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Chapter 23
The Evolution of Populations
13
• Population: is a localized group of individuals
capable of interbreeding and producing fertile
offspring
• Gene pool: all the alleles for all loci in a
population
• A locus is fixed if all individuals in a population are
homozygous for the same allele
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• Two processes, mutation and sexual reproduction,
produce the variation in gene pools that
contributes to differences among individuals
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14
• Mutation:
Point mutation:
Deletion:
Duplication (addition):
Inversion:
Transposition:
Translocation:
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15
• Gene (allele) frequency:
• Three major factors alter allele frequencies and
bring about most evolutionary change:
– Natural selection
– Genetic drift: a change in gene frequency in a
small population due to chance alone.
– Gene flow
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
15
• Hardy-Weinberg Theory: In a large population, the
frequency of all alleles will remain constant from
generation to generation according to the following
equation:
p2+ 2pq + q2= 1
p2: the frequency of homozygous dominant
individuals
2pq: the frequency of heterozygous individuals
q2: the frequency of homozygous recessive
individuals
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Fig. 23-7-1
80% CR (p = 0.8)
20% CW (q = 0.2)
Sperm
CR
(80%)
CW
(20%)
64% (p2)
CRCR
16% (pq)
CRCW
16% (qp)
CRCW
4% (q2)
CW CW
Fig. 23-7-4
20% CW (q = 0.2)
80% CR ( p = 0.8)
Sperm
(80%)
CW
(20%)
64% ( p2)
CR CR
16% ( pq)
CR CW
CR
16% (qp)
CR CW
4% (q2)
CW CW
64% CR CR, 32% CR CW, and 4% CW CW
Gametes of this generation:
64% CR + 16% CR
= 80% CR = 0.8 = p
4% CW
= 20% CW = 0.2 = q
+ 16% CW
Genotypes in the next generation:
64% CR CR, 32% CR CW, and 4% CW CW plants
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• Conditions for Hardy-Weinberg Equilibrium
• 1. The population must be large
• 2. There is no gene flow
• 3. There is no mutation
• 4. There is no natural selection
• 5. Mating is random
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Effects of Genetic Drift: A Summary
17
1. Genetic drift is significant in small populations
2. Genetic drift causes allele frequencies to
change at random
3. Genetic drift can lead to a loss of genetic
variation within populations
4. Genetic drift can cause harmful alleles to
become fixed
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17
• Founder effect: the differences in allele frequencies
in the small founder population as compared to
that in the larger parent population
• Polygenetic:
• Polymorphism:
• Cline: a gradual variation in a trait correlated with
geography.
17
• Evolutionary bottleneck: major change in allelic
frequency in a population due to environmental
crisis.
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Fig. 23-9
Original
population
Bottlenecking
event
Surviving
population
Directional, Disruptive, and Stabilizing Selection 18
• Three modes of natural selection:
– Directional selection: favors individuals at
one end of the phenotypic range
– Disruptive selection: favors individuals at
both extremes of the phenotypic range
– Stabilizing selection: favors intermediate
variants and acts against extreme phenotypes
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Fig. 23-13
Original population
Original
Evolved
population population
(a) Directional selection
Phenotypes (fur color)
(b) Disruptive selection
(c) Stabilizing
selection
Sexual Selection
18
• Sexual selection is natural selection for
mating success
• It can result in sexual dimorphism, marked
differences between the sexes in secondary
sexual characteristics
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19
• Diploidy:
• Polymorphism:
• Balanced polymorphism:
• Neutral variation:
• Sexual dimorphism:
• Sexual selection:
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Fig. 23-15
• Intrasexual selection is competition among
individuals of one sex (often males) for mates
of the opposite sex
• Intersexual selection, often called mate
choice, occurs when individuals of one sex
(usually females) are choosy in selecting their
mates
• Male showiness due to mate choice can
increase a male’s chances of attracting a
female, while decreasing his chances of
survival
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
• How do female preferences evolve?
• The good genes hypothesis suggests that if a
trait is related to male health, both the male
trait and female preference for that trait should
be selected for
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Why Natural Selection Cannot Fashion
Perfect Organisms
19
1. Selection can act only on existing variations
2. Evolution is limited by historical constraints
3. Adaptations are often compromises
4. Chance, natural selection, and the
environment interact
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