Transcript Rare migration vs. Regular Migration

Migration
Q1. What would be the trend for
allele frequencies of populations that
that are large vs. very small?
A. Large populations will have a greater range in
frequency; smaller populations will have a low
range of variation in frequency
B. Large populations will have a low range of
variation in frequency; smaller populations will
have a high range of variation in frequency
C. The trend would be very similar for both large
and small populations
Small v. Large Populations
• Greater effect on small populations. Why?
– Not as many numbers to even things out
http://www.biologylabsonline.com
Population size effect on frequency
4000
400
40
20
Migration
• In an evolutionary sense
– movement of alleles among populations-thus
changing the allele frequency among populations
• Dispersal by animals, wind, water, etc.
• Multiple Populations combining into one
– Homogenization
• Making two populations more similar
• Less variation (homozygosity)
• http://www.biologylabsonline.com
Migration Homogenizes
400 carrying capacity
20 carrying capacity
All Stands Showing
Individual Stand(s) -- Allele A
Average -- Allele A
Individual Stand(s) -- Allele a
Population size effect on frequency
Stand size 400
Without migration
25% Migration
All Stands Showing
Individual Stand(s) -- Allele A
Average -- Allele A
Individual Stand(s) -- Allele a
Population size effect on frequency
Stand size 20
Without migration
25% Migration
All Stands Showing
Individual Stand(s) -- Allele A
Average -- Allele A
Individual Stand(s) -- Allele a
Migration with other mechanisms of
evolution
Selection
&
Genetic Drift
Selection v. Migration
• Selection opposes homogenization
– Tends to fix alleles
• Phenotypes attached to particular genotypes that are
selected for survive
Q.2 If selection’s influences are
stronger for a particular allele than
migration’s influence on it then
heterozygosity will prevail
A. True
B. False
Selection with Migration
• No selection W/ migration= homogenization
• Selection stronger than migration= more diversity
(more homozygotes)
• Migration stronger than selection= less diversity
(more heterozygotes)
Migration v. Genetic Drift
• Genetic Drift
– Random fixation of alleles and loss of
heterozygosity
– Prevents complete homogenization
• As observed in Pop Gen Lab
Migration
An Evolutionary Force
What is migration, evolutionarily speaking?
A. Moving from one’s country of origin to a
foreign country to seek a better life
B. The movement of alleles from one population
to another
C. A bird flying from one lake to another within
the same geological location
D. Packing all your belongings into the U-Haul
and moving across the country
E. More than one of the above
What is Migration?
• Evolutionarily speaking: “the movement of
alleles from one population to another”
• Movement of individuals
• Transport of gametes:
• Wind
• Water
• Pollinators
How Does Migration Affect Populations?
• Takes separated populations of species and
connects them, essentially making them one
population
• Keeps allele frequencies of populations in
equilibrium, away from fixation
• Works against Natural Selection
• Works against Genetic Drift
Rare versus Regular Migration
PopGen Lab Demonstration
• Rare migration: Populations remain relatively static,
with little spread of alleles from population to
another
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Set Number of Stands to 20
Set Stand Size to 400/40
Set Migration Rate to 1%
Set Number of Generations to 300
Run Experiment and observe what happens
• As the migration rate increases we see something
else happen…
Rare versus Regular Migration
PopGen Lab Demonstration
• Regular Migration: Populations are dynamic,
with lots of movement of alleles from one
population to another
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Set Number of Stands to 20
Set Stand Size to 400/40
Set Migration Rate to 75%
Set Number of Generations to 300
Run Experiment and observe what happens
Rare versus Regular Migration
Discussion
• With rare migration events the alleles for each population act
as they would without migration. Any population could have
genetic equilibrium or tend towards fixation of one of the
alleles
• With regular migration events the movement of alleles from
one population to another causes the separate populations to
act collectively: what happens to the allele frequency in one
population happens to the allele frequency in all the others.
Migration keeps the allele frequencies in equilibrium;
however, PopGen shows an occasional fixation of one of the
alleles and loss of the other in small stand sizes, which seems
unexplainable
What is the One Island Model?
A. Includes one island among many that shows
evolution among populations
B. Explains certain populations that live only on one
island and never migrate anywhere
C. Includes migration from a mainland to a small island
that is close by
D. A brand of salad dressing competing with the
Thousand Islands™ brand
E. A model that only poses on a particular island
One Island Model
• Scenario: a small island close to the mainland
• If migration occurs from the small island to
the mainland, the population on the mainland
will not be affected much
• If migration occurs from the mainland to the
small island, the population can change
drastically
One Island Model
The Numbers (see pages 227-8)
• Suppose a small island has a species with a population
of 800 with a genotype frequency of 1.0 for a
dominant, naturally selected gene, AA (0.0 for Aa and
aa).
• If 200 individuals come from the mainland with the aa
allele the new genotype frequencies will be AA – 0.8,
Aa – 0.0, and aa 0.2.
• The allele frequencies will be: A – 0.8, and a – 0.2
• Both Hardy-Weinberg conclusions have been violated.*
Evolution has occurred.
– *If you need us to, we can explain this
A Snaky Example
• Lake Erie Water Snake, Nerodia sipedon
• Two varieties – banded and unbanded, genetically
• These snakes can be found on both the islands and on the
mainland
On the islands’ limestone rocks the young, small, banded
snakes are much more vulnerable to predation than the
unbanded snakes (see pictures).
• Why do the banded snakes exist on the islands if they are
selected against?
Migration works against selection
Discussion
• (At least part of) the answer is that every generation
several banded snakes move from the mainland to
the islands
• The migrants bring with them copies of the allele for
banded coloration
• Interbreeding contributes these copies to the
population
• Migration is working against selection preventing
fixation of any one allele.
Homogenization
Homogenization: When allele frequencies are
made similar among different populations.
Migration is a powerful homogenizer of
populations.
Red Bladder Campion
Predictions
• Young, newly established populations that are
small will have a high degree of allele variation
• Intermediate populations will be more
homogeneous in their allele frequencies
• Old populations will be more variable in their
allele frequencies.
Results
Results
• Young populations have more variation in allele
frequencies due to small population size and genetic
drift.
• Intermediate populations have very little variation
in allele frequencies due to migration.
• Older populations have greater variation in allele
frequencies due to a cessation of migration,
decreased population size, and genetic drift.
Homogenization
“If allowed to proceed unopposed by any other
mechanism of evolution, migration will
eventually homogenize allele frequencies
across populations completely.”
Bottleneck effect
Bottleneck: An evolutionary event in which a
significant percentage of a population or species is
killed or otherwise prevented from reproducing
Founder Effect: The effect of establishing a new
population by a small number of individuals, carrying
only a small fraction of the original population's
genetic variation.
Bottleneck effect
Evololution.berkley.edu
Question
Which of the following is a possible reason why
populations tend to move toward one of the
homozygotes when a bottleneck event
happens without migration?
A) Bottleneck events always target one of the
alleles.
B) The small population size allows genetic drift to
move a population towards fixing one allele.
Question
Given that migration homogenizes populations,
which allele frequency chart would best portray how
migration affects bottleneck populations?
A.
B.
Bottleneck effect
Effect of bottlenecks:
The Species may not be
able to adapt to new
selection pressures, such as
climatic change or a shift in
available resources.
Northern Elephant Seal (evolution.berkley.edu)
Conclusion
• Migration can cause allele frequencies to
change from one generation to another which
violates the first Hardy-Weinberg conclusion.
• Migration can be a powerful mechanism for
evolution.
• Migration tends to homogenize allele
frequencies across populations.