Natural Selection n Selection is the only evolutionary mechanism that leads to nonrandom changes in allele frequencies. n Natural selection can increase.

Natural Selection

Selection is the only evolutionary mechanism that leads to nonrandom changes in allele frequencies.

Natural selection can increase or decrease the amount of genetic diversity in a population.

Natural Selection

Types of natural selection:

• Directional selection decreases diversity by favoring one extreme phenotype in the population.

• Stabilizing selection decreases diversity by favoring the average phenotype over both extremes.

• Disruptive selection increases diversity by favoring both extremes, and selecting against the average.

After selection

During selectionN

um

ber

of

ind

ivid

ual

s

Before selection

Normal distribution

Directional selection changes the average value of a trait.

Value of a trait

Figure 22.7a

Body size class

Per

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tag

e o

f b

ird

s

40

35

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0

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Difference in

average

1 2 3 4 5 7 8 9 10 11 126

SurvivorsN = 1027

NonsurvivorsN = 1853

For example, directional selection caused overallbody size to increase in a Cliff Swallow population

Figure 22.7b

Normal distribution

High fitness

Value of a trait

Nu

mb

er o

f in

div

idu

als

After selection

During selection

Before selection

Stabilizing selection reduces the amount of variation in a trait.

Figure 22.8a

20

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1 2 3 4 5 6 7 8 9 10 11

2

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Birthweight (pounds)

Percen

tage o

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rtalityP

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of

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Heavymortalityonextremes

Mortality

For example, very small and very large babies are mostlikely to die, leaving a narrower distribution of birthweights.

Figure 22.8b

Goldenrod Spherical Gall

The gall fly lays an egg near the apical meristem of a goldenrod

The larva hatches from the egg and burrows into the goldenrod at the apical meristem

Secretions from the larva induce the formation of the gall by the goldenrod plant

The larva overwinters inside the gall, emerging the following spring

The Spot-winged Fly larvae have two types of enemies

Parasitoid wasps

These wasps oviposit an egg through the wall of the gall

The wasp larva hatches and then consumes the gall fly larva

Birds are also enemies of the gall fly larvae

Stabilizing selection is observed for the diameter of the galls

Galls that are thin-walled are subject to wasp parasitism

Galls that are thick-walled are subject to bird predation

Selection favors the galls of intermediate diameter

• Thick enough to discourage wasp oviposition

• Thin enough to make bird predation difficult

Value of a trait

Low fitness

Normal distribution

Before selection

During selection

After selection

Nu

mb

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f in

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Disruptive selection increases the amount of variation in a trait.

Figure 22.9a

6 7 11108 9

Beak length (mm)

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For example, only juvenile Black-bellied Seedcrackers with very longor very short beaks survived long enough to breed.

Figure 22.9b

Figure 16.18

Song Thrush

Cepaea nemoralis

Cepaea nemoralis