Download Chapter 17: Population Genetics and Speciation Section 1: Genetic

3/29/17
Section 1: Genetic Variation
Chapter 17: Population
Genetics and Speciation
Population Genetics: The study of the
frequency and interaction of alleles and
genes in populations
Normal Distribution: A line graph
showing the general trends in a set of
data of which most values are near the
mean
I. 
Population Genetics
A. Charles Darwin Knew
1. Heredity influences characteristics
2. Did not know about genes
B. We now study and predict genetic
variation and change
C. Microevolution
1. Evolution at the level of genetic
change in populations
2. Can be studied by observing
changes in the numbers and
types of alleles in populations,
called population genetics
Microevolution
D. The study of genetics and evolution
are advancing together
E. Speciation
1. Link from microevolution to
macroevolution
2. Now can study in detail
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II. Phenotypic Variation
Examples of Polygenic
A. Variety of phenotypes that exists for
a given characteristic depend on how
many genes affected
B. Polygenic characters are influenced
by several genes
1. Example
a. Human eye color
b. Height
C. Biologists study polygenic
phenotypes by measuring each
individual in the population and
then analyzing the distribution
of the measurements
1. Distribution is an overview of the
relative frequency and range of a set
of values
2. Some values in a range are more
common than others
III. Measuring Variation and
Change
D. A normal distribution or bell curve
1. Tends to cluster around the average
value in the center of the range
A. Particular combination of alleles in a
population at any one point in time
makes up a gene pool.
B. Genetic variation and change are
measured in terms of the frequency
of alleles.
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Example of Gene Pool
C. A frequency is the proportion or ratio
of a group that is of one type
1. To study genetic change
a. frequency in a population is
tracked over time
Gene Frequency
IV. Sources of Genetic
Variation
A. Evolution cannot proceed if there is
no variation
B. Major source of new alleles in natural
populations is mutation in germ cells
C. Mutation generates new alleles at a
slow rate
D. Only mutations in germ cells (egg and
sperm) are passed on to offspring
Section 2: Genetic Change
Genetic Equilibrium: A state in which
the allele frequencies of a population
remain in the same ratios from one
generation to the next.
I. Equilibrium and Change
A. Population in which no genetic change
occurred would be in a state of
genetic equilibrium
B. Genetic change in a population can
be measured
1. Change in genotype frequency
2. Change of allele frequency
3. Change in one doesn’t necessarily mean
a
change in other
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Example of Genetic
Equilibrium
C. The Hardy-Weinberg principle
predicts that the frequencies of
alleles and genotypes in a population
will not change unless at least one of
five forces acts upon the population.
Hardy-Weinberg
D. The forces that can act against
genetic equilibrium
1. Gene flow
2. Nonrandom mating
3. Genetic drift
4. Mutation
5. Natural selection
Hardy Weinberg Forces
E. Gene Flow
1. Occurs when genes are added or
removed from a population
2. Can be caused by
a. migration
b. movement of individuals from one
population to another
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Example of Gene Flow
F. Nonrandom Mating
1. In sexually reproducing populations,
any limits or preferences of mate
choice
Example
Genetic Drift
1. Chance events can cause rare
alleles to be lost from one
generation to the next, especially
when population are small.
Example of Genetic Drift
H. Mutation
•  1. Introduces New Alleles to the
Population
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Mutation Example
I. Natural Selection
1. Acts to eliminate individuals with
certain traits from a population
2. As individuals are eliminated, the
alleles for those traits become less
frequent
3. Both allele and genotype frequencies
change
Example of Natural Selection
II. Sexual Reproduction and
Evolution
A. Creates chances to recombine alleles
and increase variation in a population
B. Creates the possibility that mating
patterns or behaviors can influence the
gene pool.
1. Example, in animals, females
sometimes select mates based on
the male’s size, color, ability to gather
food, or other characteristics
a. Behavior is called sexual selection
and is an example of nonrandom
mating
b. Another example inbreeding
1) individuals either self-fertilize or
mate with others like themselves
2) more likely to occur if a population
is small
3) all members are likely to be closely
related
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III. Population Size and
Evolution
IV. Natural Selection and
Evolution
A. Strongly affects the probability of
genetic change in a population
B. Allele frequencies are more likely to
remain stable in large populations
than in smaller populations
C. Genetic drift is a strong force in small
populations and occurs when a
particular allele disappears
A. Natural selection is a result of the
following facts:
1. All populations have genetic
variation
2. Individuals tend to produce more
offspring than the environment can
support
3. Populations depend upon the
reproduction of individuals
B. How Selection Acts
C. Genetic Results of
Selection
1. Causes evolution in populations by
acting on individuals
2. Acts when individuals survive and
reproduce (or fail to do so)
3. Less “fit” individuals are less likely
to pass on their genes
D. Why Selection is Limited
1. Key lesson that scientists have
learned about evolution by natural
selection is that the environment does
the selecting.
2. Natural selection is indirect
a. Acts only to change the relative
frequency of alleles
b. Acts on genotypes by removing
unsuccessful phenotypes
1. Each allele’s frequency may increase
or decrease depending on the allele’s
effects on survival and reproduction
3. The role of Mutation
a. Only characteristics that are
expressed can be targets of natural
selection.
b. If a mutation results in rare recessive
alleles, selection cannot operate
against it.
1) For this reason, genetic disorders (cystic
fibrosis) can persist in populations
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V. Patterns of Natural
Selection
A. Three major patterns are possible in
the way that natural selection affects
the distribution of polygenic
characters over time.
1. These patterns are:
a. directional selection
b. stabilizing selection
c. disruptive selection
Directional Selection
B. Directional Selection
1. The “peak” of a normal distribution
moves in one direction along its range
a. Selection acts to eliminate an
extreme from a range of phenotypes
making them less common
C. Stabilizing Selection
1. The bell-curve shape becomes
narrower
a. Selection eliminated individuals
that have alleles for any extreme
type
b. Very common in nature
Stabilizing Selection
D. Disruptive Selection
1. The bell curve is “disrupted” and
pushed apart into two peaks
2. Selection acts to eliminate individuals
with average phenotype.
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Disruptive Selection
Section 3: Speciation
Reproductive Isolation: A state in which a
population can no longer interbreed with
other populations to produce future
generations
Subspecies: A taxonomic classification below
the level of species, refers to populations that
differ from, but can interbreed with, other
populations of the same species
I. Defining Species
A. Scientists may use more than one definition
for species
B. Definition depends on organisms and
field of science being studied
C. Generally defined as a group of
natural populations that can
interbreed and produce fertile offspring
D. Instead of, or in addition to, the
biological species concept, species
may be defined based on
1. Physical features
2. Ecological roles
3. Genetic relatedness
II. Forming New Species
A. Each population of a single species
lives in a different place
B. Natural selection acts upon each
population and tends to result in
offspring that are better adapted to
each specific environment
C. If the environments differ, adaptations
may differ
1. This is called divergence and can
lead to formation of a new species
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E. Reproductive Isolation
D. Speciation
1. Process of forming new species by
evolution from preexisting species
2. Has occurred when the effects of
evolution result in population with
unique features and is reproductively
isolated
1. State in which two populations can no
longer interbreed to produce future
offspring
2. Groups may be subject to different
forces
a. will tend to diverge over time
3. Populations of the same species may differ
enough to be considered subspecies
Example of Subspecies
F. Mechanisms of Isolation
1. Any of the following mechanisms may
contribute to the reproductive isolation
of populations
a. Geography
1) physical barrier that prevents
interbreeding
Geographical Barriers
b. Ecological Niche
1) populations use different niches
causes divergence
c. Mating Behavior and Timing
1) two populations have different
mating times
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Example of Different Niches
•  Spiny Mice- Avoid competition
•  One Population Feeds at Night
•  One Population Feeds during Day
d. Polyploidy
1) because of 2 sets of
chromosomes cannot match
chromosomes
2) becomes genetically isolated
Polyploidy in Plants
e. Hybridization
1) two species are so different
have a hybrid that is not fertile
2) horse + donkey= mule (sterile)
3) lion + tiger= liger (sterile)
Mule
Liger
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G. Extinction: The End of a
Species
Extinct Animals
1. Occurs when a species fails to
produce any more descendants
a. Can only be detected when complete
2. More than 99% of all of the species
that have ever lived have become extinct
3. Many cases of extinction are the result of
environmental change
4. If a species cannot adapt fast enough to
changes, the species may be driven to
extinction
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