Download Natural Selection

How Populations Evolve
Chapter.13
DARWIN S THEORY
OF EVOLUTION
A sea voyage helped Darwin frame his theory of evolution
•  The primary mechanism of evolutionary change producing
adaptation of organisms to their environment is natural
selection, the differential survival and reproduction of
individuals within a population
•  The Greek philosopher Aristotle viewed species as perfect
and unchanging
–  Judeo-Christianism : perfect creature from a divine creator
•  Earth is only 6000 years old. all the living organism is static in the form
•  In the century prior to Darwin, the study of fossils suggested
that species had changed over time
•  Jean Baptiste Lamarck suggested that life on Earth evolves
–  His proposed mechanisms:
•  Use and disuse
•  Inheritance of acquired characteristics
4
A sea voyage helped Darwin frame his theory of evolution
•  He came to realize that the Earth was very old and that, over
time, present day species have arisen from ancestral species
by natural processes
•  In 1859, Darwin published On the Origin of Species by
Means of Natural Selection, presenting a strong, logical
explanation of descent with modification, evolution by the
mechanism of natural selection
•  Darwin observed that
–  Organisms produce more offspring than the environment can
support
–  Organisms vary in many traits
Darwin proposed natural selection as the mechanism of evolution
• 
• 
• 
Darwin reasoned that traits that increase their chance of surviving and
reproducing in their environment tend to leave more offspring than
others
As a result, favorable traits accumulate in a population over generations
–  Natural Selection
–  descent with modification
Darwin found convincing evidence for his ideas in the results of artificial
selection, the selective breeding of domesticated plants and animals
•  Note these important points
–  Individuals do not evolve: populations evolve
–  Natural selection can amplify or diminish only heritable traits;
acquired characteristics cannot be passed on to offspring
–  Evolution is not goal directed and does not lead to perfection;
favorable traits vary as environments change
–  Ex. Sickle cell anemia
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Artificial selection
Cabbage
Selection for
apical (tip) bud
Brussels
sprouts Selection for
axillary (side)
buds
Broccoli
Selection
for flowers
and stems
Selection
for stems
Selection
for leaves
Kale
Wild mustard
Kohlrabi
(a) A flower mantid in Malaysia
(b) A leaf mantid in Borneo
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RESULTS
Beak
10
On native species,
southern Florida
8
Number of individuals
6
4
2
0
Museum-specimen average
10
On introduced species,
central Florida
8
6
4
2
0
6
7
8
9
Beak length (mm)
10
11
10
Natural selection as the mechanism of evolution
•  Rosemary and Peter Grant have worked on Darwin s finches
in the Galápagos for over 20 years
–  In wet years, small seeds are more abundant and small beaks are
favored
–  In dry years, large strong beaks are favored because large seeds
remain
Figure 22.6
(b) Insect-eater
(a) Cactus-eater
(c) Seed-eater
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Scientists can observe natural selection in action
•  Development of pesticide resistance in insects
–  Initial use of pesticides favors those few insects that have genes for
pesticide resistance
–  With continued use of pesticides, resistant insects flourish and
vulnerable insects die
–  Proportion of resistant insects increases over time
•  Natural selection is an editing process than a creative
mechanism
–  The presence of the pesticide leads to natural selection for insects
already in the population who have those alleles.
•  Natural selection is contingent on time and place
–  It favors those characteristics in a varying population that fit the current
local environment.
•  DDT resistant houseflies
Chromosome with allele
conferring resistance
to pesticide
Additional
applications will
be less effective, and
the frequency of
resistant insects in
the population
will grow
Pesticide application
Survivors
The evolution of drug resistant bacteria
•  The bacterium Staphylococcus aureus is commonly found on people
•  One strain, methicillin-resistant S. aureus (MRSA) is a dangerous
pathogen
•  S. aureus became resistant to penicillin in 1945, two years after it was
first widely used
•  S. aureus became resistant to methicillin in 1961, two years after it was
first widely used
•  Methicillin works by inhibiting a protein used by bacteria in their cell walls
•  MRSA bacteria use a different protein in their cell walls
•  When exposed to methicillin, MRSA strains are more likely to survive and
reproduce than nonresistant S. aureus strains
•  MRSA strains are now resistant to many antibiotics
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The study of fossils provides strong evidence for evolution
•  The fossil record shows that organisms have
evolved in a historical sequence
–  The oldest known fossils are prokaryote cells
–  The oldest eukaryotic fossils are a billion years younger
–  Multicellular fossils are even more recent
•  Many fossils link early extinct species with species
living today
–  A series of fossils documents the evolution of whales from
a group of land mammals
Pakicetus (terrestrial)
Rhodocetus (predominantly aquatic)
Pelvis and Dorudon (fully aquatic)
hind limb
Pelvis and
hind limb
Balaena (recent whale ancestor)
A mass of other evidence reinforces the evolutionary view of life
•  Biogeography, the geographic distribution of species, suggested to
Darwin that organisms evolve from common ancestors
–  Darwin noted that animals on islands resemble species on nearby mainland
more closely than they resemble animals on similar islands close to other
continents
–  Galapagos species evolved from animals of South America
–  New species due to adaptation of new environment.
•  Ex. Marsupials (유대류) in Australia
•  Comparative anatomy is the comparison of body structures in different
species
–  Anatomical similarity : sign of common descent
•  Homology is the similarity in characteristics that result from common
ancestry
–  Vertebrate forelimbs (Decent with modification)
Vertebrate forelimbs (Decent with modification)
Humerus
Radius
Ulna
Carpals
Metacarpals
Phalanges
Human
Cat
Whale
Bat
A mass of other evidence reinforces the evolutionary view of life
•  Comparative embryology is the comparison of early stages of
development among different organisms
–  Many vertebrates have common embryonic structures, revealing
homologies
–  When you were an embryo, you had a tail and pharyngeal pouches
(just like an embryonic fish)
•  Some homologous structures are vestigial organs
–  For example, the pelvic and hind-leg bones of some modern whales
Vestigial organs
Pharyngeal
Pouches
(인두낭)
Post-anal
tail
Chick embryo
Human embryo
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A mass of other evidence reinforces the evolutionary view of life
•  Molecular biology: Comparisons of DNA and amino
acid sequences between different organisms reveal
evolutionary relationships
–  All living things share a common DNA code for the
proteins found in living cells
–  We share genes with bacteria, yeast, and fruit flies
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Phylogenetic trees
•  Darwin was the first to represent the history of life as
a tree
–  Homologous structures and genes can be used to
determine the branching sequence of an evolutionary tree
Lungfishes
Amniotes
Mammals
2
Tetrapod limbs
Amnion
Lizards
3
and snakes
4
Crocodiles
Ostriches
6
Feathers
Hawks and
other birds
Birds
5
Tetrapods
Amphibians
1
THE EVOLUTION
OF POPULATIONS
Populations are the units of evolution
•  A population is a group of individuals of the same species living in the
same place at the same time
•  Evolution is the change in heritable traits in a population over
generations
•  Populations may be isolated from one another (with little interbreeding),
or individuals within populations may interbreed
•  A gene pool is the total collection of genes in a population at any one time
•  Microevolution is a change in the relative frequencies of alleles in a
gene pool over time
–  Variation in heritable traits is a prerequisite for evolution
•  Population genetics studies how populations change genetically over time
•  The modern synthesis connects Darwin s theory with population genetics
Microevolution
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Mutation and sexual reproduction produce genetic variation, making evolution possible
•  Mutation in gametes producing cells
•  Mutation, or changes in the nucleotide sequence of DNA, is
the ultimate source of new alleles
–  Occasionally, mutant alleles improve the adaptation of an individual to
its environment and increase its survival and reproductive success
(for example, DDT resistance in insects)
•  Chromosomal duplication is an important source of genetic
variation
–  If a gene is duplicated, the new copy can undergo mutation without
affecting the function of the original copy
–  For example, an early ancestor of mammals had a single gene for an
olfactory receptor
–  The gene has been duplicated many times, and humans now have
1,000 different olfactory receptor genes
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Mutation and sexual reproduction produce genetic
variation, making evolution possible
•  Sexual reproduction shuffles alleles to produce new
combinations
–  Homologous chromosomes sort independently as they separate
during anaphase I of meiosis
–  During prophase I of meiosis, pairs of homologous chromosomes
cross over and exchange genes
–  Further variation arises when sperm randomly unite with eggs in
fertilization
•  How many possible combinations of chromosomes are
possible in a human sperm or egg due to independent
assortment during meiosis?
– 
– 
– 
– 
23 combinations
46 combinations
232 = 529 combinations
223 = ~ 8 million combinations
A1
Parents
A1
×
A2
A3
Meiosis
Gametes
A1
A2
A3
Random
fertilization
Offspring,
with new
combinations
of alleles
A1
A2
A1
and
A3
Hardy-Weinberg equation
•  Sexual reproduction alone does not lead to evolutionary
change in a population
–  Although alleles are shuffled, the frequency of alleles and genotypes in
the population does not change
–  Similarly, if you shuffle a pack of cards, you ll deal out different hands,
but the cards and suits in the deck do not change
•  The Hardy Weinberg principle states that allele and genotype
frequencies within a sexually reproducing, diploid
population will remain in equilibrium unless outside forces
act to change those frequencies
•  Imagine that there are two alleles in a blue-footed booby
population: W and w
–  W is a dominant allele for a nonwebbed booby foot
–  w is a recessive allele for a webbed booby foot
Phenotypes
Genotypes
WW
Ww
ww
Number of animals
(total = 500)
320
160
20
Genotype frequencies
320
––– =
500
Number of alleles
in gene pool
(total = 1,000)
Allele frequencies
160
––– =
500
0.64
640 W
800
=
1,000
0.32
160 W + 160 w
0.8 W
200
=
1,000
20
–––
500 =
40 w
0.2 w
0.04
The Hardy-Weinberg equation
•  Consider the gene pool of a population of 500 boobies
–  320 (64%) are homozygous dominant (WW)
–  160 (32%) are heterozygous (Ww)
–  20 (4%) are homozygous recessive (ww)
•  Frequency of dominant allele (W) = 80% = p
–  80% of alleles in the booby population are W
•  Frequency of recessive allele (w) = 20% = q
–  20% of alleles in the booby population are w
•  Frequency of all three genotypes must be 100% or 1.0
–  p2 + 2pq + q2 = 100% = 1.0
–  homozygous dominant + heterozygous + homozygous recessive =
100%
•  What about the next generation of boobies?
–  Probability that a booby sperm or egg carries W = 0.8 or 80%
–  Probability that a sperm or egg carries w = 0.2 or 20%
Gametes reflect
allele frequencies
of parental gene pool
W egg
p = 0.8
Eggs
w egg
q = 0.2
Sperm
W sperm
w sperm
p = 0.8
q = 0.8
WW
p2 = 0.64
Ww
pq = 0.16
wW
qp = 0.16
ww
q2 = 0.04
Next generation:
Genotype frequencies 0.64 WW
Allele frequencies
0.32 Ww
0.8 W
0.04 ww
0.2 w
The Hardy-Weinberg equation
•  If a population is in Hardy-Weinberg equilibrium,
allele and genotype frequencies will not change
unless something acts to change the gene pool
•  For a population to remain in Hardy-Weinberg
equilibrium for a specific trait, it must satisfy five
conditions:
–  Very large population
–  No gene flow between populations
–  No mutations
–  Random mating
–  No natural selection
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The Hardy-Weinberg equation
•  Public health scientists use the Hardy-Weinberg equation to
estimate frequencies of disease-causing alleles in the human
population
•  One out of 3,300 Caucasian newborns in the United States
have cystic fibrosis
–  This disease, which causes digestive and respiratory problems, is
caused by a recessive allele
•  The frequency of individuals with this disease is
approximately q2 = 1/3300 = 0.0003
–  The frequency of the recessive allele is q = .0174 or 1.7%
•  The frequency of heterozygous carriers of cystic fibrosis is
2pq = 2 x 0.983 x 0.017 = 0.034
•  Around 3.4% of Caucasian Americans are carriers for cystic
fibrosis
Phenylketonuria (PKU) in Hardy-Weinberg equilibrium
•  The occurrence of PKU is 1 per 10,000 births
–  q2 = 0.0001
–  q = 0.01
•  The frequency of normal alleles is
–  p = 1 – q = 1 – 0.01 = 0.99
•  The frequency of carriers is
–  2pq = 2 × 0.99 × 0.01 = 0.0198
–  or approximately 2% of the U.S. population
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MECHANISMS
OF MICROEVOLUTION
Natural selection, genetic drift, and gene flow
can alter allele frequencies in a population
•  If the five conditions for the Hardy-Weinberg
equilibrium are not met in a population, the
population s gene pool may change
–  Mutations are rare and random and have little effect on the
gene pool
–  If mating is nonrandom, allele frequencies won t change
much (although genotype frequencies may)
•  The three main causes of evolutionary change are
–  Natural selection
–  Genetic drift : bottleneck effect, founder effect
–  Gene flow
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
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54
Founder effect
•  The founder effect occurs when a few individuals become
isolated from a larger population
•  Allele frequencies in the small founder population can be
different from those in the larger parent population
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Bottleneck effect
•  The bottleneck effect is a sudden reduction in population
size due to a change in the environment
•  The resulting gene pool may no longer be reflective of the
original population s gene pool
•  If the population remains small, it may be further affected by
genetic drift
•  Understanding the bottleneck effect can increase
understanding of how human activity affects other species
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Gene Flow
•  Gene flow consists of the movement of alleles among populations
•  Alleles can be transferred through the movement of fertile individuals or
gametes (for example, pollen)
•  Gene flow tends to reduce variation among populations over time
•  Gene flow can decrease the fitness of a population
•  Consider, for example, the great tit (Parus major) on the Dutch island of
Vlieland
–  Mating causes gene flow between the central and eastern
populations
–  Immigration from the mainland introduces alleles that decrease
fitness
–  Natural selection selects for alleles that increase fitness
–  Birds in the central region with high immigration have a lower
fitness; birds in the east with low immigration have a higher fitness
Figure 23.12
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Survival rate (%)
50
Population in which the
surviving females
eventually bred
Central
Eastern
Central
population
NORTH SEA
Eastern
population
Vlieland,
the Netherlands
40
2 km
30
20
10
0
Females born
in central
population
Females born
in eastern
population
Parus major
59
Adaptive evolution
•  Genetic drift, Gene flow and even mutation can cause
microevolution.
–  Chance event and only blind luck improves a population to the
environment.
•  Natural selection only leads to adaptive evolution.
–  Better fit between organisms and environment.
•  An individual s fitness is the contribution it makes to the gene
pool of the next and subsequent generations
•  Struggle for existence or survival of the fitness
–  such individuals may produce more offspring than others due to
natural selection
•  Ex) wing color of moth, variations in flower color or fragrance
–  The fittest individuals are those that pass on the most genes to the
next generation
Natural selection can alter variation in a population in three ways
•  Stabilizing selection favors intermediate phenotypes, acting
against extreme phenotypes
•  Stabilizing selection is very common, especially when
environments are stable
•  Example of stabilizing selection
–  In Swiss starlings, clutch size varies from 1 to 8; the average clutch
size is 4
–  Researchers marked chicks from different clutch sizes and recaptured
fledglings after 3 months
–  Birds from clutches with 3, 4, or 5 birds had higher recapture rates
than birds from clutches of 1 or 2 or 6, 7, or 8
Frequency of
individuals
Original
population
Phenotypes (fur color)
Original
Evolved
population population
Stabilizing selection
Directional selection
Disruptive selection
Natural selection can alter variation in a population in three ways
•  Directional selection acts against individuals at one of the
phenotypic extremes
•  Directional selection is common during periods of
environmental change, or when a population migrates to a
new and different habitat
–  Ex. Fur color matched for better camouflage of a certain environment.
–  Insect resistant to the pesticide
•  Disruptive selection favors individuals at both extremes of the
phenotypic range
–  Mice colonized a patchy habitat where a background of light soil was
studded with areas of dark rocks.
–  African seedcracker finches, large-billed birds and small billed birds
Sexual selection
•  In many animal species, males and females show distinctly different
appearance, called sexual dimorphism
–  Form of male adornment (ex. Manes on lions, antlers on deer or colorful
plumage in birds) : secondary sex structure
–  Normally males are the showier sex among vertebrates
•  Sexual selection: a form of natural selection in which individuals with
certain characteristics are more likely than other individuals to obtain
mate.
•  Intrasexual competition involves competition for mates, usually by
males
–  Using secondary sex structure
–  Winning individual garners a harem of mates
•  In intersexual competition (or mate choice), individuals of one sex
(usually females) are choosy in picking their mates, often selecting flashy
or colorful mates ( choose me sign)
Figure 23.15
65
Sexual selection
•  Mate attracting feature (ex. showy plumage of peacock) may
make male birds more visible to predators
–  But due to higher reproduction successful rate, the trait will be
reinforced over the generation.
–  Female perpetuates the alleles that caused her to make that choice
and allows a male with such trait to perpetuate his alleles.
•  Such showy secondary structure of male indicates good
genes .
–  A bright beaks or long tails ! indicates overall male health
–  A male gray tree frog : with long mating call
•  Offspring of long-calling male frogs grew bigger faster and survived better
•  long mating call indicates overall genetic quality
•  human??
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Figure 23.16
EXPERIMENT
Recording of LC
male s call
Recording of SC
male s call
Female gray
tree frog
LC male gray
tree frog
SC male gray
tree frog
SC sperm × Eggs × LC sperm
Offspring of
SC father
Offspring of
LC father
Survival and growth of these half-sibling offspring compared
RESULTS
Offspring Performance
1995
1996
Larval survival
LC better
NSD
Larval growth
NSD
LC better
Time to metamorphosis
LC better
(shorter)
LC better
(shorter)
NSD = no significant difference; LC better = offspring of LC males superior to
offspring of SC males.
The evolution of antibiotic resistance in bacteria
•  The excessive use of antibiotics is leading to the evolution of
antibiotic-resistant bacteria
–  Livestock producers, overprescription of antibiotics, patient misuse of
antibiotics
•  As a result, natural selection is favoring bacteria that are
resistant to antibiotics
–  A gene to encode an enzyme to break down penicillin
•  Beta-lactamase (penicillin)
–  Mutation in ribosome (erythromycin)
–  Natural selection for antibiotic resistance is particularly strong in
hospitals
–  Many hospital-acquired infections are resistant to a variety of
antibiotics
•  Super-bacteria
•  100,000 people die each year in the USA
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The evolution of antibiotic resistance in bacteria
•  The fruit fly Drosophila melanogaster has an allele that
confers resistance to DDT and similar insecticides
•  Laboratory strains of D. melanogaster have been established
from flies collected in the wild in the 1930s (before the
widespread use of insecticides) and the 1960s (after 20
years of DDT use)
•  Lab strains established in the 1930s have no alleles for DDT resistance;
in lab strains established in the 1960s, the frequency of the DDTresistance allele is 37%
70
Diploidy and balancing selection preserve genetic variation
• 
Why doesn’t natural selection act to eliminate genetic variation in populations, retaining only
the most favorable alleles?
• 
Diploidy preserves variation by hiding recessive alleles
–  A recessive allele is only subject to natural selection when it influences the phenotype in
homozygous recessive individuals
–  In a heterozygote, a recessive allele is hidden or protected from natural selection.
–  it may prove advantageous when the environmental change.
• 
Balancing selection : occurs when natural selection maintains stable frequencies of two or
more phenotype forms in a population
–  Heterozygous individuals have greater reproductive success
–  For example, sickle cell anemia
• 
In frequency-dependent selection, two different phenotypes are maintained in a population
–  For example, Indonesian silverside fishes
• 
Neutral variation : Some variations may be neutral, providing no apparent advantage or
disadvantages
–  For example, human variation in fingerprints
Left-mouthed
P. microlepis
Frequency of
left-mouthed individuals
1.0
Right-mouthed
P. microlepis
0.5
0
1981 82
83
84
85
86
87
Sample year
88
89
90
Natural selection cannot fashion perfect organisms
•  Selection can only act on existing variation
–  Natural selection favors only the fittest variations from the phenotypes
that are available, which may not be the ideal traits.
–  New advantageous alleles do not arise on demand
•  Evolution is limited by historical constraints
–  Birds arose as the forelimb of a small dinosaur evolved into a wing
–  Four legs with a set of wings?
•  Adaptations are often compromises
–  Blue-footed booby
•  Chance, natural selection and the environment interact
No perfect organisms
Natural selection operates on a better than basis
Imperfections of the organisms : evidence for evolution