Transcript Population Genetics ppt - Liberty Union High School District

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Reminder:
•A group of the
Populations
same species living
in an area
No two individuals
are exactly alike
(variations)
More Fit
individuals survive &
pass on their traits
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Reminder:Speciation
•Formation of new
species
•One species may
split into 2 or more
species
A species may
evolve into a new
species
Requires very long
periods of time
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Modern Synthesis Theory of Evolution
• TODAY’S theory on evolution
• 1. GENES are responsible for the
inheritance of characteristics
• 2. POPULATIONS, not individuals, evolve
due to natural selection & genetic drift
• 3. SPECIATION usually is due to the
gradual accumulation of small genetic
changes
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What causes
populations of
organisms to
EVOLVE???
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There are 5 factors that can lead
to or cause EVOLUTION!!!
1.
2.
3.
4.
5.
Genetic Drift
Natural Selection
Gene Flow
Mutation
Sexual Selection
(non-random mating)
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see chapter 11.3
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Causes of Evolution
Genetic Drift
the change in the gene pool of
a small population due to
chance
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Factors that Cause Genetic Drift
• Bottleneck Effect
- a drastic reduction in population about 10,000
years ago
- Reduced genetic variation
- Smaller population have a hard time adapting
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Factors that cause Genetic Drift
Founder’s Effect
Genetic drift that occures after a
small number of individuals colonize a
new area
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Causes of Evolution
Natural Selection
success in reproduction based on
heritable traits results in selected
alleles being passed to relatively
more offspring (Darwinian
inheritance)
- Cause ADAPTATION of Populations
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Causes of Evolution:
Gene Flow
Changes in the gene pool of a population
because of the introduction of genes
from another population by migration.
• Example: the gene pool in southeast Asia
was changed when U.S. Soldiers had
children with Vietnamese women during the
Vietnam war.
Causes of Evolution:
Mutation
a change in an organism’s DNA
Mutations can be transmitted in gametes
to offspring
Non-random mating
- Mates are chosen on the basis of
the best traits
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Having a lot of genetic
variation within a population
is GOOD!!!
Chap 11.1
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The Gene Pool
Chapter 11.1
•Members of a species
can interbreed &
produce fertile
offspring
Species have a shared
gene pool
Gene pool – all of the
alleles of all individuals
in a population
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Allele Frequencies
Different combinations of alleles in a gene pool can be
formed when organisms mate and have offspring.
The Allele Frequency is the measure of how common a
certain allele is in the population.
Allele Frequency = the number of times an allele occurs
the total number of alleles for that gene
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Let’s calculate the allele frequency for:
You have a population of mice. There are 30 mice total
and 15 mice are black (BB), 10 mice are brown (Bb)
and 5 mice are white (bb).
What is the allele frequency of B?
What is the allele frequency of b?
You can track the frequency of an allele over time to see
the changes in populations
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Answer this!!
If a certain trait’s allele frequency is
100%, describe the genetic
variation of that allele in the
population.
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2 main sources for variation in
allele frequencies
1. Mutation
2. Recombination through Meiosis
Q? Why aren’t mutations in somatic cells
sources of genetic variation??
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Evolution Through Natural Selection
Modes of Natural Selection
1. Directional Selection
-
Favors individuals at one end or the other end of
the phenotypic range
Most common during times of environmental change
or when moving to new habitats
example, colored animals like mice.
At times black is favored, other times white is
favored
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Modes of Natural Selection
2. Disruptive selection
- Favors both extremes over intermediate
phenotypes
- Occurs when environmental change favors an
extreme phenotype
- Can be a pathway for species formation
WITHOUT geographic isolation
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Modes of Natural Selection
3. Stabilizing Selection
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Favors intermediate over extreme phenotypes
Reduces variation and maintains the cureent
average
Example: Human birth weight
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How does Natural Selection
Affect Allele Frequency??
1. Mutations provide the raw
material on which natural selection
acts
2. Evolution depends on variations
because it is the only way that
differences among organisms are
created
3. Acts on Populations not
individuals by changing the % of
alleles in the population
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How Do New Species Occur?
ISOLATION!!!!!
(nobody likes to be alone!!)
1. Behavioral Barriers:
When members of a population can’t breed due to
behavioral changes in their mating habits.
2. Geographic Barriers:
When Organisms are isolated due to changes in geography
or environment that have separated one group into two.
3.Temporal Barriers:
When timing of reproductive behaviors are different.
EX: different pollination periods, different mating
times.
THIS ALL CONTRIBUTES TO REPRODUCTIVE
ISOLATION & FORMATION OF NEW SPECIES!!!
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New Species Evolve in Patterns:
Convergent Evolution:
Different species must
adapt to similar environments. Evolution towards
SIMILAR characteristics in UNRELATED species.
ex: Dolphin/Shark
Divergent Evolution:
Closely related species evolve
In different directions. Evolution towards
DIFFERENT characteristics in RELATED species.
ex: Kit fox/Red fox
Co-Evolution:
When 2 or more different species
evolve in response to changes in the other.
ex: Acacia plant/ants
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Repeating Patterns of Speciation
Throughout the history of EARTH
Punctuated Equilibrium
Adaptive Radiation
A pattern of BURSTS
of evolutionary
activity followed by
long periods of
stability.
The diversification of
one ancestral species
into many descendent
species.
EX:
EX: mammals
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Genetic Variations,
(changes in allele frequencies)
within Populations
creates Evolution of
Species!!
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