chapter 23. population genetics - explore...
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2004-2005AP Biology
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 ¶sites
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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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