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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
• Natural selection does act on individuals, however, to see
the impact of natural selection we need to look at how a
population of organisms changes over a long period of
time.
• It is the population, not its individuals, that evolves.
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
• What is microevolution?
• Microevolution is change in the genetic makeup of
a population from generation to generation
Another way of saying it is:
It is the evolution of populations. It is defined as a
change in the allele frequencies in a population.
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Population Genetics
• The Origin of Species convinced most biologists that
species are the products of evolution, but acceptance of
natural selection as the main mechanism of evolution
was more difficult.
•
Darwin could not explain how chance variations arise in
a population or how these variations are transmitted
from parents to offspring because he had no
understanding of Genetics. Gregor Mendel and Charles
Darwin were contemporaries but Mendel’s discoveries
were unappreciated at the time even though his
principles of heredity would have given credibility to
natural selection.
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
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The modern evolutionary synthesis integrated Darwinian
selection and Mendelian inheritance
• 1. When Mendel’s research was rediscovered in the
early twentieth century, many geneticists believed that
the laws of inheritance conflicted with Darwin’s theory of
natural selection.
• Darwin emphasized those characters that vary along a
continuum. These characters are influenced by multiple
loci while Mendel investigated “either-or” traits.
• An important turning point for evolutionary theory was
the birth of population genetics, which emphasizes the
extensive genetic variation within populations and
recognizes the importance of quantitative characters (
those that vary along a continuum).
• Advances in population genetics in the 1930s allowed
the ideas of Mendelian Genetics and Darwinism to be
reconciled and provided a genetic basis for variation and
natural selection.
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
The modern synthesis
• The modern synthesis is a comprehensive theory of
evolution, the modern synthesis, took form in the early
1940s and it integrated ideas from paleontology,
taxonomy, biogeography, and population genetics.
• The architects of the modern synthesis included
geneticists Theodosius Dobzhansky and Sewall Wright,
biogeographer and taxonomist Ernst Mayr,
paleontologist George Simpson, and botanist G. L.
Stebbins.
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What is emphasized in this theory?
• The modern synthesis emphasizes:
1.The importance of populations as the units of evolution.
2. Natural selection as the most important mechanism of
evolution.
3.The idea of gradualism to explain how large changes can
evolve as an accumulation of small changes over long
periods of time.
• While many evolutionary biologists are now challenging
some of the assumptions of the modern synthesis, it has
shaped most of our ideas about how populations evolve.
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
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MAP
AREA
CANADA
ALASKA
LE 23-3
Beaufort Sea
Porcupine
herd range
Fairbanks
Fortymile
herd range
Whitehorse
A population’s gene pool is defined by its allele frequencies
• A population is a group of individuals that belong to the
same species and live in the same place.
• A species is a group of populations whose individuals have
the potential to interbreed and produce fertile offspring.
• Populations of a species may be isolated from each other
(they exchange genetic material rarely), or they may be
integraded with low densities in an intermediate region.
• Members of a population are far more likely to breed with
members of the same population
• Gene pool. is the total aggregate of genes in a population
at any one time. A gene pool consists of all alleles at all
gene loci in all individuals of a population.
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The Hardy-Weinberg Theorem
• The Hardy-Weinberg theorem describes a population that
is not evolving.
• The Hardy-Weinberg theorem describes the gene pool of
a non-evolving population.
• It states that frequencies of alleles and genotypes in a
population’s gene pool will 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
• In other words, the shuffling of alleles after meiosis and
random fertilization should have no effect on the overall
gene pool of a population.
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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
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Hardy-Weinberg Equilibrium
• Hardy-Weinberg equilibrium describes a
population in which random mating occurs
• It describes a population where allele frequencies
do not change
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Hardy-Weinberg equilibrium
• All we get by meiosis and sexual reproduction is a
reshuffling of the genes that are there but no new ones
are introduced so the processes of meiosis and random
fertilization just maintain the same allele and genotype
frequencies that existed in the previous generation.
• The population is in a state of equilibrium called HardyWeinberg equilibrium.
• Theoretically, the allele frequencies should remain the
same forever.
• The Hardy-Weinberg theorem states that the processes
involved in a Mendelian system have no tendency to
alter allele frequencies from one generation to another.
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
An example:
• We can use the Hardy-Weinberg theorem to estimate the percentage of
the human population that carries the allele for a particular inherited
disease, phenyketonuria (PKU) in this case.
• About 1 in 10,000 babies born in the United States is born with PKU,
which results in mental retardation and other problems if left untreated.
The disease is caused by a recessive allele.
• From the epidemiological data, we know that frequency of homozygous
recessive individuals (q2 in the Hardy-Weinberg theorem) = 1 in 10,000 or
0.0001.
• The frequency of the recessive allele (q) is the square root of 0.0001 =
0.01.
• The frequency of the dominant allele (p) is p = 1 - q or 1 - 0.01 = 0.99.
• The frequency of carriers (heterozygous individuals) is 2pq = 2 x 0.99 x
0.01 = 0.0198 or about 2%.
• Thus, about 2% of the U.S. population carries the PKU allele.
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
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Why the Hardy-Weinberg equilibrium does not represent a real population?
• Natural selection is clearly a violation of the conditions
necessary for the Hardy-Weinberg equilibrium.
• The Hardy-Weinberg equilibrium expects that all
individuals in a population have equal ability to survive
and produce viable, fertile offspring. However, in a real
population there will be variety among individuals,
natural selection will lead some individuals to leave
more offspring than others.
• Natural selection results in some alleles being passed
along to the next generation in numbers disproportionate
to their frequencies in the present generation. This is
because natural selection accumulates and maintains
favorable genotypes in a population.
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
Populations at Hardy-Weinberg equilibrium must satisfy five
conditions.
• Very large population size. In small populations,
chance fluctuations in the gene pool, genetic drift, can cause
genotype frequencies to change over time.
• No migrations. Gene flow, the transfer of alleles due to the
movement of individuals or gametes into or out of our target
population can change the proportions of alleles.
• No net mutations. If one allele can mutate into another, the gene
pool will be altered.
• Random mating. If individuals pick mates with certain genotypes,
then the mixing of gametes will not be random and the HardyWeinberg equilibrium does not occur.
• No natural selection. If there is differential survival or mating
success among genotypes, then the frequencies of alleles in the
next variation will deviate from the frequencies predicted by the
Hardy-Weinberg equation.
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
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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
mutations
• A mutation is a change in an organism’s DNA.
• A new mutation that is transmitted in gametes can immediately change
the gene pool of a population by substituting the mutated allele for the
older allele.
• Each individual has thousands of genes, any one of which could
experience a mutation.
• Populations are composed of thousands or millions of individuals that
may have experienced mutations.
• Over the long term, mutation is a very important to evolution because it
is the source of genetic variation that serves as the raw material for
natural selection.
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
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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
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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
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Sexual Recombination
• Sexual recombination is far more important than
mutation in producing the genetic differences that
make adaptation possible
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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
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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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Causes of Microevolution
• What is microevolution?
Microevolution is a generation-to-generation change
in a population’s allele frequencies
• The Hardy-Weinberg theory provides a baseline against
which we can compare the allele and genotype
frequencies of an evolving population.
• The two main causes of microevolution are genetic
drift and natural selection
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
What factors can change the allele frequencies in a population?
• Genetic drift
• Natural selection
• Gene flow
• Mutation
• All represent departures ( the opposite) from the
conditions required for the Hardy-Weinberg equilibrium.
• Natural selection is the only factor that adapts a
population to its environment,
and it always favors the favorable traits.
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
How can genetic drift affect a population?
• The other three ( genetic drift, gene flow and mutations)
may effect populations in positive, negative, or neutral
ways.
• Genetic drift occurs when changes in gene frequencies
from one generation to another occur because of chance
events that occur when populations are small in size.
The smaller the sample, the greater the chance of
deviation from an idealized result.
•
For example, one would not be too surprised if a coin
produced seven heads and three tails in ten tosses, but
you would be surprised if you saw 700 heads and 300
tails in 1000 tosses—you expect 500 of each.
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
Types of Genetic Drift
• The bottleneck effect occurs when the numbers of individuals in a
larger population are drastically reduced by a disaster.
• By chance, some alleles may be overrepresented and others
underrepresented among the survivors. Some alleles may be
eliminated altogether.
• Bottlenecking is an important concept in conservation biology of
endangered species. Populations that have suffered bottleneck
incidents have lost at least some alleles from the gene pool. This
reduces individual variation and adaptability.
• The founder effect occurs when a new population is started by only
a few individuals that do not represent the gene pool of the larger
population.
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
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
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LE 23-8
Original
population
Bottlenecking
event
Surviving
population
• Understanding the bottleneck effect can increase
understanding of how human activity affects other
species
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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
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Gene Flow (another factor that changes allele frequency in a population)
• Gene flow consists of genetic additions or subtractions
from a population, resulting from movement of fertile
individuals.
• Gene flow causes a population to gain or lose alleles
• Gene flow tends to reduce differences between
populations over time.
•
Gene flow is genetic exchange due to migration of fertile
individuals between populations.
• The migration of people throughout the world is
transferring alleles between populations that were once
isolated, increasing gene flow.
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Concept 23.4: Natural selection is the primary
mechanism of adaptive evolution (important factor to change
allele frequency in a population)
• Natural selection accumulates and maintains
favorable genotypes in a population
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Natural Selection happens because of genetic variation
• Genetic variation occurs within and between
populations
• The variation among individuals in a population is a
combination of heritable and non-heritable traits.
• Phenotype, the observable characteristics of an organism (
its appearance), is
the cumulative product of an inherited genotype and many
environmental influences.
• Only the genetic component of variation can affect
evolution.This is because only heritable traits pass from
generation to generation.
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
What is polygenic inheritance?
• When two or more genes determine a single
trait.
• Quantitative characters are those that vary along
a continuum within a population. Examples of
this are height, color
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Genetic Variation
• Genetic variation occurs in individuals in
populations of all species
• It is not always heritable
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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
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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
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What is polymorphism?
• Polymorphism occurs when two or more discrete
characters are present and noticeable in a population.
Polymorphism applies only to discrete characters, not
quantitative characters, such as human height, which
varies among people in a continuum.
• Discrete characters, such as flower color, are usually
determined by a single locus (gene) with different
alleles.( without intermediates)
• The contrasting forms are called morphs, as in the redflowered and white-flowered morphs in our wildflower
population.
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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
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Variation Between Populations
• Most species exhibit geographic variation
differences between gene pools of separate
populations or population subgroups
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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
What process is responsible for nearly all the genetic variation in a population?
The contribution of mutation to variation is very small. In
organisms with sexual reproduction, nearly all the
variation results from new combinations of alleles by
sexual recombination, in other words, most of the
genetic differences among individuals are due to gene
recombination of the existing alleles from the population
gene pool.
• Random segregation of homologous chromosomes and
random union of gametes creates a unique assortment
of alleles in each individual.
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Lets answer these questions to review what we studied so far.
• 1.What is meant by “the modern synthesis?
• 2.What is a population? A gene pool? A species?
• 3.What is the Hardy-Weinberg theorem? What does the
Hardy-Weinberg model describe?
• 4.What are some practical uses of the Hardy-Weinberg
equation? Why is it important?
The equation allows the calculation of allele frequencies in a gene pool, if the
genotype frequencies are known. It is important in the study of evolution because it
tells you what happens in non evolving populations and provides a reference point or
a base line to compare to natural populations whose gene pools may be changing.
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
More questions….
•
5. What conditions a population must meet to maintain a Hardy-Weinberg equilibrium?
Must be very large, isolated, no mutations, random mating, and no natural selection
•
6. What five factors can cause microevolution change in a population?
Microevolution can be caused by genetic drift, gene flow,mutation, non random mating and
natural selection. Each of these is a deviation from the Hardy-Weinberg equilibrium. Of these 5
causes for microevolution, only natural selection leads to adaptations.
•
7. What is genetic drift? Why is the effect of genetic drift so great in small populations?
Changes in the gene pool of a small population due to chance. The larger the population, the
less important is the effect of genetic drift.
•
8. What are two situations which result in populations small enough for genetic drift to be
important?.
The bottleneck effect ( size of the population is reduced drastically by a natural disaster which
kills non selectively. The surviving population is unlikely to have the same genetic make up as
the original one) and the founder effect ( when a few individuals colonize a new habitat)
•
9. How does the bottleneck effects the gene pool of a population?
It reduces its genetic variability since some alleles may be absent.
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
10. What is gene flow? What are the consequences of gene flow
on different populations?
Gene flow is the migration of fertile individuals (gametes) between
populations. A population may gain or lose alleles this way and results in
reducing differences between populations. Extensive gene flow can merge
two neighboring populations into one.
• 11. What is the affect of mutations on populations?
A mutation changes one allele for another. Mutations are rare and have little
effect on large populations. If a new mutation increases in frequency is only
because that individual produced a larger percentage of offsprings than the
rest of the population due to natural selection.
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
What are the SOURCES OF GENETIC VARIATION ?
• Mutation and sexual recombination generate genetic
variation
• Mutations are changes in the nucleotide sequence of DNA.
• New alleles originate only by mutation but mutations of
individual genes are rare and random
 Only mutations in cell lines that produce gametes can be
passed along to offspring. Mutations in somatic cells are
lost when the individual dies.
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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
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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
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Directional, Disruptive, and Stabilizing Selection
• Selection favors certain genotypes by acting on
the phenotypes of certain organisms
• Three modes of selection:
– Directional
– Disruptive
– Stabilizing
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• 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
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Diploidy
• Diploidy maintains genetic variation in the form of
hidden recessive alleles
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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
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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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• An example of heterozygous advantage:
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
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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
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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
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• Intrasexual selection is competition among
individuals of one sex for mates of the opposite
sex
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• 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
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The Evolutionary Enigma of Sexual Reproduction
• Sexual reproduction produces fewer reproductive
offspring than asexual reproduction, a so-called
“reproductive handicap”
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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
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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