10-17 Lecture Notes

7/30/2019 10-17 Lecture Notes

1/15

a ura

e ec on

ev s e

o ay sques ons:

I. Howcanweapp yt eHar yWe n erg

principle?

. ow oesna ura se ec onac onquan a ve

variation?


2/15

Conclusions,underHardyWeinbergconditions:

andq,

then

the

genotype

frequencies

are

given

by

p2,

2pq,andq2.Iftheallelefrequenciesinapopulationaregivenbyp, q,

andr,

then

the

genotype

frequencies

are

given

by

2,

2pq,2pr,2qr,q2, andr2.(Andsoon,forasmanyallelesasarepresent.)

Whenallindividualsinthepopulationbreedandproduce

thesamenumberofoffs rin theallelefre uenciesinthe

populationwillnotchange,generationaftergeneration.

(Theyremainatp ,q,andrforever.)


3/15

TheHardyWeinbergprinciplefurnishesanullmodelin

populationgenetics.

Itassumesthat,withrespecttothegeneinquestion,

thereis:

No

selection

Nogeneflow

No eneticdrift

Randommating


4/15

So,weneedtoshiftourthinking

1.Evolution=achangeinallelefrequencies

2.Whenyouworkonpopulationgenetics,youneedto

thousandsofmatings inapopulationNOTthe

frequenciesofgenotypesfromaparticularmating.

changetopopulationthinking

Think about "everybody" now.


5/15

We survey 100 people. We determine genotype at the MN blood type locus.

We get:

-MM: 18%

-MN: 50%

-NN: 32%

What is the observed frequency of M and N alleles.

fr (M)= 18% +1/2(50%)=43%=.43

fr (N)=1-.43=.57

Given the observed allele frequency, what do we expect to happen under the

HWE (Hardy-Weinberg Principle) conditions?

MM: .185

MN: .49

NN: .325

The ratios are very similar (within 5%), so we therefore can conclude that

there is no significant difference. These are the null.


6/15

AA: .37

Aa: .33

aa: .30

fr

(A) .37

1/2*.33

=.465

fr

(a)=.

535


7/15

I.Howdoesselectionactonquantitativetraits?

.Peter and Rosemary Grantexperiment.


8/15

1.

What

is

directional

about

directional

selection?

2.Whathappenstooverallvariation?

3.Doallelefrequencieschange?Explain.

4. If drou ht conditions continued would overall

variationeventuallyNOTbenormallydistributed?

5.IftheenvironmentchangedandNONEofthealleles

,

One side of the normal distribution is selected for, and the graph

will shift toward that direction as the average does. Mean of trait

of population shift to one direction.

The overall genetic variation will decrease.

Yes, alleles that allows for fitness will be selected for.

No, the normal distribution will remain. Even with reduced

variation, the mutation, recombinants, and independent assortment

will create enough variation. Hard for polygenic trait not to benormally distributed.

Extinction.


9/15

Longtermchangesinthispopulation:

6.What

happenedhere?

7.and

here?

.an

ere

Extreme conditions, lots of

droughts.

High rainfall.

Extreme conditions again?


10/15

B.Stabilizingselection

Study

organism:

Mayflies

males

form

mating

swarms.

Femalesflyintotheswarms,copulateintheair,and

t en ayt e reggs nwaternear y.

malesthatcopulatesuccessfully.


11/15

Thedata:blackbars=allmales

openbars=copulatingmales


12/15

Upshot:

Iftheenvironmentisnotchangingverymuchovertime,

whatpatternofselectionareyoulikelytosee?

Iftheenvironmentischangingdramaticallyovertime,

whatpatternofselectionareyoulikelytosee?


13/15


14/15


15/15

P-Value: Probability of observing your data by chance.

-Low p-value (< or = to .05): Very low probability of observing this by

chance-->no support for null.

R^2: Only in a linear regression, looking at two continuous variables. It

quantifies % of variation in the dependent variable that is explained by the

independent variable.

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10-17 Lecture Notes