Problem of the day

Imagine a locus with three alleles (A1,A2,A3). The Imagine a locus with three alleles (A1,A2,A3). The frequency of alleles are summarized by p, q, frequency of alleles are summarized by p, q, and r, respectively (i.e., the frequency of allele and r, respectively (i.e., the frequency of allele A1 is p).A1 is p).

The expected frequency of A1A3 heterozygotes is The expected frequency of A1A3 heterozygotes is (at Hardy Weinberg equilibrium):(at Hardy Weinberg equilibrium):

A)A) (p2 * r2) / 2pr(p2 * r2) / 2pr B) 2*p*r C) 2*p*q*r D) 2*p*qB) 2*p*r C) 2*p*q*r D) 2*p*q

E) We are not able to figure that out given the information E) We are not able to figure that out given the information providedprovided

Problem of the dayThe expected frequency of A1A3 heterozygotes is The expected frequency of A1A3 heterozygotes is

(at Hardy Weinberg equilibrium):(at Hardy Weinberg equilibrium):

A)A) (p2 * r2) / 2pr(p2 * r2) / 2pr B) 2*p*rB) 2*p*r C) 2*p*q*r D) C) 2*p*q*r D) 2*p*q2*p*q

E) We are not able to figure that out given the E) We are not able to figure that out given the information providedinformation provided

p q r

p

q

r

Problem of the day• A population is at Hardy – Weinberg

equilibrium for two traits, each with two alleles. For the first trait (A/a), p = 0.9, q=0.1. For the second trait (B/b), p = 0.4, q=0.6 The traits sort independently. What is the frequency of individuals that are homozygous recessive for both traits (aabb)?

• 1) 0.1, 2) 0.01, 3) 0.36, 4) 0.036, 5) 0.0036• 6) There is not sufficient information to figure

this out

Problem of the day• A population is at Hardy – Weinberg

equilibrium for two traits, each with two alleles. For the first trait (A/a), p = 0.9, q=0.1. For the second trait (B/b), p = 0.4, q=0.6 The traits sort independently. What is the frequency of individuals that are homozygous recessive for both traits (aabb)?

• 1) 0.1, 2) 0.01, 3) 0.36, 4) 0.036, 5) 0.0036• 6) There is not sufficient information to figure

this outBreaking the problem down: A) the probability of aa is Breaking the problem down: A) the probability of aa is q^2 = 0.01. B) The probability of bb is q^2 = 0.36. C) The q^2 = 0.01. B) The probability of bb is q^2 = 0.36. C) The probability of both events happening is 0.01 * 0.36 = probability of both events happening is 0.01 * 0.36 = 0.00360.0036

Population Genetics: Drivers of EvolutionPopulation Genetics: Drivers of Evolution

MutationMutation Rare on a per locus basisRare on a per locus basisOften deleteriousOften deleteriousPoint mutation or chromosomalPoint mutation or chromosomalIntegral for evolutionIntegral for evolution

Gene flowGene flowMovement of alleles into or out of a Movement of alleles into or out of a population (populations are not closed)population (populations are not closed)

Genetic DriftGenetic DriftFounder effectsFounder effectsPopulation BottlenecksPopulation Bottlenecks

Non-Random MatingNon-Random MatingPositive assortative mating (inbreeding)Positive assortative mating (inbreeding)Negative assortative mating (outbreeding)Negative assortative mating (outbreeding)

Causes of EvolutionCauses of EvolutionBottleneck Hall of FameBottleneck Hall of Fame

Jan 2009Jan 2009

Both American and European Bison Both American and European Bison suffered severe population bottleneckssuffered severe population bottlenecks

Feb 4: Natural Selection Feb 4: Natural Selection

Types of natural selection, including...Types of natural selection, including...1.1. stabilizing selectionstabilizing selection2.2. directional selectiondirectional selection3.3. disruptive selectiondisruptive selection4.4. balancing selectionbalancing selection5.5. frequency-dependent selectionfrequency-dependent selection6.6. sexual selectionsexual selection7.7. kin selection (and the evolution of cooperation)kin selection (and the evolution of cooperation)

Causes of EvolutionCauses of Evolution MutationMutation Gene flowGene flow Genetic driftGenetic drift Non-random matingNon-random mating Natural SelectionNatural Selection

1.1. ____________________________: : The differential contribution of offspring to The differential contribution of offspring to the next generation by various genetic types belonging to a the next generation by various genetic types belonging to a populationpopulation

2.2. Natural selection acts on __________, Natural selection acts on __________,

3.3. ________________ is the is the relativerelative reproductive contribution of a reproductive contribution of a phenotype to subsequent generationsphenotype to subsequent generations

4.4. Changes in the Changes in the ______________________ success of different HERITABLE success of different HERITABLE aspects of phenotypes in a population leads to changes in aspects of phenotypes in a population leads to changes in the frequencies of the alleles that contribute to a the frequencies of the alleles that contribute to a phenotype (i.e. evolution by natural selection)phenotype (i.e. evolution by natural selection)

Causes of Evolution, Terminology REVIEWCauses of Evolution, Terminology REVIEW

Evolution, Phenotypes (or individuals), Fitness, RelativeEvolution, Phenotypes (or individuals), Fitness, Relative

The ConsequenceThe Consequence

Natural selection increases Natural selection increases

the the frequencyfrequency, across generations, , across generations,

of of individualsindividuals with with more advantageous more advantageous allelesalleles (or genotypes) (or genotypes)

relative to individualsrelative to individuals with less with less advantageous alleles or genotypesadvantageous alleles or genotypes

Studying Natural Selection:Studying Natural Selection:

If the trait favored by selection is discrete and controlled by one or If the trait favored by selection is discrete and controlled by one or a few alleles—like flower color or seed color in peas—then we can a few alleles—like flower color or seed color in peas—then we can examine evolution by studying examine evolution by studying the change in ________________ the change in ________________ over timeover time

But many traits are But many traits are ________________________, and controlled by many genes , and controlled by many genes (i.e. (i.e. ______________________) and also affected by the environment) and also affected by the environment

Here we look at the change in Here we look at the change in the phenotypic distribution of the the phenotypic distribution of the population over timepopulation over time

We will now examine the types of natural selection, and how they We will now examine the types of natural selection, and how they act on quantitative traitsact on quantitative traits

Causes of Evolution, E. Natural SelectionCauses of Evolution, E. Natural Selection

Allele frequencies, quantitative, polygenicAllele frequencies, quantitative, polygenic

Height (feet)Height (feet)

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A Histogram showing A Histogram showing a normal distribution of a normal distribution of heightheight

Mean = 5.6Mean = 5.6

Human height Human height is a is a polygenicpolygenic and and quantitative quantitative charactercharacter

Causes of Evolution, Natural SelectionCauses of Evolution, Natural SelectionHuman height as a heuristicHuman height as a heuristic

Imagine that fitness Imagine that fitness is all about airplane is all about airplane travel. travel.

natural selection natural selection selects against selects against everyone over 6’ (can’t everyone over 6’ (can’t fit in seat) and fit in seat) and everyone under 5.3’ everyone under 5.3’ (can’t get bag in (can’t get bag in overhead overhead compartment)?compartment)?

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II. Causes of Evolution, E. Natural SelectionII. Causes of Evolution, E. Natural Selection

Imagine that fitness Imagine that fitness is all about Cooper is all about Cooper Mini’sMini’s

Natural selection Natural selection strongly disfavors strongly disfavors those over 5.75’?those over 5.75’?

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II. Causes of Evolution, E. Natural SelectionII. Causes of Evolution, E. Natural Selection

Imagine that fitness is Imagine that fitness is all standing out in a all standing out in a crowd. crowd.

What if natural What if natural selection disfavored selection disfavored the middle—everyone the middle—everyone over 5.42’ and under over 5.42’ and under 5.83’?5.83’?

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II. Causes of Evolution, E. Natural SelectionII. Causes of Evolution, E. Natural Selection

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1. Stabilizing selection1. Stabilizing selection5

.00

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3. Disruptive selection3. Disruptive selection Diversifying selectionDiversifying selection

___________ ___________ favors average phenotype favors average phenotype

__________ __________ favors individuals that are EITHER above favors individuals that are EITHER above OR below the average phenotypeOR below the average phenotype

___________ ___________ favors individuals that are BOTH above favors individuals that are BOTH above AND below the averageAND below the average

Causes of Evolution, 3 common types of Natural Selection Causes of Evolution, 3 common types of Natural Selection based on directionality with quantitative traitsbased on directionality with quantitative traits

Stabilizing selectionStabilizing selection::

1.1. favors individuals with the average phenotypefavors individuals with the average phenotype

2.2. reduces phenotypic variance for that traitreduces phenotypic variance for that trait

Stabilizing Selection does not significantly change the mean; it Stabilizing Selection does not significantly change the mean; it reduces the phenotypic variancereduces the phenotypic variance

Phenotypic trait (z)Phenotypic trait (z)

Rel

ativ

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itnes

sR

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ive

Fitn

ess

FITNESS FUNCTIONFITNESS FUNCTION

before selectionbefore selectionafter selectionafter selection

freq

uenc

yfr

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Phenotypic trait (z)Phenotypic trait (z)

RESPONSE FUNCTIONRESPONSE FUNCTION

Causes of Evolution, Natural SelectionCauses of Evolution, Natural SelectionStabilizing SelectionStabilizing Selection

CASE STUDY: Stabilizing selection on human birth weightCASE STUDY: Stabilizing selection on human birth weight

Infants pre-mature Infants pre-mature

Infants can’t Infants can’t get out!get out!

““Many obstetricians and some midwives recommend inducing labor if you are near or at full-term, and they Many obstetricians and some midwives recommend inducing labor if you are near or at full-term, and they think the baby is larger than average -- macrosomia, literally, “big body.” Typically, they use an estimation think the baby is larger than average -- macrosomia, literally, “big body.” Typically, they use an estimation that the baby weighs or will soon weigh 4,000 grams (8 lbs. 13 oz.) as the threshold.”that the baby weighs or will soon weigh 4,000 grams (8 lbs. 13 oz.) as the threshold.”

fromfrom parenting.ivillage.comparenting.ivillage.com

Causes of Evolution, Natural SelectionCauses of Evolution, Natural SelectionStabilizing SelectionStabilizing Selection

March, 2010March, 2010

Directional selectionDirectional selection::

1.1. shifts the mean value of a trait shifts the mean value of a trait

2.2. usually reduces phenotypic variance for that traitusually reduces phenotypic variance for that trait

Causes of Evolution, Natural SelectionCauses of Evolution, Natural SelectionDirectional SelectionDirectional Selection

FITNESS FUNCTIONFITNESS FUNCTION

Directional Selection favors more _______________ individuals Directional Selection favors more _______________ individuals (e.g. largest or smallest) and leads to a change in mean(e.g. largest or smallest) and leads to a change in mean

after after selectionselection

RESPONSE FUNCTIONRESPONSE FUNCTION

before selectionbefore selectionafter

CASE STUDY Directional selectionCASE STUDY Directional selection in the peppered moth, in the peppered moth, Biston betularia H.B.D. KettlewellBiston betularia H.B.D. Kettlewell

““typica” formtypica” form ““carbonaria” formcarbonaria” form

Causes of Evolution, Natural SelectionCauses of Evolution, Natural SelectionDirectional SelectionDirectional Selection

0019591959

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% M

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% M

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amount of soot

Directional selection in peppered moths: as soot declines so does Directional selection in peppered moths: as soot declines so does the dark (carbonaria) morphsthe dark (carbonaria) morphs

Directional Directional selection acting selection acting over many over many generations can generations can lead to lead to major major evolutionary evolutionary changechange

Causes of Evolution, Natural SelectionCauses of Evolution, Natural SelectionDirectional SelectionDirectional Selection

Figure 22.12 Natural Selection Can Operate on Quantitative Variation in Several Ways

If individuals at both extremes have higher fitness…

Phenotypic trait (z)

Fit

ness

Disruptive/diversifyingDisruptive/diversifying selectionselection favors individuals favors individuals at the extremes of a phenotypic distribution.at the extremes of a phenotypic distribution.

…variation in the population is increased, and a bimodal pattern may result.

Phenotypic trait (z)

BEFOREBEFORE AFTERAFTER

Fre

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Disruptive/diversifyingDisruptive/diversifying selection selection … …

1.1. has little effect on the mean value of a trait has little effect on the mean value of a trait

2.2. usually INCREASES phenotypic variance for that traitusually INCREASES phenotypic variance for that trait

3.3. maintains genetic variation in a population (it is a form of maintains genetic variation in a population (it is a form of BALANCING SELECTION).BALANCING SELECTION).

Disruptive selection: individuals at either extreme are more successful than average individuals.

CASE Study: Disruptive selection in CASE Study: Disruptive selection in black-bellied seed crackersblack-bellied seed crackers

Soft, small seedsSoft, small seeds

Large, hard Large, hard seedsseeds

Smith 1993EastEastAfricaAfrica

Disruptive selection results in a bimodal Disruptive selection results in a bimodal phenotypic distribution & maintains genetic phenotypic distribution & maintains genetic variation.variation.

Fitness functionFitness function

Magalhaes IS, Mwaiko S, Schneider MV, et al.2009. Divergent selection and phenotypic plasticity during incipient speciation in Lake Victoria cichlid fish Journal Of Evolutionary Biology  22: 260-274.

Hendry AP, Huber SK, De Leon LF, et al.2009. Disruptive selection in a bimodal population of Darwin's finches. Proceedings Of The Royal Society B-biological Sciences  276: 753 –759 Published: FEB 22 2009

Character displacementin mustelids Mustellids Mustellids

appear to appear to have have communities communities structured by structured by competition. competition. The diameter The diameter of their upper of their upper canine canine predicts a predicts a more regular more regular distribution distribution than than expected by expected by chancechance

Dayan & Simberloff ‘94

Balancing Selection 2Balancing Selection 2: Heterozygote Advantage/Heterosis: Heterozygote Advantage/HeterosisWarfarin resistance in Norway ratsWarfarin resistance in Norway rats

•Vitamin K complexes with Vitamin K complexes with coagulants in the blood to stop coagulants in the blood to stop bleeding. bleeding.

•Warfarin is an anticoagulant/blood Warfarin is an anticoagulant/blood thinner that prevents vitamin K thinner that prevents vitamin K from complexing with coagulants from complexing with coagulants in the blood.in the blood.

•Non-resistant rats that eat Non-resistant rats that eat warfarin bleed to death.warfarin bleed to death.

•In 1959, warfarin-resistant rats In 1959, warfarin-resistant rats appeared in rat populations in the appeared in rat populations in the United KingdomUnited Kingdom

•Warfarin resistance conferred Warfarin resistance conferred by a single mutation at the W-by a single mutation at the W-locus:locus:

WWss W Wrr

•WWssWWss homozygotes homozygotes susceptiblesusceptible•WWssWWrr heterozygotes resistant heterozygotes resistant•WWrrWWrr homozygotes resistant homozygotes resistant

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Q: Why didn’t the warfarin resistance allele (WQ: Why didn’t the warfarin resistance allele (Wrr) go to fixation?) go to fixation?

A: WA: WrrWWss heterozygotes have higher fitness than either heterozygotes have higher fitness than either homozygotehomozygote. .

Figure 22.12 Natural Selection Can Operate on Quantitative Variation in Several Ways

GenotypeGenotype Resistance to warfarin Resistance to warfarin Vitamin K Vitamin K

requirement requirement

WsWs susceptible 1x

WsWr resistant 2-3x

WrWr resistant 20x

A: WA: WrrWWss heterozygotes have higher fitness than either heterozygotes have higher fitness than either homozygote. homozygote.

Here’s why WHere’s why WrrWWss heterozygotes have higher fitness than either heterozygotes have higher fitness than either homozygote.homozygote.

1.1. Rats need vitamin K in their food, and vitamin K is Rats need vitamin K in their food, and vitamin K is almost always in short dietary supply.almost always in short dietary supply.

2.2. The allele that confers warfarin resistace (WThe allele that confers warfarin resistace (Wrr) also makes ) also makes it more difficult to absorb dietary vitamin K.it more difficult to absorb dietary vitamin K.

3.3. So, inSo, individuals carrying the Wdividuals carrying the Wrr allele need LOTS more allele need LOTS more vitamin K in their diets than individuals with the Wvitamin K in their diets than individuals with the Wss allele.allele.

Heterozygote advantage & warfarin Heterozygote advantage & warfarin resistance in Norway rats: the data resistance in Norway rats: the data please…please…

WsWs WsWr WrWr TOTAL

H-W Expected Frequencies

0.115 0.4480.448 0.437 1.000

ObservedFrequencies

0.054 0.5680.568 0.378 1.000

Observed f(WObserved f(Wss) = ) = 0.3390.339Observed f(WObserved f(Wrr) = 0.661) = 0.661

There are 12% more There are 12% more heterozygotes than expected at H-heterozygotes than expected at H-W equilibrium.W equilibrium.

Frequency-dependent selection in scale-eating cichlidsFrequency-dependent selection in scale-eating cichlids

Hori (1993) Science 260: 216

Perissodus microlepisPerissodus microlepis

Left-mouthed Left-mouthed P. P. microlepismicrolepis attack attack right flank of preyright flank of prey

Right-mouthed Right-mouthed P. microlepisP. microlepis attack left flank attack left flank of preyof prey

Sample YearSample Year

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Positive versus negative frequency dependence

• Positive frequency dependence– The more common a particular trait, the more

the trait is favored

• Negative frequency dependence– The less common a particular trait, the more

the trait is favored.

Negative Frequency Dependent Selection

Gamoetophytic self-incompatibility genesGamoetophytic self-incompatibility genes

Almonds (and many, many other plants)Almonds (and many, many other plants)

An agricultural problem

• Gametophytic self-incompatibility allows individuals to avoid selfing and maintains high genetic variability.

• S-RNase allelic diversity can be high (7-15 different alleles).

• With many clones of the same tree, even if it grows well, it will not produce almonds because of gametophytic self-incompatibility.

• Now, imagine a wild population. If one genotype has the highest fitness, then it may spread. However, fitness can be reduced by increasing the frequency of inbreeding and self-incompatibility genes being expressed. This fosters the maintenance of variability.

Positive Frequency Dependent Selection

Snail chiralitySnail chiralitySpeciation and gene flow Speciation and gene flow between snails of opposite between snails of opposite chirality. Davison et al. PLoS chirality. Davison et al. PLoS Biology 2005.Biology 2005.

Japanese Land Snails, Euhadra (not pictured).

Snails of opposite chirality cannot interbreed.

Positive frequency dependent selection could drive one or the other extinct by initial chance events.

A word about Heritability

• Heritability is mentioned in your lab manual.– The proportion of phenotypic variability that is

attributable to genetic variation.– As an equation:

– Phenotype (P) = Genotype (G) + Environment (E). – Var(P) = Var(G) + Var(E) + 2 Cov(G,E).

Natural Selection

• http://www.youtube.com/watch?v=R_RXX7pntr8