Chapter 23Chapter 23The Evolution of The Evolution of

PopulationsPopulations

Population GeneticsPopulation Genetics

Darwin and Mendel Darwin and Mendel – population geneticspopulation genetics

Gene pool and Allele frequency Gene pool and Allele frequency – fixed fixed – HeterozygousHeterozygous– Mind your p’s and q’s!Mind your p’s and q’s!

LE 23-4

Generation3 25% CRCR

Generation4

50% CRCW 25% CWCW

50% CW

gametes50% CR

come together at random

25% CRCR 50% CRCW 25% CWCW

Alleles segregate, and subsequentgenerations also have three typesof flowers in the same proportions

gametes

Generation2

Generation1

CRCR CWCW

genotypegenotypePlants mate

All CRCW

(all pink flowers)

50% CR 50% CW

gametes gametes

come together at random

X

Calculate allele frequencies:

500 total flowers

320 red

160 pink

20 white

Freq CR:

FreqCW:

The Hardy-Weinberg TheoremThe Hardy-Weinberg Theorem

Allele frequencies in a population Allele frequencies in a population remain unchangedremain unchanged– Basis for understanding long-term Basis for understanding long-term

evolutionary changesevolutionary changes H-W equilibrium – pH-W equilibrium – p22 + 2pq + q + 2pq + q22 = 1 = 1

– p + q = 1p + q = 1

LE 23-5Gametes for each generation are

drawn at random from the gene poolof the previous generation:

80% CR (p = 0.8) 20% CW (q = 0.2)

SpermCR

(80%)CW

(20%)

pqp2

16%CRCW

64%CRCR

Eg

gs

CW

(20%

)C

R

(80%

)

16%CRCW

qp4%

CWCW

q2

Conditions for Hardy-WeinbergConditions for Hardy-Weinberg

Large population sizeLarge population size No gene flowNo gene flow No mutationsNo mutations Random matingRandom mating No natural selectionNo natural selection

Sources of VariationSources of Variation

MutationsMutations– Point mutationPoint mutation– Alter gene number or sequenceAlter gene number or sequence

duplicationduplication

– Mutation ratesMutation rates Sexual recombinationSexual recombination

Altering Populations Gene Pool: Altering Populations Gene Pool: The Big PlayersThe Big Players

Natural selectionNatural selection Genetic DriftGenetic Drift

– Bottleneck effectBottleneck effect– Founder effectFounder effect

Gene flowGene flow

LE 23-7

CRCR CRCR CWCW CRCR

CRCW

CRCR

CRCW

CWCW

CWCW

CRCW CRCW

CRCRCRCW

CRCWCRCR

CRCR

CRCW

CWCW

CRCW

CRCR

Only 5 of10 plantsleaveoffspring

Only 2 of10 plantsleaveoffspring

CRCR

CRCR

CRCR CRCR

CRCR

CRCR CRCR

CRCR

CRCR

CRCR

Generation 2p = 0.5q = 0.5

Generation 3p = 1.0q = 0.0

Generation 1p (frequency of CR) = 0.7q (frequency of CW) = 0.3

LE 23-8

Originalpopulation

Bottleneckingevent

Survivingpopulation

Natural Selection and Adaptive Natural Selection and Adaptive EvolutionEvolution

Genetic variationGenetic variation– PolymorphismPolymorphism– Geographic variationGeographic variation

Closer look at Natural SelectionCloser look at Natural Selection– Evolutionary fitnessEvolutionary fitness– Types of selectionTypes of selection

Preservation of genetic variationPreservation of genetic variation Sexual SelectionSexual Selection

LE 23-10

1 2.4 3.14 5.18 6 7.15

8.11 9.12 10.16 13.17 19 XX

1

9.10 11.12 13.17 15.18 XX

2.19 3.8 4.16 5.14 6.7

LE 23-11Heights of yarrow plants grown in common garden

Sierra NevadaRange

Great BasinPlateau

Seed collection sites

100

50

0

3,000

2,000

1,000

0

Mea

n h

eig

ht

(cm

)A

lti t

ud

e (m

)

LE 23-12a

Original population

Phenotypes (fur color)

Fre

qu

ency

of

ind

ivid

ual

s

LE 23-12b

Originalpopulation

Evolvedpopulation

Directional selection Disruptive selection Stabilizing selection

LE 23-13

Frequencies of thesickle-cell allele

0–2.5%

2.5–5.0%

5.0–7.5%

7.5–10.0%

10.0–12.5%

>12.5%

Distribution ofmalaria caused byPlasmodium falciparum(a protozoan)

LE 23-14

Parental population sample

Experimental group sample

On pecking a mothimage the blue jayreceives a food reward.If the bird does notdetect a moth oneither screen, it pecksthe green circle tocontinue a new setof images (a newfeeding opportunity).

Plain background Patterned background

0.6

0.5

0.4

0.3

0.2

0 20 40 60 80 100

Generation number

Frequency-independent control

Ph

eno

typ

icva

riat

ion