population genetics unrevised

7/27/2019 Population Genetics Unrevised

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The Gene Pool

Members of a speciescaninterbreed&producefertileoffspringSpecies have asharedgene poolGene poolall of the

alleles of all individualsin a population


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The Gene Pool

Different speciesdoNOT exchange

genesbyinterbreeding

Different speciesthat interbreedoften produce

sterile or less viableoffspring e.g. Mule


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Populations

A group of thesame species living

in an area

No two individualsare exactly alike(variations)

MoreFitindividuals survive &pass on their traits


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Speciation

Formation of newspeciesOne species maysplitinto 2 or more

speciesA species mayevolve into a new

speciesRequires verylongperiods of time


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Modern Synthesis Theory

CombinesDarwinianselectionandMendelian inheritance

Population genetics-

study of geneticvariation within apopulation

Emphasis onquantitativecharacters (height,size )


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1940scomprehensivetheory of evolution(Modern SynthesisTheory)

Introduced by Fisher &Wright

Until then, many did notaccept that Darwins

theory of naturalselection could driveevolution

S. Wright

A. Fisher


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TODAYStheory on evolutionRecognizes thatGENESare responsible for

the inheritance of characteristics

Recognizes thatPOPULATIONS, notindividuals, evolve due to natural selection& genetic drift

Recognizes thatSPECIATIONusually is dueto thegradual accumulation of small geneticchanges


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Microevolution

Changes occur in gene poolsdue tomutation, natural selection, geneticdrift, etc.

Gene pool changes cause moreVARIATION in individualsin thepopulationThis process is called

MICROEVOLUTIONExample:Bacteriabecoming unaffected

by antibiotics(resistant)


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The Hardy-Weinberg Principle

Used to describe anon-evolvingpopulation.

Shuffling of allelesby meiosis and

random fertilization haveno effecton the overall gene pool.

Natural populationsare NOT

expected to actually be in Hardy-Weinberg equilibrium.


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Deviationfrom Hardy-Weinbergequilibrium usuallyresults inevolution

Understanding a non-evolvingpopulation, helps us to understandhow evolution occurs


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5 Assumptions of the H-W Principle

1.Large population size- small populations have fluctuations in allelefrequencies (e.g., fire, storm).

2.No migration

- immigrants can change the frequency of anallele by bringing in new alleles to apopulation.

3.No net mutations- if alleles change from one to another, thiswill change the frequency of those alleles


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5 Assumptions of the H-W Principle

3.Random mating

- if certain traits are more desirable,then individuals with those traits will beselected and this will not allow for random

mixing of alleles.4.No natural selection- if some individuals survive and reproduceat a higher rate than others, then their

offspring will carry those genes and thefrequency will change for the nextgeneration.


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Traits Selected for Random Mating

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The gene pool of a NON-EVOLVINGpopulation remains CONSTANT over multiplegenerations (allele frequency doesnt change)

The Hardy-Weinberg Equation:1.0 = p2+ 2pq + q2

Where:

p2

= frequency of AA genotype2pq= frequency of Aa

q2= frequency of aa genotype17


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Determining the Allele Frequency usingHardy-Weinberg:

1.0 = p + qWhere:

p= frequency of A allele

q= frequency of a allele

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Allele Frequencies Define Gene Pools

As there are 1000 copies of the genes for color,the allele frequencies are (in both males and females):

320 x 2 (RR) + 160 x 1 (Rr) = 800 R; 800/1000 = 0.8(80%) R160 x 1 (Rr) + 20 x 2 (rr) = 200 r; 200/1000 = 0.2(20%) r

500 flowering plants

480 red flowers 20 white flowers

320 RR 160 Rr 20 rr


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Microevolutionof Species


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Causes of Microevolution

Genetic Drift- the change in the gene pool of a small

population due to chance

Natural Selection- success in reproduction based on heritabletraits results in selected alleles being passed torelatively more offspring (Darwinian inheritance)

- Cause ADAPTATION of Populations

Gene Flow-is genetic exchange due to the migration offertile individuals or gametes between

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Causes of Microevolution

Mutation-

a change in an organisms DNA

- Mutations can be transmitted in gametes tooffspring

Non-random mating- Mates are chosen on the basis of the besttraits

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Genetic Drift


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Factors that Cause Genetic Drift

Bottleneck Effect

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a drastic reduction in population (volcanoes,earthquakes, landslides )

- Reduced genetic variation

- Smaller population may not be able to adapt to newselection pressures

Founder Effect- occurs when a new colony is started by a few

members of the original population-

Reduced genetic variation

- May lead to speciation

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Loss of Genetic Variation

Cheetahshave little genetic variation intheir gene pool

This can probably be attributed to a

population bottleneckthey experiencedaround 10,000 years ago, barelyavoiding extinction at the end of the

last ice age

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Founders Effect

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Modes of NaturalSelection


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Modes of Natural Selection

Directional Selection

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Favors individuals at one end of the phenotypicrange

- Most common during times of environmental changeor when moving to new habitats

Disruptive selection

- Favors extreme over intermediate phenotypes

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Occurs when environmental change favors anextreme phenotype

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DirectionalSelection


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M d f N t l S l ti


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Modes of Natural Selection

Stabilizing Selection

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Favors intermediate over extreme phenotypes

- Reduces variation and maintains the cureentaverage

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Example: Human birth weight

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Variations inPopulations


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Geographic Variations

Variation in a speciesdue toclimate oranother geographicalcondition

Populations live indifferent locations

Example:Finches of

Galapagos Islands &South America

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Heterozygote Advantage

Favorsheterozygotes (Aa)

Maintainsboth alleles (A,a)instead ofremoving less successful alleles from apopulation

Sickle cell anemia

> Homozygotes exhibit severe anemia, haveabnormal blood cell shape, and usually diebefore reproductive age.

> Heterozygotes are less susceptible tomalaria

Sickle Cell and Malaria


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Sickle Cell and Malaria


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Coevolution

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population genetics unrevised

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