
into two subgroups
specializing in
different seeds.
Beak Size
Number of Birds
in Population
Key
Number of Birds
in Population
Largest and smallest seeds become more common.
Beak Size
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Genetic drift = random change in allele
frequencies
In small populations, individuals that carry a
particular allele may leave more descendants
than other individual, just by chance. Over time,
a series of chance occurrences can cause an
allele to become common in a population.
Genetic drift can happen when a small group of
individuals colonize a new habitat carrying
different relative frequencies that the larger
population.
Founder effect = allele frequencies change as a
result of the migration of a small subgroup of a
population
Section 16-2
Sample of
Original Population
Descendants
Founding Population A
Founding Population B
Section 16-2
Sample of
Original Population
Descendants
Founding Population A
Founding Population B
Section 16-2
Sample of
Original Population
Descendants
Founding Population A
Founding Population B
 The
conditions that must be met to avoid
evolution:
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Random mating
Large population
No movement into or out of the population
No mutations
No natural selection
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Speciation = formation of new species
Species = group of organisms that breed with
one another and produce fertile offspring (share
a common gene pool)
As new species evolve, populations become
reproductively isolated from each other: When 2
populations can’t breed and produce fertile
offspring, resulting in separate gene pools
Behavioral isolation: Capable of breeding, but have
different courtship rituals or behaviors
– Geographic isolation: Separate by geographic barriers
– Temporal isolation: Reproduce at different times
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Speciation in the Galapagos finches occurred by:
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Founding a new population: A small population of finches
migrates to a different island
Geographic isolation: Finches don’t usually fly over open
water, so stayed on own island (separate gene pool)
Changes in the new population’s gene pool: Adapted to
new environment (directional selection) to be more fit
Reproductive isolation: Differences in phenotypes and
mating rituals may turn different finches off to one
another
Ecological competition: Similar finches compete, so
individuals that are most different from each other have
the highest fitness, because less competition.
Continued Evolution: Process repeats and over many
generations, it produced the 13 different finch species