Download Evolution Population Genetics - Liberty Union High School District

3/30/2014
Targets: Alleles and Genes
Evolution
Population Genetics
Vocabulary Review
 Population – all of the individuals of a single species
that live together in one place.
 Natural Selection – populations changing in
response to their environment as individuals adapt
and leave more offspring.
 Allele – alternative forms of a gene that code for the
same trait (Ex: eye color has blue, brown, green,….)
You have two alleles for each trait, one from your
mom and one from your dad.
 Phenotype – the physical traits expressed for each
allele
Ex: Genotype = BB Phenotype = Brown eyes
Examples:
- during cold winters, penguins that
are short and round might survive the
cold better.
- during famine, tall and slender
penguins might be able to dive deeper
for additional food sources.
1. I can explain how genetic variation in a
species increases chances for survival
2. I can write an example of how lethal alleles
are stored in a gene pool
3. I can define allele frequency
4. I can explain why natural selection acts on
phenotype rather than genotype
Genetic variation increases the chance of survival
for a species
•Natural Selection selects for favorable
phenotypes of a population
•The more variety there is within a
population, the more likely that population
will survive changes.
Genetic variation is stored in a population's gene
pool
•A gene pool is the combined alleles of all the
individuals in a population.
•The more alleles for a trait, the greater
the genetic variation
•Allele frequency is the relative amount of an
allele in a population.
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Allele Frequency
How to calculate allele frequency in a
population:
•Count the number of times a specific allele
occurs in a gene pool
•Divide by the total number of alleles
•Answer expressed as a percentage (%)
How are lethal alleles stored in a gene pool?
Most lethal alleles are recessive and do
not appear until later in life
*Most people don’t know they are carriers
*Don’t know they have it till later in life (have
already passed on the gene to kids)
*Both parents must carry the recessive gene
(only 25% chance of passing on)
How is genetic variation created?
Two main sources of new variation in a
population:
Mutations
Recombination (during meiosis)
What is the allele frequency for b?
What is the allele frequency for B?
7/14 = 50%
Targets: Effects of Mutations
5. I can explain how mutations can
increase genetic variation.
6. I can evaluate the impact that
mutations can have in a gene pool.
Mutations
•A mutation is a random change in DNA
•New mutations are constantly being generated,
adding new alleles to the gene pool, thus
increasing genetic variation in the population
•Change can be positive, negative or neutral
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•Mutations alter genotype
•DNA is changed
•add variety to a population
•Natural Selection acts on phenotype.
•DNA is NOT changed
•New variations ARE NOT added
•Favorable traits (either from mutations or
meiosis) are passed on
Natural Selection acts on the distribution of
traits within a population
 Within a population there is a range of phenotypes
for each trait.
 These ranges often have more of some phenotypes
and less of others.
 Natural Selection can change the distribution in 3
ways:
Stabilizing
Directional
Diversifying/Disruptive
•
Stabilizing Selection
 Extremes from both ends of the frequency
distribution are eliminated.
 Most common form of natural selection
Directional Selection
•Favors phenotypes at one extreme of a trait’s range.
•What we usually think of as natural selection.
•Example: increase of drug-resistant bacteria
 Example: birth weight of babies
Disruptive Selection
•Both extreme phenotypes are favored at expense of
middle phenotype.
•Favoring both extremes can lead to new species
formation.
•Example: small-medium-large beak size of finch
Targets: Gene Flow
7. I can define gene flow
8. I can define and give examples of
genetic drift.
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Gene Flow
The movement of alleles from one population
to another (migration).
- can occur in plants and animals
- increases variation in receiving
population
** a lack of gene flow between 2 populations
increases the chance they will evolve into 2
different species.
Genetic Drift
Change in allele frequency due to chance.
- causes a loss of diversity
- greatly affects smaller populations
Two types of genetic drift:
Founder Effect
Bottleneck Effect
Founder Effect
A small group of individuals break away and
colonize a new area.
- gene pool of small group different from
original group.
Example: Ellis-van Creveld Syndrome
Amish population in Pennsylvania
Bottleneck Effect
Occurs after an event greatly reduces the
size of a population:
- overhunting
- predation
- catastrophe
New population no longer has many alleles
that old population has.
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Effects of Genetic Drift
• Loss of genetic variation
• Lethal alleles become more common
•Founder Effect Video
Types of Isolation
Geographic Isolation
Barriers divide a population: rivers, mountains, dried
lake beds.
Reproductive Isolation
• Members of a population are unable to mate
successfully with each other.
• Could be due to different mating seasons, different
mating calls, variations that favor different
environmental factors.
• Last step in becoming different species.
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