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

All Stands Showing

Individual Stand(s) -- Allele A

Average -- Allele A

Individual Stand(s) -- Allele a

20 carrying capacity

Population size effect on frequencyStand size 400

Without migration

All Stands Showing

Individual Stand(s) -- Allele A

Average -- Allele A

Individual Stand(s) -- Allele a

25% Migration

Population size effect on frequencyStand size 20

Without migration

All Stands Showing

Individual Stand(s) -- Allele A

Average -- Allele A

Individual Stand(s) -- Allele a

25% Migration

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. TrueB. 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

• 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

• 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 MigrationDiscussion• 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 ModelThe 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

mainlandOn 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

QuestionWhich 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

Northern Elephant Seal (evolution.berkley.edu)

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.

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.