Evolution as Genetic Change

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Evolution as Genetic Change
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A. Natural selection acts on phenotypes
(appearances) of organisms
B. Determines which alleles are passed from one
generation to another
C. Can change the relative frequencies of alleles
in a population over time (Microevolution)
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Three effects of selection on a characteristic
1. Directional Selection 2. Stabilizing
Selection 3. Disruptive Selection
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Evolution of Traits
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A. Can lead to changes in allele frequencies and
thus to evolution
1. One allele could become more predominant and
the other possibly extinct over time
a. Example Red and black lizards living on
black rocks
B. Also affects polygenic traits (like mouse fur)
1. Traits that are controlled by more than one
gene
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Gene Pool

• Gene Pool A pool of genetic resources that is
  shared by all members of a population and is
  passed on to the next generation.

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What is the Hardy-Weinberg Principle? http//highe
red.mcgraw-hill.com/sites/0072439564/student_view0
/animations_quizzing.html
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A. Hardy-Weinberg Principle
1. States that allele frequencies in a
population will remain c constant unless one or
more factors cause those frequencies to change
2. Five conditions required to maintain genetic
equilibrium from generation to generation,
otherwise the population will change/evolve
a. Random mating
b. Large population
c. No movement in/out of population
(immigration/emigration)
i. New alleles could be brought in from the
outside
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d. No mutations
i. Mutations cause changes in alleles causing
new alleles to appear in the population
e. No natural selection
i. All genotypes have equal probability and no
phenotype can have an advantage over the other
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Solving for the Hardy-Weinberg Principle
p2 2pq q2 1 and p q 1
Sample Problem
1 in 1700 US Caucasian newborns have cystic
fibrous. C for normal is dominant over c for
cystic fibrous.
1. When counting the phenotypes in a population
why is cc the most significant?
When dominance is the case, CC and Cc are
phenotypically the same. So of the 1700 cases,
CC and Cc cannot be counted separately. There is
only one phenotype for the recessive trait, cc.
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Solving for the Hardy-Weinberg Principle
p2 2pq q2 1 and p q 1
Sample Problem
1 in 1700 US Caucasian newborns have cystic
fibrous. C for normal is dominant over c for
cystic fibrous.
2. What percent of the above population have
cystic fibrous (cc or q2)?
q2 1/1700 0.00059 x 100 0.059
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3. From the above numbers you should be able to
calculate the expectant frequencies of all the
following (assuming a Hardy-Weinberg equilibirum)
ALLELE FREQUENCY CALCULATIONS
Why calculate "q" first?
Since you know what q2 is (cc), then you can
easily find q by finding the square root of q2.
q square root of 0.00059
q 0.024
You also can determine the percent of the
population that has the cystic fibrous allele
0.024 x 100 2.4
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Why is it now easy to find "p"?
If p q 1 then p 1 q is also correct.
p 1 q
p 1 0.024
p 0.976
You can now determine what percent of the
population that has the dominant normal allele
0.976 x 100 97.6
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4. Now that you know that p .976 and q .024.
The following genotypes can be found.
GENOTYPE FREQUENCY CALCULATIONS
Normal homozygous dominate (or CC) p2
p2 0.976 x 0.976
p2 0.953
You can now determine what percent of the
population that has a normal genotype.
0.953 x 100 95.3
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Carriers of cystic fibrous 2pq
2pq 2(0.976 x 0.024)
2pq 0.0468

You can now determine what percent of the
population has the carrier (heterozygous)
genotype.
0.0468 x 100 4.68
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6. How many of the 1700 of the population are
homozygous Normal?
0.953 x 1700 1620

7. How many of the 1700 in the population are
heterozygous (carrier)?
0.0468 x 1700 79.56
8. It has been found that a carrier is better
able to survive diseases with severe diarrhea.
What would happen to the frequency of the "c" if
there was a epidemic of cholera or other type of
diarrhea producing disease?
Carriers of the trait (Cc) would survive over
CC, thereby increasing the frequency of the
recessive allele c and reducing the dominant
allele C in the process.
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Your turn!!!
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How realistic is the Hardy-Weinberg principle?
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What is Genetic Drift?
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A. A random change in allele frequencies over the
generations, brought about by chance alone.
B. Impact is minor in large populations, but is
significant in small populations.
C. In the absence of other forces, random change
in allele frequencies leads to the homozygous
condition and a loss of genetic diversity over
the generations.
1. This happens in all populations it just
happens faster in small ones
D. Genetic drift is pronounced when very few
individuals rebuild a population or start a new
one
E. Inbreeding form of genetic drift in a small
population.
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Bottleneck Effect Ex earthquakes, floods, fires,
etc. Result the genetic makeup of the small
group of survivors will/will not be
representative of the genetics of the original
population
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Founder Effect
Occurs when a few individuals from a larger pop
colonize an isolated area
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Polymorphism
Morphs when two or more forms of a discrete
character are represented in a pop, this is what
the different forms are called If two or more
distinct morphs are each represented in high
frequencies, population is polymorphic
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How are New Species Formed?
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A. Called Speciation
B. Begins with reproductive isolation
1. Members of 2 populations cannot interbreed
and produce fertile offspring
a. Remember a population is referring to
individuals of the same species living in one
area
2. Both populations have separate gene pools
caused by...
a. Behavioral isolation
i. produced by behavioral differences
b. Temporal isolation
i. produced by different mating times
c. Geographic isolation
i. produced by physical separation
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3. Results in independently evolving populations
which eventually can generate two different
species
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Steps in speciation of Galapagos finches.

• 1. founders arrive
• 2. separation of populations
• 3. changes in the gene pool
• 4. reproductive isolation
• 5. ecological competition
• 6. continued evolution

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Patterns of EvolutionMacroevolution

• Macroevolution the large scale evolutionary
  changes that take place over long periods of time
• Mass Extinctions
• Adaptive Radiation small group of species
  evolved into several different forms
• Convergent evolution unrelated orgs look
  similar because of similarity in environments
• Coevolution process by which two species evolve
  in response to each other over time
• Ex. Yucca plant and Yucca Moth
• Punctuated Equilibrium long stable periods of
  no changes followed by brief periods of rapid
  change
• Gradualism slow, minute changes that build up
  over time

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Mass Extinctions

• Extinctions occur all the time
• More than 99 of all species that ever lived are
  extinct today
• Usually, extinctions occur at a constant rate
• Several times, however, huge numbers of species
  have disappeared in mass extinctions
• Paleontologists think that most mass extinctions
  in the past were caused
• by multiple factors
• Asteroids
• Volcanic activity
• Changing position of continents
• Changing sea levels

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Adaptive Radiation

• The evolution of many diversely adapted species
  from a common ancestor upon introduction to
  various new environmental opportunities and
  challenges is called adaptive radiation.

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Convergent Evolution

• Convergent evolution describes 2 unrelated
  species that share similar traits.
• These similarities are not due to common
  ancestry, but rather a result of similar
  environmental factors.

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Coevolution

• Coevolution describes the evolution of one
  species in response to new adaptations that
  appear in another species of which the first
  shares close interaction

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Punctuated Equilibrium Gradualism

• Punctuated equilibrium is a pattern of evolution
  that occurs in spurts. There is a period of very
  little change, and then one or a few huge changes
  occur, often through mutations in the genes of a
  few individuals.
• Typically occurs when new niches become available
  following a mass extinction

Gradualism is selection and variation that
happens more gradually. Change is slow, constant,
and consistent.
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Speciation Flow Chart
Section 17-4
Species
that are
in
under
under
form
in
in
can undergo
can undergo
can undergo
can undergo
can undergo
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