1 random genetic drift : chance as an evolutionary force random genetic drift is the random change...

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Random Genetic Drift : Chance as an Evolutionary Force

• Random Genetic Drift is the random change in allele frequencies from one generation to the next that is caused by the finite size of the breeding population of parents.

• By chance, some parents have more offspring than other parents.

• By chance, some parents have fewer offspring or no offspring at all.

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• Random Genetic Drift occurs in ALL countable, finite populations.

• RGD is STRONGER in Small Populations (0 < N < 500).

• RGD is WEAKER in Large Populations (500 < N).

• The Strength of RGD is proportional to (1/2N) in a diploid population and to (1/N) in a haploid population.

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1) WITHIN populations RGD DECREASES GENETIC VARIATION :: Random Genetic Drift makes a population less genetically less variable and makes the individuals in the population more Homozygous. The ultimate outcome: ALL HERITABLE VARIATION IS LOST!!

The population becomes either p* = 0.0 (allele is lost) or p* = 1.0 (allele is FIXED).

2) BETWEEN populations RGD INCREASES GENETIC VARIATION: Random Genetic Drift makes two populations become genetically different from one another.

RGD and its Evolutionary Consequences

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Generation 1 Generation 2

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No Chance Events5

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Chance Events

Random GeneticDrift

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5FrequenciesStay Exactly

the Same

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Generation 1

Generation 2

DifferentChance Events

Lead toDifferent Outcomes

One Starting Point

Many Possible Outcomes

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Random Genetic Drift

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RGD for A FewGenerations

With NOMigration

START: All Populations GeneticallyIdentical

END: Populations GeneticallyDifferent from one another

RGD INCREASES Variation AMONG Populations

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Average of MANYIndependent

Chance Events

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Average Effect ofRandom Genetic Drift

On Allele Frequencies = ZEROAverage P = 0

On AVERAGEFrequencies

Stay the Same

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No Chance Events5

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Random GeneticDrift

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5FrequenciesStay Exactly

the Same

On AVERAGEFrequencies

Stay the Same

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Q: If the frequency of an allele, B, at time 0, equals 0.65, what is probability that RGD will result in Fixation of the B allele? That is, B will go from a frequency of 0.65 to a frequency of 1.00?

A) 0.00

B) 0.50

C) 0.65

D) 1.00

E) None of the above

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RGD for A FewGenerations

With NOMigration

START: All Populations GeneticallyIdentical

END: Populations GeneticallyDifferent from one another

RGD INCREASES Variation AMONG Populations

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RGD for ManyGenerations

With NOMigration

All Populations GeneticallyIdentical

Populations GeneticallyDifferent from one another

RGD DECREASES Variation WITHIN Populations

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RGD and Migrationfor Many

Generations


Populations SomewhatGenetically Different

Migration OPPOSES RGD and LIMITS its effects

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Random Genetic Drift : Effects

1. On average WITHIN one population, RGD DECREASES genetic variation:

A) Random Genetic Drift makes INDIVIDUALS more homozygous, more genetically similar.

B) the POPULATION becomes genetically less variable. C) RGD causes ALLELES to become FIXED (Pa = 1) or LOST (Pa = 0).D) RGD diminishes the heritable variation within a population, which limits

NATURAL SELECTION.

2. On average AMONG a group of populations, RGD INCREASES genetic variation: A) RGD makes different populations genetically more different from one another.

3. GENETIC FIXITY of SPECIES is NOT possible as long as there are a lot of populations, because RGD will do something different in each population. There is continual change of allele frequencies in finite populations by chance, RGD.

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Generation 1

Generation 2

ManyDifferent

Chance Events

One Starting Point

Three Equally Possible Outcomes

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Random Genetic Drift

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Migration as an Evolutionary Force• Migration is the Movement of Individuals between

populations.• When individuals move from one population to

another and then breed with the residents of the new population, ‘Gene Flow’ can change frequency of alleles in the new population.

– immigration movement of individuals INTO a population

– emigration movement of individuals OUT OF a

population

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Evolutionary Consequences of Migration (Gene Flow)

1) Gene Flow between populations reduces the genetic differences between the populations.

2) Gene Flow can be a constraint on evolution when immigrants carry genes into a population that are not adapted for the ecological conditions of the population.

alleles that are good in one population may be bad in another population

3) Gene Flow can accelerate evolution when immigrants carry genes into a population that are adapted for the conditions of the population.

beneficial alleles from other populations can be brought in faster by migration than they can arise by mutation.

Migration does NOT create new genetic variation; it moves around already existing variation.

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pGreen = 1.00pRED = 0.00

Population 1 Population 2

Exchange Migrants

M = 0.20


The two Populationsare genetically veryDifferent BEFORE

GENE FLOW

The two Populationsare genetically more

similar AFTER GENE FLOW

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p’Green = (1 – M)pGREEN,1 + (M)pGREEN,2

= 0.80*(1.00) + 0.20*(0.00) = 0.80

ΔpGREEN,1 = M (p2 – p1) < 0, Green DECREASED

What is Allele Frequency AFTER Migration in Population 1?


M = 0.20From Pop2

p’Green = ???p’RED = ???

Population 1Population 1

19ΔpGREEN,2 = M (p1 – p2) > 0, GREEN INCREASED

What is Allele Frequency AFTER Migration in Population 2?

pGreen,2 = 0.00pRED,2 = 1.00

M = 0.20From Pop 1.

p’Green = ???p’RED = ???

p’Green,2 = (1 – M)pGREEN,2 + (M)pGREEN,1

= 0.80*(0.00) + 0.20*(1.00) = 0.20

Population 2Population 2

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Random Genetic Drift versus Migration

• RGD makes populations genetically different.

• Migration makes populations genetically similar.

• Together, RGD and Migration place a limit on the degree of genetic differences between populations.

• Where is the limit or Balance between these two evolutionary forces?

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RGD and Migrationfor Many

Generations


Populations SomewhatGenetically Different

Migration opposes RGD and LIMITS its effects

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RGD for ManyGenerations


Populations GeneticallyDifferent from one another

Could this happen with Migration? Answer: ????

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Random Genetic Drift and Migration: Effects

1. On average WITHIN one population, RGD DECREASES genetic variation, Migration INCREASES genetic variation:

A) RGD makes INDIVIDUALS more homozygous, Migration makes INDIVIDUALS more heterozygous.

B) the POPULATION reaches a STABLE LEVEL of genetic variation where RGD and Migration are balanced.

C) Migration PREVENTS RGD from making ALLELES become FIXED (Pa = 1) or LOST (Pa = 0).

D) Migration can replenish the heritable variation within a population lost by RGD.

2. On average AMONG a group of populations, RGD INCREASES genetic variation, Migration DECREASES genetic variation:

1 random genetic drift : chance as an evolutionary force random genetic drift is the random change...

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