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Chapter 23
The Evolution of Populations
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Overview:
• Populations evolve, not individuals
• Natural selection acts on individuals
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• Genetic variations in populations
– Contribute to evolution
Figure 23.1
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Concept 23.1:
• Population genetics provides a foundation for
studying evolution
• Microevolution
– Is change in the genetic makeup of a
population from generation to generation
Figure 23.2
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The Modern Synthesis
• Population genetics
– Is the study of how populations change
genetically over time
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• The modern synthesis
– Integrates Mendelian genetics with the
Darwinian theory of evolution by natural
selection
– Focuses on populations as units of evolution
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Gene Pools and Allele Frequencies
• A population
– Is a localized group of individuals that are capable of
interbreeding and producing fertile offspring
MAP
AREA
•
Fairbanks
Fortymile
herd range
•
Whitehorse
Figure 23.3
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• The gene pool
– Is the total aggregate of genes in a population
at any one time
– Consists of all gene loci in all individuals of the
population
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The Hardy-Weinberg Theorem
• The Hardy-Weinberg theorem
– Describes a population that is not evolving
– States that the frequencies of alleles and
genotypes in a population’s gene pool remain
constant from generation to generation
provided that only Mendelian segregation and
recombination of alleles are at work
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• Mendelian inheritance
– Preserves genetic variation in a population
Generation
1
CW CW
CRCR
genotype
genotype
Plants mate
Generation
2
All CRCW
(all pink flowers)
50% CR
gametes
50% CW
gametes
Come together at random
Generation
3
25% CRCR
50% CRCW
50% CR
gametes
25% CWCW
50% CW
gametes
Come together at random
Generation
4
25% CRCR 50% CRCW 25% CWCW
Figure 23.4
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Alleles segregate, and subsequent
generations also have three types
of flowers in the same proportions
Hardy-Weinberg Equilibrium
• Hardy-Weinberg equilibrium
– Describes a population in which random
mating occurs
– Describes a population where allele
frequencies do not change
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A population in Hardy-Weinberg equilibrium
Gametes for each generation are drawn at random from
the gene pool of the previous generation:
80% CR (p = 0.8)
20% CW (q = 0.2)
Sperm
CR
(80%)
CW
(20%)
p2
Eggs
CR
(80%)
pq
CW
(20%)
p2
64%
CRCR
16%
CRCW
16%
CRCW
qp
4%
CW CW
q2
If the gametes come together at random, the genotype
frequencies of this generation are in Hardy-Weinberg equilibrium:
64% CRCR, 32% CRCW, and 4% CWCW
Gametes of the next generation:
16% CR from
64% CR from
+
CRCW homozygotes
CRCR homozygotes
4% CW from
CWCW homozygotes
+
16% CW from
CRCW heterozygotes
=
80% CR = 0.8 = p
=
20% CW = 0.2 = q
With random mating, these gametes will result in the same
mix of plants in the next generation:
Figure 23.5
64% CRCR, 32% CRCW and 4% CWCW plants
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• If p and q represent the relative frequencies of
the only two possible alleles in a population at
a particular locus, then
– p2 + 2pq + q2 = 1
– And p2 and q2 represent the frequencies of the
homozygous genotypes and 2pq represents
the frequency of the heterozygous genotype
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Conditions for Hardy-Weinberg Equilibrium
• The Hardy-Weinberg theorem
– Describes a hypothetical population
• In real populations
– Allele and genotype frequencies do change
over time
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• The five conditions for non-evolving
populations are rarely met in nature
– Extremely large population size
– No gene flow
– No mutations
– Random mating
– No natural selection
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Concept 23.2:
• Two processes, mutation and sexual
recombination
– Produce the variation in gene pools that
contributes to differences among individuals
and may lead to evolution.
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Mutation
• Mutations
– Are changes in the nucleotide sequence of DNA
– Cause new genes and alleles to arise
Figure 23.6
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Point Mutations
• A point mutation
– Is a change in one base in a gene
– Can have a significant impact on phenotype
– Is usually harmless, but may have an adaptive
impact
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Mutations That Alter Gene Number or Sequence
• Chromosomal mutations that affect many loci
– Are almost certain to be harmful
– May be neutral and even beneficial
• Gene duplication or deletion
– Duplicates or deletes chromosome segments
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Mutation Rates
• Mutation rates
– Tend to be low in animals and plants
• Average about one mutation in every
100,000 genes per generation
– Are more rapid in microorganisms
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Sexual Recombination
• In sexually reproducing populations, sexual
recombination
– Is far more important than mutation in
producing the genetic differences that make
adaptation possible
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Concept 23.3:
• Three major factors alter allele frequencies and
bring about most evolutionary change
– Natural selection
– Genetic drift
– Gene flow
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Natural Selection
• Differential success in reproduction
– Results in certain alleles being passed to the
next generation in greater proportions
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Genetic drift
– Describes how allele frequencies can fluctuate
unpredictably from one generation to the next
– Tends to reduce genetic variation
– May result from very small population size
CWCW
CRCR
CRCR
Only 5 of
10 plants
leave
offspring
CRCW
CWCW
CRCR
CRCR
CRCW
CWCW
CRCR
CRCW
CRCW
CRCR
CWCW
CRCW
CRCR
CRCR
CRCW
Generation 1
p (frequency of CR) = 0.7
q (frequency of CW) = 0.3
CRCR
CRCR
CRCR
CRCR
CRCR
CRCR
CRCR
CRCR
CRCW
CRCW
Generation 2
p = 0.5
q = 0.5
Figure 23.7
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Only 2 of
10 plants
leave
offspring
CRCR
CRCR
Generation 3
p = 1.0
q = 0.0
The Bottleneck Effect
• In the bottleneck effect
– A sudden change in the environment may
drastically reduce the size of a population
– The gene pool may no longer be reflective of
the original population’s gene pool
(a) Shaking just a few marbles through the
narrow neck of a bottle is analogous to a
drastic reduction in the size of a population
after some environmental disaster. By chance,
blue marbles are over-represented in the new
population and gold marbles are absent.
Original
population
Figure 23.8 A
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Bottlenecking
event
Surviving
population
• Understanding the bottleneck effect
– Can increase understanding of how human
activity affects other species
(b) Similarly, bottlenecking a population
of organisms tends to reduce genetic
variation, as in these northern
elephant seals in California that were
once hunted nearly to extinction.
Figure 23.8 B
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The Founder Effect
• The founder effect
– Occurs when a few individuals become
isolated from a larger population
– Can affect allele frequencies in a population
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Gene Flow
• Gene flow
– Causes a population to gain or lose alleles
– Results from the movement of fertile
individuals or gametes
– Tends to reduce differences between
populations over time
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Concept 23.4:
• Natural selection is the primary mechanism of
adaptive evolution
• Natural selection
– Accumulates and maintains favorable
genotypes in a population
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Variation Within a Population
• Both discrete and quantitative characters
contribute to variation within a population
– Discrete characters can be classified on an eitheror basis
– Quantitative characters vary along a continuum
within a population
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Variation Between Populations
• Most species exhibit geographic variation
– Differences between gene pools of separate
populations or population subgroups
Figure 23.10
1
2.4
3.14
5.18
8.11
9.12
10.16
13.17
1
2.19
3.8
4.16
9.10
11.12
13.17
15.18
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6
7.15
19
XX
5.14
6.7
XX
The Preservation of Genetic Variation
• Various mechanisms help to preserve genetic
variation in a population
– Diploidy
• Maintains genetic variation in the form of
hidden recessive alleles
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• Balancing selection
– Occurs when natural selection maintains
stable frequencies of two or more phenotypic
forms in a population
– Leads to a state called balanced polymorphism
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Heterozygote Advantage
• Some individuals who are heterozygous at a
particular locus have greater fitness than
homozygotes
• Natural selection will tend to maintain two or more
alleles at that locus
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• The sickle-cell allele
– Causes mutations in hemoglobin but also
confers malaria resistance
– Exemplifies the heterozygote advantage
Distribution of
malaria caused by
Plasmodium falciparum
(a protozoan)
Figure 23.13
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Frequencies of the
sickle-cell allele
0–2.5%
2.5–5.0%
5.0–7.5%
7.5–10.0%
10.0–12.5%
>12.5%
Sexual Selection
• Sexual selection
– Is natural selection for mating success
– Can result in sexual dimorphism, marked
differences between the sexes in secondary
sexual characteristics
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The Evolutionary Enigma of Sexual Reproduction
• Sexual reproduction
– Produces fewer reproductive offspring than asexual
reproduction, a so-called reproductive handicap
Sexual reproduction
Asexual reproduction
Generation 1
Female
Female
Generation 2
Male
Generation 3
Generation 4
Figure 23.16
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• If sexual reproduction is a handicap, why has it
persisted?
– It produces genetic variation that may aid in
• disease resistance and
• promoting other advantageous adaptations.
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