10-22 Lecture Notes.pdf

7/30/2019 10-22 Lecture Notes.pdf

1/12

u a o nan

ene c

r

o ay sques ons:

I.How oesmutat oncausec anges na e e

frequencies?. ow oesgene c r causec anges na e e

frequencies?


2/12

I. Howdoesmutationchangeallelefrequencies?

A.

How

often

do

mutations

occur?DatafromArabidopsisthaliensis (amustard).Theycanself.Experimental

design:

Start

with

1individual;

let

it

grow

in

optimumconditions soil,light,moisture,space andself

fertilize.

Choose1seedatrandomtobethenextgenerationletit

row choose1seedatrandometc.totalof30

generations5lines(replicates)


3/12

Sequencetheentiregenomeatgenerations1and30

.

w

u

Estimatean

average

of

7x109 mutations

per

base

per

eneration

Haploid

genome

size

is

~1.2

x

10

8

bases

1.Whatistheaverage#ofmutations/haploidgenome?

2.Whatisaveragenumberofmutations/seed?

You multiply the above two #s will give you about .84 mutations per

haploid genome.

Since there are two haploid in the diploid, you can say it is twice the #

of mutations will give you 1.68 mutations.


4/12

Takehomemessages:

. a ana as , genes, enmu a onc angesallele

frequencies

~4.2

x103%/generation

3.Howmanygenerationswouldittaketocreatea1%

changein

aparticular

gene?

Important:By

itself,

mutations don't necessary cause evolution, it helpscreates new alleles for selection.


5/12

BUT

erearenew

Important:Mutationconstantlycreates new allele.


6/12

.

w

u

Prediction:

Datafrom

an

experiment

on

C.elegans (similarsetupto

A.thaliana=mutationaccumulation)Control

lines

(large n

,

competition,lotsof

selection) Lifespa

Experimentallines

no selection

mutationsaccumulate)

Time


7/12

II.Howdoesgeneticdriftchangeallelefrequencies?

A.Drifthappens:ChancevariationinRSsamplingerror...luck

Experiment1:Startingpopulationsize=2

om: H T a : H T

= =

Kids

F s#1: #3: #5:

#2:

#4:

#6:

H T


8/12

Experiment2:Startingpopulationsize~500

oms: H T a s: H Tfreq

(AH) = freq(AT)=

Flipacoinoruse2littlepiecesofpaperonewithH

andone

with

Tto

make

your

F1 offspring

Clickers: freq(AH) =

2. AHAT3. ATAT

freq(AT)=


9/12

Punchlines:

s r more mpor an nsma popu a onsor arge

populations?

,

generation(especiallyinsmallpopulations)?

It's effect is stronger in smaller populations compared to bigger

populations. This is due to sampling error.

Loss (fr=0) or fixation (fr=1) of an allele.


10/12

Man

eventscan

cause

drift

DuringtheVietNamwar,U.S.soldiersboardedahelicopterand

cameunderfireafterliftingoff.Oneguygothitandkilled.

,

asmallgroupofG.magnirostris arriveonDaphneMajorandstartanew

population.

Whenwhites

brought

smallpox

and

measles

to

North

America,

up

to90%ofNativeAmericansinsomecommunitiesdied.

Small effect: but the alleles don't get passed on.

Only 10% of the allele remains, there was a certain randomness to which

Native Americans that die. However, they were also under selection for

disease resistant allele. Genes not under selection were subject todrift.

Small sample from source population, founder effect.


11/12

Considerthefollowingexperiment:100fruitfly

popu at ons

nt e

a ,

eac

cons st ng

o

ema es

an

males.Inthisstartingpopulation,allelesA1 andA2 areeach

. .

Letthembreed.Randoml ick4femaleand4male

offspringtoformthenextgenerationineachpopulation.

Note:this

is

aTINY

population.

Repeatforatotalof16generations,ineachofthe100

populations.

alleleA1 atvariousfrequencies,from0(lost)to1.0(fixed).


12/12

C.Consequences

of

genetic

drift

. ,

2.With

respect

to

genetic

variation,

drift

3.Whyisdriftaconcernforendangeredspecies?

It is random with respect to fitness. Alleles get lucky, whether they arebeneficial or deleterious.

It lowers overall genetic variation.

Because the population is smaller, so they're more susceptible to sampling

error.

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10-22 Lecture Notes.pdf