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Chapter 23.

Population Genetics

“I’m from theshallow end of thegene pool”

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Essential Questions How can we measure evolutionary

change in a population? What produces the variation that

makes evolution possible? What are the primary

mechanisms ofadaptive evolution?

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Population genetics provides afoundation for studying evolution

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Smallest unit of evolution Individuals are selected Populations evolve

Bent Grassgrowing on minetailings; onlyindividualstolerant of toxicheavy metals willgrow from theseeds blown infrom nearby field

Individuals are selected; populations evolve.The Bent Grass (Agrostis tenuis) in the foreground of the photo isgrowing on the tailings of an abandoned mine. These plants tolerateconcentrations of heavy metals that are toxic to other plants of thesame species in the pasture beyond the fence. Many seeds from thepasture drift onto the tailings, but only those with genes that enablethem to tolerate metallic soil survive and reproduce.

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Evolution since Darwin comprehensive theory of

evolution took form in early 1940s integration of natural selection &

Mendelian inheritance (genetics) aka Neo-Darwinism R.A. Fisher J.B.S. Haldane Theodosius Dobzhansky Ernst Mayr Sewall Wright George Gaylord Simpson Ledyard Stebbins

Modern Synthesis

Dobzhansky

Mayr

Ernst Mayr and the Evolutionary SynthesisErnst Mayr helped define the modern synthesis of evolutionary theory,proposing the "Biological Species Concept." In particular, his work onspecies and speciation helped scientists understand the progressand mechanisms of evolution from one species to another, and theimportance of the species unit as "the keystone of evolution."Ironically, one great unsolved problem in Darwin's master work, Onthe Origin of Species, was just that: How and why do speciesoriginate? Darwin and his later followers were faced with a seemingparadox. They described evolution as a continuous, gradual changeover time, but species are distinct from each other, suggesting thatsome process has created a discontinuity, or gap, between them.Credit for doing the most to crack this puzzle goes to Ernst Mayr,perhaps the greatest evolutionary scientist of the twentieth century.Along with Theodosius Dobzhansky, George Gaylord Simpson, andothers, Mayr achieved the "modern synthesis" in the 1930s and1940s that integrated Mendel's theory of heredity with Darwin's theoryof evolution and natural selection

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Populations & gene pools Concepts

a population is a localized group ofinterbreeding individuals

gene pool is collection of alleles in thepopulation remember difference between alleles & genes!

allele frequency is frequency of allele in apopulation how many A vs. a in whole population

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Evolution of populations Evolution implies a change in allele

frequencies in a population hypothetical: what would it be like if

allele frequencies didn’t change? non-evolving population

very large population size (no genetic drift) no migration (in or out) no mutation random mating (no competition) no natural selection

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Hardy-Weinberg equilibrium Hypothetical, non-evolving population

preserves allele frequencies Serves as a model

natural populations rarely in H-W equilibrium useful model to measure if forces are acting

on a population

W. Weinbergphysician

G.H. Hardymathematician

G.H. Hardy (the English mathematician) and W. Weinberg (theGerman physician) independently worked out the mathematicalbasis of population genetics in 1908. Their formula predicts theexpected genotype frequencies using the allele frequencies in adiploid Mendelian population. They were concerned with questionslike "what happens to the frequencies of alleles in a population overtime?" and "would you expect to see alleles disappear or becomemore frequent over time?"

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Hardy-Weinberg theorem Alleles

frequency of dominant allele = p frequency of recessive allele = q

frequencies must add to 100%, so:p + q = 1

Individuals frequency of homozygous dominant = p2

frequency of homozygous recessive = q2

frequency of heterozygotes = 2pq frequencies must add to 100%, so:

p2 + 2pq + q2 = 1

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Calculating frequency of alleles Example:

a wildflower population with 2 flowercolors allele for red flower color (R) is

completely dominant to the allele forwhite flowers (r)

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Calculating frequency of alleles Population of 500 plants

what is the allele frequency? what % of gene pool is red allele vs.

white allele? remember diploid = 1000 alleles

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Calculating frequency of alleles RRR: 320 x 2 = 640Rr: 160 x 1 =160R = 800/1000 = 80%p = 0.8

rrr: 20 x 2 = 40Rr: 160 x 1 =160r = 200/1000 = 20%q = 0.2

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Application of HW theorem What is the frequency of an allele in the

population example:

what % of the human population carriesallele for PKU (phenylketonuria )

~ 1 in 10,000 babies born in the US isborn with PKU, which results in mentalretardation & other problems if untreated

disease is caused by a recessive allele

PKU (phenylketonuria) is a rare, inherited metabolic disease thatresults in mental retardation and other neurological problems whentreatment is not started within the first few weeks of life. When a verystrict diet is begun early and well-maintained, affected children canexpect normal development and a normal life span.The disease arises from the absence of a single enzyme(phenylalanine hydroxylase). This enzyme normally converts theessential amino acid, phenylalanine, to another amino acid, tyrosine.Failure of the conversion to take place results in a buildup ofphenylalanine. Through a mechanism that is not well understood, theexcess phenylalanine is toxic to the central nervous system andcauses the severe problems normally associated with PKU.PKU is carried through an autosomal recessive gene. The incidenceof carriers in the general population is approximately one in fiftypeople, but the chance that two carriers will mate is only one in 2500.Carrier tests are available only through PKU treatment programs.

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Application of HW theorem PKU

frequency of homozygous recessive individuals(q2) = 1 in 10,000 or 0.0001

frequency of recessive allele (q): √ q2 → √0.0001 = 0.01

frequency of dominant allele (p): p = 1 – q → 1 – 0.01 = 0.99

frequency of carriers, heterozygotes (2pq):2 x (0.99 x 0.01) = 0.0198 or ~2%

~2% of the US population carries the PKU allele

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Hardy-Weinberg equilibrium Implications of HW theorem

in H-W population, all alleles remain atthe same frequencies

if allele frequencies change, thenpopulation is not in equilibrium &evolution is occurring

population biologistsmeasure & study sampling of individuals

& genetic testing measure from year to year

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Using H-W theorem Microevolution

generation to generationchange in a population’sallele frequencies

Measuring changes inpopulation from generation togeneration

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Mutation & sexual recombinationproduce the variation that makesevolution possible

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Mutation Mutation creates variation

new genes & new alleles originate onlyby mutation

only mutations to sex cells can bepassed on

Mutation changes DNA sequence changes amino acid sequence changes protein

change structure? change function? changes in protein may change

phenotype & therefore change fitness most mutations are deleterious

Every individual has hundreds of mutations

1 in 100,000 bases copied3 billion bases in human genomeBut most happen in introns, spacers, junk ofvarious kind

Not every mutation has a visible effect.

Some effects on subtle.May just affect rate of expression of a gene.

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Types of mutations Point mutations

sickle cell anemia Duplications

hemoglobin chains, fetal hemoglobin olfactory receptors immunoglobulins tRNAs & rRNAs

Rearrangements translocations

Beneficial increases in gene number appear to have played a majorrole in evolution. For example, the remote ancestors of mammalscarried a single gene for detecting odors that has been duplicatedthrough a variety of mutational mechanisms. As a result, modernhumans have close to 1,000 olfactory receptor genes, and mice have1,300. About 60% of human olfactory receptor genes have beeninactivated by subsequent mutations, whereas mice have lost only20% of theirs—a remarkable demonstration that a versatile sense ofsmell is more important to mice than it is to us!

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Sexual recombination Sex spreads variation

sex causes recombination segregation & independent assortment

offspring have new combinations oftraits = new phenotypes

Sexual reproduction recombinesalleles into new arrangementsin every offspring

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Selection & Variation Natural selection requires a source of

variation within the population there have to be differences some individuals are more fit

than others Genetic variation is

the substrate fornatural selection

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Types of selection The effect of selection depending on

what is “fit”

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Directional selection

Directional selection for beaksize in Galápagos populationof medium ground finchDrier years = thicker shelledseeds = select stronger billedbirds

Environment favors one extreme

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Diversifying selection

small billedsoft seeds

large billedhard seeds

Environment favors extremes

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Variation Discrete vs. quantitative characters

red vs. white flower color = discrete human height = quantitative

Polymorphic morphs

distinct types in a population

Geographic variation clines

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Polymorphic

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ClinesPlant height varies with altitude, but stillsame population

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Preserving variation Diploidy

genetic variation— even lethal alleles—are hidden in heterozygotes

Balancing Selection balanced polymorphism

maintaining 2 or more phenotypesthrough selection

heterozygote advantage frequency-dependent selection

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Heterozygote advantage heterozygotes have a greater fitness

maintain both alleles in population sickle cell anemia

heterozygotes areprotected severesteffects of malaria &do not develop sicklecell disease

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Frequency-dependent selection Fitness of any morph

decrease if itbecomes toocommon selection against

more abundantphenotype

consider action ofboth predators &parasites

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Sexual selection Natural selection for mating success

competition amongst males for females ritual displays & battles between males

female choice courtship

displays toattract females

Blue Footed Boobycourtship display

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Female choice rules animal kingdom!

Sexual dimorphism

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Males may go to extremes

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How can such a male evolve?

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Limitations of Natural Selection Natural selection cannot fashion perfect

organisms evolution is limited by genetic constraints

legacy of ancestral genes existing variations may not be ideal

adaptations are often compromises adaptation for one situation may be limitation

for another chance & natural selection interact

the founders may not be the fittest

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