Download Natural Selection

Biology 102 Week 1
Descent with Modification: A
Darwinian View of Life.
The Evolution of Populations.
PowerPoint Lectures for
Biology, Seventh Edition
Neil Campbell and Jane Reece
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Overview: Darwin Introduces a Revolutionary
Theory
• A new era of biology began on November 24,
1859, the day Charles Darwin published On the
Origin of Species by Means of Natural Selection
• The Origin of Species focused biologists’ attention
on the great diversity of organisms
Video: Galápagos Marine Iguana
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
• Darwin made two major points in his book:
– Many current species are descendants of
ancestral species
– Natural selection is a mechanism for this
evolutionary process
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Concept 22.1: The Darwinian revolution challenged traditional
views of a young Earth inhabited by unchanging species
• To understand why Darwin’s ideas were
revolutionary, we must examine them in relation to
other Western ideas about Earth and its life
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
LE 22-2
Linnaeus (classification)
Hutton (gradual geologic change)
Lamarck (species can change)
Malthus (population limits)
Cuvier (fossils, extinction)
Lyell (modern geology)
Darwin (evolution, natural selection)
Mendel (inheritance)
Wallace (evolution, natural selection)
American Revolution
U.S. Civil War
French Revolution
1850
1900
1750
1800
1795 Hutton proposes his theory of gradualism.
1798 Malthus publishes “Essay on the Principle of Population.”
1809 Lamarck publishes his theory of evolution.
1830 Lyell publishes Principles of Geology.
1831–1936 Darwin travels around the world on HMS Beagle.
1837 Darwin begins his notebooks on the origin of species.
1844 Darwin writes his essay on the origin of species.
1858 Wallace sends his theory to Darwin.
1859 The Origin of Species is published.
1865 Mendel publishes inheritance papers.
Resistance to the Idea of Evolution
• The Origin of Species
– Shook the deepest roots of Western culture
– Challenged a worldview that had been
prevalent for centuries
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
The Scale of Nature and Classification of Species
• The Greek philosopher Aristotle viewed species as
fixed and unchanging
• The Old Testament holds that species were
individually designed by God and therefore perfect
• Carolus Linnaeus interpreted organismal
adaptations as evidence that the Creator had
designed each species for a specific purpose
• Linnaeus was a founder of taxonomy, the branch
of biology concerned with classifying organisms
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Fossils, Cuvier, and Catastrophism
• The study of fossils helped to lay the groundwork
for Darwin’s ideas
• Fossils are remains or traces of organisms from
the past, usually found in sedimentary rock, which
appears in layers or strata
Video: Grand Canyon
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
• Paleontology, the study of fossils, was largely
developed by French scientist Georges Cuvier
• Cuvier advocated catastrophism, speculating that
each boundary between strata represents a
catastrophe
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Theories of Gradualism
• Gradualism is the idea that profound change can
take place through the cumulative effect of slow
but continuous processes
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
• Geologists Hutton and Lyell perceived that
changes in Earth’s surface can result from slow
continuous actions still operating today
• This view strongly influenced Darwin’s thinking
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Lamarck’s Theory of Evolution
• Lamarck hypothesized that species evolve
through use and disuse and the inheritance of
acquired traits
• The mechanisms he proposed are unsupported by
evidence
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Concept 22.2: In The Origin of Species, Darwin
proposed that species change through natural selection
• As the 19th century dawned, it was generally
believed that species had remained unchanged
since their creation
• However, a few doubts about the permanence of
species were beginning to arise
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Darwin’s Research
• As a boy and into adulthood, Charles Darwin had
a consuming interest in nature
• After receiving his B.A. degree, he was accepted
on board the HMS Beagle, which was embarking
on a voyage around the world
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
The Voyage of the Beagle
• During his travels on the Beagle, Darwin collected
specimens of South American plants and animals
• He observed adaptations of plants and animals
that inhabited many diverse environments
• His interest in geographic distribution of species
was kindled by a stop at the Galápagos Islands
near the equator west of South America
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
LE 22-5
England
EUROPE
NORTH
AMERICA
PACIFIC
OCEAN
ATLANTIC
OCEAN
AFRICA
Galápagos
Islands
HMS Beagle in port
Equator
SOUTH
AMERICA
Darwin in 1840,
after his return
AUSTRALIA
Cape of
Good Hope
Tasmania
Cape Horn
Tierra del Fuego
New
Zealand
Video: Albatross Courtship Ritual
Video: Blue-footed Boobies Courtship Ritual
Video: Galápagos Island Overview
Video: Galápagos Sea Lion
Video: Soaring Hawk
Video: Galápagos Tortoise
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Darwin’s Focus on Adaptation
• In reassessing his observations, Darwin perceived
adaptation to the environment and the origin of
new species as closely related processes
• From studies made years after Darwin’s voyage,
biologists have concluded that this is indeed what
happened to the Galápagos finches
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
LE 22-6
Cactus eater. The long,
sharp beak of the
cactus ground finch
(Geospiza scandens)
helps it tear and eat
cactus flowers and
pulp.
Seed eater. The large
ground finch (Geospiza
magnirostris) has a large
beak adapted for cracking
seeds that fall from plants
to the ground.
Insect eater. The green warbler finch
(Certhidea olivacea) used its narrow,
pointed beak to grasp insects.
• In 1844, Darwin wrote an essay on the origin of
species and natural selection but did not introduce
his theory publicly, anticipating an uproar
• In June 1858, Darwin received a manuscript from
Alfred Russell Wallace, who had developed a
theory of natural selection similar to Darwin’s
• Darwin quickly finished The Origin of Species and
published it the next year
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
The Origin of Species
• Darwin developed two main ideas:
– Evolution explains life’s unity and diversity
– Natural selection is a cause of adaptive
evolution
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Descent with Modification
• The phrase descent with modification summarized
Darwin’s perception of the unity of life
• The phrase refers to the view that all organisms
are related through descent from an ancestor that
lived in the remote past
• In the Darwinian view, the history of life is like a
tree with branches representing life’s diversity
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
LE 22-7
Sirenia
Hyracoidea (Manatees
(Hyraxes) and relatives)
0
10,000
2
5.5
24
34
Elephas Loxodonta Loxodonta
cyclotis
maximus africana
(Africa)
(Africa)
(Asia)
Natural Selection and Adaptation
• Evolutionary biologist Ernst Mayr has dissected
the logic of Darwin’s theory into three inferences
based on five observations
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
• Observation #1: For any species, population sizes
would increase exponentially if all individuals that
are born reproduced successfully
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
• Observation #2: Populations tend to be stable in
size, except for seasonal fluctuations
• Observation #3: Resources are limited
• Inference #1: Production of more individuals than
the environment can support leads to a struggle
for existence among individuals of a population,
with only a fraction of their offspring surviving
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
• Observation #4: Members of a population vary
extensively in their characteristics; no two
individuals are exactly alike
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
• Observation #5: Much of this variation is heritable
• Inference #2: Survival depends in part on inherited
traits; individuals whose inherited traits give them
a high probability of surviving and reproducing are
likely to leave more offspring than other individuals
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
• Inference #3: This unequal ability of individuals to
survive and reproduce will lead to a gradual
change in a population, with favorable
characteristics accumulating over generations
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Artificial Selection
• In artificial selection, humans have modified other
species over many generations by selecting and
breeding individuals with desired traits
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
LE 22-10
Terminal
bud
Lateral
buds
Brussels sprouts
Cabbage
Leaves
Flower
clusters
Kale
Cauliflower
Stem
Flowers
and
stems
Broccoli
Wild mustard
Kohlrabi
Summary of Natural Selection
• Natural selection is differential success in
reproduction from interaction between individuals
that vary in heritable traits and their environment
• Natural selection produces an increase over time
in adaptation of organisms to their environment
• If an environment changes over time, natural
selection may result in adaptation to these new
conditions
Video: Seahorse Camouflage
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
LE 22-11
A flower mantid
in Malaysia
A stick mantid
in Africa
Concept 22.3: Darwin’s theory explains a wide
range of observations
• Darwin’s theory of evolution continues to be tested
by how effectively it can account for additional
observations and experimental outcomes
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Natural Selection in Action
• Two examples provide evidence for natural
selection: the effect of differential predation on
guppy populations and the evolution of drugresistant HIV
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Differential Predation in Guppy Populations
• Researchers have observed natural selection
leading to adaptive evolution in guppy populations
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
LE 22-12a
Pools with killifish
but no guppies prior
to transplant
Predator: Killifish; preys
mainly on small guppies
Experimental
transplant of
guppies
Guppies:
Larger at
sexual maturity
than those in
“pike-cichlid pools”
Predator: Pike-cichlid; preys mainly on large guppies
Guppies: Smaller at sexual maturity than
those in “killifish pools”
200
160
120
80
40
185.6
161.5
67.5 76.1
Males
Females
Age of guppies
at maturity (days)
Mass of guppies
at maturity (mg)
LE 22-12b
100
80
60
40
20
85.792.3
58.2
48.5
Males
Females
Control population:
Guppies from pools with
pike-cichlids as predators
Experimental population:
Guppies transplanted to
pools with killifish as
predators
The Evolution of Drug-Resistant HIV
• The use of drugs to combat HIV selects for viruses
resistant to these drugs
• The ability of bacteria and viruses to evolve rapidly
poses a challenge to our society
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
LE 22-13
Percent of HIV resistant to 3TC
100
Patient
No. 1
Patient No. 2
75
50
Patient No. 3
25
0
0
2
4
6
Weeks
8
10
12
Homology, Biogeography, and the Fossil Record
• Evolutionary theory provides a cohesive
explanation for many kinds of observations
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Homology
• Homology is similarity resulting from common
ancestry
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Anatomical Homologies
• Homologous structures are anatomical
resemblances that represent variations on a
structural theme present in a common ancestor
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
LE 22-14
Human
Cat
Whale
Bat
• Comparative embryology reveals anatomical
homologies not visible in adult organisms
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
LE 22-15
Pharyngeal
pouches
Post-anal
tail
Chick embryo (LM)
Human embryo
• Vestigial organs are remnants of structures that
served important functions in the organism’s
ancestors
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Molecular Homologies
• Examples of homologies at the molecular level are
genes shared among organisms inherited from a
common ancestor
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Homologies and the Tree of Life
• The Darwinian concept of an evolutionary tree of
life can explain homologies
• Anatomical resemblances among species are
generally reflected in their molecules, genes, and
gene products
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
LE 22-16
Species
Percent of Amino Acids That Are
Identical to the Amino Acids in a
Human Hemoglobin Polypeptide
Human
100%
Rhesus monkey
95%
87%
Mouse
69%
Chicken
54%
Frog
Lamprey
14%
Biogeography
• Darwin’s observations of biogeography, the
geographic distribution of species, formed an
important part of his theory of evolution
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
• Some similar mammals that have adapted to
similar environments have evolved independently
from different ancestors
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
LE 22-17
NORTH
AMERICA
Sugar
glider
AUSTRALIA
Flying
squirrel
The Fossil Record
• The succession of forms observed in the fossil
record is consistent with other inferences about
the major branches of descent in the tree of life
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
• The Darwinian view of life predicts that
evolutionary transitions should leave signs in the
fossil record
• Paleontologists have discovered fossils of many
such transitional forms
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
What Is Theoretical about the Darwinian View of
Life?
• In science, a theory accounts for many
observations and data and attempts to explain and
integrate a great variety of phenomena
• Darwin’s theory of evolution by natural selection
integrates diverse areas of biological study and
stimulates many new research questions
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Chapter 23
The Evolution of Populations
PowerPoint Lectures for
Biology, Seventh Edition
Neil Campbell and Jane Reece
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