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Chapter 23
The Evolution of Populations
PowerPoint Lectures for
Biology, Seventh Edition
Neil Campbell and Jane Reece
Lectures by Chris Romero
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Overview: The Smallest Unit of Evolution
• One misconception is that organisms evolve, in
the Darwinian sense, during their lifetimes
• Natural selection acts on individuals, but only
populations evolve
• Genetic variations in populations contribute to
evolution
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
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
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
The Modern Synthesis
• Population genetics is the study of how
populations change genetically over time
• Population genetics integrates Mendelian genetics
with the Darwinian theory of evolution by natural
selection
• This modern synthesis focuses on populations as
units of evolution
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Gene Pools and Allele Frequencies
• A population is a localized group of individuals
capable of interbreeding and producing fertile
offspring
• The gene pool is the total aggregate of genes in a
population at any one time
• The gene pool consists of all gene loci in all
individuals of the population
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
MAP
AREA
CANADA
ALASKA
LE 23-3
Beaufort Sea
Porcupine
herd range
Fairbanks
Fortymile
herd range
Whitehorse
The Hardy-Weinberg Theorem
• The Hardy-Weinberg theorem describes a
population that is not evolving
• It states that 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
• Mendelian inheritance preserves genetic variation
in a population
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
LE 23-4
Generation
1
X
CRCR
genotype
Generation
2
Plants mate
CWCW
genotype
All CRCW
(all pink flowers)
50% CW
gametes
50% CR
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
Alleles segregate, and subsequent
generations also have three types
of flowers in the same proportions
Preservation of Allele Frequencies
• In a given population where gametes contribute to
the next generation randomly, allele frequencies
will not change
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Hardy-Weinberg Equilibrium
• Hardy-Weinberg equilibrium describes a
population in which random mating occurs
• It describes a population where allele frequencies
do not change
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
• 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
LE 23-5
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
CW
(20%)
p2
pq
64%
CRCR
16%
CRCW
(20%)
CR
(80%)
CW
Eggs
(80%)
qp
4%
CWCW
16%
CRCW
q2
Conditions for Hardy-Weinberg Equilibrium
• The Hardy-Weinberg theorem describes a
hypothetical population
• In real populations, allele and genotype
frequencies do change over time
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
• 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
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Population Genetics and Human Health
• We can use the Hardy-Weinberg equation to
estimate the percentage of the human population
carrying the allele for an inherited disease
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Concept 23.2: Mutation and sexual recombination
produce the variation that makes evolution possible
• Two processes, mutation and sexual
recombination, produce the variation in gene pools
that contributes to differences among individuals
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Mutation
• Mutations are changes in the nucleotide sequence
of DNA
• Mutations cause new genes and alleles to arise
Animation: Genetic Variation from Sexual Recombination
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Point Mutations
• A point mutation is a change in one base in a
gene
• It is usually harmless but may have significant
impact on phenotype
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Mutations That Alter Gene Number or Sequence
• Chromosomal mutations that delete, disrupt, or
rearrange many loci are typically harmful
• Gene duplication is nearly always harmful
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Mutation Rates
• Mutation rates are low in animals and plants
• The average is about one mutation in every
100,000 genes per generation
• Mutations are more rapid in microorganisms
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Sexual Recombination
• Sexual recombination is far more important than
mutation in producing the genetic differences that
make adaptation possible
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Concept 23.3: Natural selection, genetic drift, and
gene flow can alter a population’s genetic composition
• Three major factors alter allele frequencies and
bring about most evolutionary change:
– Natural selection
– Genetic drift
– Gene flow
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Natural Selection
• Differential success in reproduction results in
certain alleles being passed to the next generation
in greater proportions
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Genetic Drift
• The smaller a sample, the greater the chance of
deviation from a predicted result
• Genetic drift describes how allele frequencies
fluctuate unpredictably from one generation to the
next
• Genetic drift tends to reduce genetic variation
through losses of alleles
Animation: Causes of Evolutionary Change
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
LE 23-7
CWCW
CRCR
CRCR
CRCW
Only 5 of
10 plants
leave
offspring
CRCR
CWCW
CRCW
CWCW
CRCR
CRCW
CRCW
CRCR
CRCR
CRCR
CRCW
CRCW
Generation 1
p (frequency of CR) = 0.7
q (frequency of CW) = 0.3
CWCW
CRCR
Only 2 of
10 plants
leave
offspring
CRCR
CRCR
CRCR
CRCR
CRCR
CRCR
CRCR
CRCR
CRCW
CRCW
Generation 2
p = 0.5
q = 0.5
CRCR
CRCR
Generation 3
p = 1.0
q = 0.0
The Bottleneck Effect
• The bottleneck effect is a sudden change in the
environment that may drastically reduce the size
of a population
• The resulting gene pool may no longer be
reflective of the original population’s gene pool
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
LE 23-8
Original
population
Bottlenecking
event
Surviving
population
• Understanding the bottleneck effect can increase
understanding of how human activity affects other
species
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
The Founder Effect
• The founder effect occurs when a few individuals
become isolated from a larger population
• It can affect allele frequencies in a population
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Gene Flow
• Gene flow consists of genetic additions or
subtractions from a population, resulting from
movement of fertile individuals or gametes
• Gene flow causes a population to gain or lose
alleles
• It tends to reduce differences between populations
over time
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Concept 23.4: Natural selection is the primary
mechanism of adaptive evolution
• Natural selection accumulates and maintains
favorable genotypes in a population
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Genetic Variation
• Genetic variation occurs in individuals in
populations of all species
• It is not always heritable
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
LE 23-9
Map butterflies that
emerge in spring:
orange and brown
Map butterflies that
emerge in late summer:
black and white
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
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Polymorphism
• Phenotypic polymorphism describes a population
in which two or more distinct morphs for a
character are represented in high enough
frequencies to be readily noticeable
• Genetic polymorphisms are the heritable
components of characters that occur along a
continuum in a population
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Measuring Genetic Variation
• Population geneticists measure polymorphisms in
a population by determining the amount of
heterozygosity at the gene and molecular levels
• Average heterozygosity measures the average
percent of loci that are heterozygous in a
population
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Variation Between Populations
• Most species exhibit geographic variation
differences between gene pools of separate
populations or population subgroups
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
LE 23-10
1
2.4
3.14
8.11
9.12
10.16
5.18
6
13.17
19
1
2.19
3.8
4.16
9.10
11.12
13.17
15.18
5.14
7.15
XX
6.7
XX
• Some examples of geographic variation occur as
a cline, which is a graded change in a trait along a
geographic axis
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
LE 23-11
Heights of yarrow plants grown in common garden
Mean height (cm)
100
50
0
3,000
2,000
1,000
Sierra Nevada
Range
0
Seed collection sites
Great Basin
Plateau
A Closer Look at Natural Selection
• From the range of variations available in a
population, natural selection increases
frequencies of certain genotypes, fitting organisms
to their environment over generations
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Evolutionary Fitness
• The phrases “struggle for existence” and “survival
of the fittest” are commonly used to describe
natural selection but can be misleading
• Reproductive success is generally more subtle
and depends on many factors
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
• Fitness is the contribution an individual makes to
the gene pool of the next generation, relative to
the contributions of other individuals
• Relative fitness is the contribution of a genotype to
the next generation, compared with contributions
of alternative genotypes for the same locus
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Directional, Disruptive, and Stabilizing Selection
• Selection favors certain genotypes by acting on
the phenotypes of certain organisms
• Three modes of selection:
– Directional
– Disruptive
– Stabilizing
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
• 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 © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Frequency of
individuals
LE 23-12
Original
population
Evolved
population
Directional selection
Original population
Phenotypes (fur color)
Disruptive selection
Stabilizing selection
The Preservation of Genetic Variation
• Various mechanisms help to preserve genetic
variation in a population
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Diploidy
• Diploidy maintains genetic variation in the form of
hidden recessive alleles
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Balancing Selection
• Balancing selection occurs when natural selection
maintains stable frequencies of two or more
phenotypic forms in a population
• Balancing selection leads to a state called
balanced polymorphism
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
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
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
• The sickle-cell allele causes mutations in
hemoglobin but also confers malaria resistance
• It exemplifies the heterozygote advantage
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
LE 23-13
Frequencies of the
sickle-cell allele
0–2.5%
2.5–5.0%
5.0–7.5%
Distribution of
malaria caused by
Plasmodium falciparum
(a protozoan)
7.5–10.0%
10.0–12.5%
>12.5%
Frequency-Dependent Selection
• In frequency-dependent selection, the fitness of
any morph declines if it becomes too common in
the population
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
LE 23-14
On pecking a moth
image the blue jay
receives a food reward.
If the bird does not
detect a moth on
either screen, it pecks
the green circle to
continue a new set
of images (a new
feeding opportunity).
Parental population sample
0.6
Phenotypic
variation
Experimental group sample
0.5
0.4
Frequencyindependent control
0.3
0.2
0
Plain background
Patterned background
20
40
60
Generation number
80
100
Neutral Variation
• Neutral variation is genetic variation that appears
to confer no selective advantage
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Sexual Selection
• Sexual selection is natural selection for mating
success
• It can result in sexual dimorphism, marked
differences between the sexes in secondary
sexual characteristics
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
• Intrasexual selection is competition among
individuals of one sex for mates of the opposite
sex
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
• Intersexual selection occurs when individuals of
one sex (usually females) are choosy in selecting
their mates from individuals of the other sex
• Selection may depend on the showiness of the
male’s appearance
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
The Evolutionary Enigma of Sexual Reproduction
• Sexual reproduction produces fewer reproductive
offspring than asexual reproduction, a so-called
“reproductive handicap”
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
LE 23-16
Asexual reproduction
Female
Sexual reproduction
Generation 1
Female
Generation 2
Male
Generation 3
Generation 4
• Sexual reproduction produces genetic variation
that may aid in disease resistance
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Why Natural Selection Cannot Fashion Perfect
Organisms
• Evolution is limited by historical constraints
• Adaptations are often compromises
• Chance and natural selection interact
• Selection can only edit existing variations
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
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