Transcript 16-2 Evolution as Genetic Change Outline 16-2

Outline 16-2: Evolution as
Genetic Change
16-2 Evolution as Genetic Change
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16-2 Evolution as Genetic Change 16-2
16-2
Evolution
Evolution
as Genetic
as Genetic
Change
Change
I. Variation & Gene Pools
A. A gene pool consists of all the genes, of
all possible alleles, that are present in a
population of a species.
1. The relative frequency of an allele is
the relative proportion of an allele
compared to the total number of all
alleles.
a. It is often expressed as a
percentage.
B. In genetic terms, evolution is any
change in the relative frequency of
alleles in a population.
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16-2 Evolution as Genetic Change
16-2 Evolution as Genetic
Change
II. Natural Selection & Gene Pools
A. Evolution never acts directly on genes.
Why?
1. It is entire organisms that either live
or die
a. If an individual dies without
reproducing, it does not contribute
its alleles to the population’s gene
pool.
b. If an individual produces many
offspring, its alleles stay in the gene
pool and may increase in frequency.
2. Thus, only populations can evolve, not
individuals
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16-2 Evolution as Genetic Change
Natural Selection on
Single-Gene Traits
III. Examples of Natural Selection at Work
A. A lizard population:
1. They are normally brown but have mutations
that produce red and black forms.
2. Suppose red lizards are more visible to
predators.
a. Less likely to survive & reproduce so allele
for red color will decrease in population
3. Black lizards may warm up faster on cold
days. This may give them energy to avoid
predators.
a. More likely to survive & reproduce so allele
for black color will increase in population Slide
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16-2 Evolution as Genetic Change
Natural Selection on
Single-Gene Traits
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16-2 Evolution as Genetic Change
Natural Selection on
Single-Gene Traits
B. Results of Natural Selection over Time
1. Natural selection can affect the distribution of
phenotypes in one of three ways:
a. Directional Selection
When individuals at one end of the curve
have higher fitness than individuals in the
middle or at the other end.
Birds with
larger beaks
survive better
& beak size
increases
over time
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16-2 Evolution as Genetic Change
Natural Selection on
Polygenic Traits
b. Stabilizing Selection
When individuals near the center of the curve
have higher fitness than individuals at either end
of the curve.
Human babies
born of an
average mass
are more likely
to survive than
either smaller or
larger babies
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16-2 Evolution as Genetic Change
Natural Selection on
Polygenic Traits
c. Disruptive Selection
When individuals at both ends of the curve
have higher fitness than individuals near the
middle.
If the pressure of natural selection is strong
enough and long enough, the curve will split,
creating two distinct phenotypes.
If average seeds
become scarce, the
population might split
into two groups: one of
small beaks & one of
large beaks
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