Transcript Genetic Evolution Lecture

Chapter 16
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Chapter 1 said that evolution
was change over time.
Chapter 15 said that evolution
was change of a GROUP over
time.
Chapter 16 says that evolution,
at the genetic level, is any
change in the relative frequency
of alleles in a population.
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Traits are characteristics
that are controlled by
genes.
The different types of a
gene are called alleles.
In the simplest form,
some alleles are
dominant (A) while
others are recessive (a).
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The phenotype or physical appearance of
something is determined by the living thing’s
genotype.
The genotype is the two allele code. There
are three possible genotypes:
◦ Homogyzous dominant (AA) = looks dominant
◦ Heterozygous (Aa) = looks dominant
◦ Homozygous recessive (aa) = looks recessive
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Genetic variation describes the
differences of genes within a
group. For example, everyone
in this class is a human, but
none (or few) of us are identical.
Genetic variation is caused by:
◦ Mutations: random change in
genetic material
◦ Gene Shuffling: recombination of
genes in sexual reproduction
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A gene pool describes
all the possible alleles
in a group.
◦ Example:
 Imagine that there is a
swimming pool with a
bunch of floating letters
in it. Some are B’s and
others are b’s. One day
there is a change in the
environment, and most
of the b’s leave. What is
left?
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Allele frequency aka relative frequency is the
percentage of one allele in a gene pool. For
example, 50% of the alleles might have been
B’s, but after the change, it might have
dropped to 10%.
Recall that only GROUPS can evolve, not
individuals. If this is true, then genetic
evolution can only occur if there is a change
in the allele frequency of the gene pool.
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A single-gene trait is a
characteristic that is controlled
only by one gene. Individuals
will either look dominant OR
recessive.
Example:
◦ Some people have widow’s peaks
and others do not.
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If natural selection favors a particular
phenotype, then individuals with that
phenotype will be able to have more
offspring, thereby changing the allele
frequency.
Example:
 There are black mice and light brown in a field. A
recent drought has killed the grass, turning it light
brown. Which type of mouse would survive? Which
type of mouse would be more likely to reproduce?
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Recall from the previous unit that
some traits are controlled by
multiple genes. This creates diverse,
intermediate phenotypes.
◦ Example:
 Although adult humans can be any height,
humans usually fall into a range of heights.
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When these intermediate phenotypes
are graphed, it forms a bell curve
because most individuals are usually
within a broad range of phenotypes.
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Changes in the environment may favor one or
more parts of the bell curve. There are three
major types of such selection curves:
◦ Directional Selection
◦ Stabilizing Selection
◦ Disruptive Selection
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Directional Selection occurs when individuals
at one end of the spectrum have higher
fitness.
As a result, the entire bell curve shifts.
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Stabilizing Selection occurs when individuals
in the middle phenotypes have the most
fitness.
As a result, the entire bell curve heightens in
the middle.
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Disruptive Selection occurs when individuals
at upper and lower ends of the curve are the
most fit.
As a result, the entire bell has two peaks.
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Natural selection usually pressures the
survival of a species. HOWEVER, sometimes
there is a random change in the allele
frequency called genetic drift.
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Example:
 The United States is approximately 72%
white/Caucasian and 13% black / African American. In
contrast, Thornwood High School is 90.5% black, 5.8%
Hispanic, and 1.8% white. How would the allele
frequency change if only Thornwood students were
able to reproduce? How does this demonstrate genetic
drift?
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According to the
Hardy-Weinberg
Principle, allele
frequencies will stay
constant unless there
is pressure to change.
If evolution describes
genetic change, then
this genetic
equilibrium explains
when allele
frequencies remain
constant.
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There are 5 requirements to maintain genetic
equilibrium between generations:
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1. mating is random
2. large population
3. no migration / immigration
4. no mutations
5. no natural selection (all phenotypes are equally
likely to survive)
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A species is a group of living
things that can reproduce and
have fertile offspring. The
process of making a new species
over time is speciation.
Speciation requires reproductive
isolation that allows members
from a species to evolve
separately from each other so
that they can longer reproduce
with each other.
Isolation Mechanisms
◦ Behavioral
◦ Geographic
◦ Temporal
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Even though individuals from
the same group might have
the ability to reproduce, they
choose not to because they
prefer different behaviors.
After several generations of
breeding based on these
behaviors, they can become
separate species.
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Sometimes a change in the
environment can create a
physical separation (e.g.
river, mountain).
Over time, the groups on
either side of the divide will
reproduce, thereby creating
different species.
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For some species, individuals would mate,
but the timing is off. This time separation
creates new species as new generations are
born.
◦ Example:
 If one bug reproduces in the spring and another type
of bug only reproduces in the fall, then they cannot
reproduce together.
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Darwin studied the myriad of finches in the Galapagos Islands.
Here is a summary of how they evolved.
◦ 1. The “founding” finches arrived in the islands.
◦ 2. Some of the earliest finches were isolated from each other.
◦ 3. Due to natural selection pressures, there were changes in the gene pool.
◦ 4. As time passed, the new generations of finches were not able reproduce
with each other (reproductive isolation).
◦ 5. The different species of finches eventually had to compete for
resources.
◦ 6. Evolution continues.