natural selection n selection is the only evolutionary mechanism that leads to nonrandom changes in...
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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
cen
tag
e o
f b
ird
s
40
35
30
25
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10
5
0
40
35
30
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5
0
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
15
10
5
0
1 2 3 4 5 6 7 8 9 10 11
2
3
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Birthweight (pounds)
Percen
tage o
f mo
rtalityP
erce
nta
ge
of
Po
pu
lati
on
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
er o
f in
div
idu
als
Disruptive selection increases the amount of variation in a trait.
Figure 22.9a
6 7 11108 9
Beak length (mm)
10
0
20
30
Nu
mb
er o
f in
div
idu
als
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
