Evolution
A Scientific Explanation
for Similarities and
Differences between
Organisms
Darwin’s Observations
• Living Organisms resemble extinct fossil
forms.
Darwin’s Observations
• Progressive changes from simpler to more complex
organisms can be seen in the fossil record.
Darwin’s Observations
• Different island species resemble each other.
Darwin’s Observations
• Island Populations resemble those
on nearby land.
The Galapagos finches resembled the
grassquit found on the coast of Ecuador.
Darwin’s Observations
• Lands with similar climates have unrelated
plants and animals.
Darwin’s Observations
• Plants and animals of each continent
are distinctive.
A Flowchart of Evolutionary Reasoning
Potential for
rapid reproduction
Relatively constant
resources and
population over time
Competition for
survival and
reproduction (1)
Variability in
structures and
behaviors
NATURAL SELECTION
On average, the fittest
organisms leave
the most offspring (2)
Some variability is
inherited
(observations)
(conclusions)
EVOLUTION:
The genetic makeup of the
population changes over time,
driven by natural selection (3)
Darwin’s Theory of Evolution
• Evolution occurs by natural
selection.
• Evolution = progressive change in
the characteristics of organisms as
a result of changes in genetic
composition
Darwin’s Theory of Evolution
• Natural selection = mechanism for
evolution
• Process by which those individuals
whose traits adapt them to their
environment leave a larger number
of offspring
• An increase in frequency
of genotypes that confer a
favorable advantage
in a given environment.
Adaptations
• Traits that make a species survival more likely
• Can be
– structural: physical features
– functional: physiological,
biochemical, metabolic features
– behavioral mechanisms
Natural Selection favors individuals
that are well-adapted to the current
environment.The adaptations are not
“perfect” solutions.
Applying Your Knowledge
1. Adaptation
2. Evolution
3. Natural selection
A. The mechanism for evolution is
B. A progressive change in the
characteristics of organisms is
C. A trait that makes a species
survival more likely is called a(n)
Evidence for Evolution: Fossil Record
Evidence for Evolution: Geological
• Sufficient time for evolution based
on age of the earth
Evidence for Evolution:
Comparative Anatomy
• Structures that do not show a common
origin
– Analogous structures: superficially similar
• Structures that show a common origin
– Homologous Structures: same
evolutionary origin despite differences in
function
– Vestigial Structures: serve no purpose but
are homologous to structures in related
organisms
Analogous Structures
Not homologous;
analogous
Not homologous;
not analogous
Homologous;
analogous
Homologous;
not analogous
Homologous Structures
Flying
Swimming Running Grasping
Vestigial
Structures
Remnants of
hindlimb seen in
boa and whale
Functional hindlimb
in salamander
Evidence for Evolution: Embryology
• Similar embryonic patterns of
development
Evidence for Evolution: Genetics
• Mutation generates diversity
• Meiosis and Fertilization generate new
combinations due to
– Crossing Over
– Alternate patterns of chromosome
segregation
– Unique genotype of fertilizing sperm
combined with unique genotype of egg
Evidence for Evolution: Biochemistry
• Molecular similarities between
different organisms
Evidence for Evolution: Molecular Biology
• Similarities in sequence measured by
ease of separating DNA strands by heat
Evidence for Natural Selection
• Artificial Selection
Evidence for Natural Selection
• Current Observations
– Development of insecticide resistance in
western corn rootworm larvae
Chlorpyrifos
Terbufos
Methyl Parathion
Tefluthrin
Carbofuran
Western corn rootworm larval bioassay results R LD50/S LD50.
0
5
10
15
Resistance Ratios
20
Natural Selection Data, p. 115
Color 1
P S O
Color 2
P S O
Color 3
P S O
Color 4
P S O
Color 5
P S O
20 __
5 15
__
20 __
0 __
0 20 __
1 __
3
20 __
9 __
27
2nd 40
__ 12
__ 36
__ 20
__ __
6 18
__
__
0 __
0 __
0 __
4 __
0 __
0
__ __
36
7 21
__
0
__
28
__
1st 20 10
__ 30
__
3rd
48
__
24
__
0
__
4th
5th
S= Survivors Must Total 25
O= Offspring = 3 x Survivors (Place on cloth)
P= Pre-hunt = Survivors + Offspring
Natural Selection Data, p. 115
Pre-Hunt Population
70
60
50
40
Color 1
30
Color 3
20
10
0
1
2
3
4
5
Breeding Season
Plot Pre-Hunt population values
for most and least successful colors
Types of Natural Selection
Large Size Average Size Extreme Size
Favored
Favored
Favored
Time
Before
Selection
Mean
changed
over time
After
Selection
Variation
reduced
over time
Directional Stabilizing
Selection
Selection
Variation
becomes
polarized
Disruptive
Selection
Types of Selection
• Stabilizing: eliminates extremes
Stabilizing Selection & Balanced Polymorphism
SS
Homozygous
Normal
SS’
Heterozygous
S’S’
Homozygous
Defective
Dies of malaria
Lives and
reproduces
Dies of
sickle-cell anemia
SS
SS’
S’S’
Dies of malaria
Lives and
reproduces
Dies of
sickle-cell anemia
SS
SS’
S’S’
Types of Selection
• Disruptive: increases both extremes
Types of Selection
• Directional: increases one extreme
Applying Your Knowledge
1. Stabilizing Selection
2. Disruptive Selection
3. Directional Selection
Which type of selection has occurred if
• The background is sandy with dark
rocks and snails are found with
either dark or light shell colors?
• After spraying with malathion, more
fruit flies are found to be resistant to
this insecticide?
Patterns of Evolution
• Divergent
– different phenotypes arise as related
species encounter environmental
differences
Patterns of Evolution
• Convergent
– similar phenotypes arise in unrelated
species as a result of environmental
similarities
North American
Desert Plants
Cactus
African
Desert Plants
Euphorbs
Patterns of Evolution
• Convergent
– similar phenotypes arise in unrelated
species as a result of environmental
similarities
Meadowlark of
North America
Yellow-Throated
Longclaw of Africa
Patterns of Evolution
• Coevolution: species adjust
together to maintain relationship
Predators
and their Prey
Flowering plants
and their Pollinators