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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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    Modern Synthesis Theory

    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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    Modern Synthesis Theory

    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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    The Hardy-Weinberg Principle

    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 Hardy-Weinberg Principle

    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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    The Hardy-Weinberg Principle

    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

    populations23

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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

    -

    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

    -

    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 usuall