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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
– 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
•
Mutations = any change in a sequence
of DNA
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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)
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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Think of a deck of cards – there are many possible combinations, but
frequency remains same
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.
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Natural selection can affect the distributions of
phenotypes in any of three ways:
– 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
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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
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Behavioral isolation (sympatric): Capable of breeding,
but have different courtship rituals or behaviors
Geographic isolation (allopatric): Separate by
geographic barriers
Temporal isolation (sympatric): Reproduce at different
times